EXPLORATION AND MINING
REPORT 800C Predevelopment Studies for Mine Methane Management and Utilisation
H Guo, C Mallett, S Xue, B Poulsen, H Kahraman, B Zhou, R Sliwa, R Worrall
C S I RO MARCH 2001 C S I RO
Exploration & Mining
EXPLORATION AND MINING REPORT 800C
Predevelopment Studies for Mine Methane Management and Utilisation
H Guo, C Mallett, S Xue, BA Poulsen, H Kahraman, B Zhou, R Sliwa, R Worrall B Shen, D Adhikary, J Boland and M Cunnington
March 2001
CONFIDENTIAL REPORT
This report is not to be given additional distribution or cited in other documents without the consent of CSIRO Exploration and Mining and JCOAL.
CSIRO Exploration & Mining PO Box 883, Kenmore, Queensland, Australia 4069 TABLE OF CONTENTS
1 ACKNOWLEDGEMENTS...... 9
2 INTRODUCTION...... 10
2.1 Project Background...... 10
2.2 Scope of Work...... 10
3 SUMMARY...... 12
3.1 Northern Hunter Valley Coal and Gas Resources...... 12 3.2 Gas Recovery Technologies ...... 14 3.3 Gas Utilisation Technologies ...... 14 3.4 Government Policy and Regulation...... 15 3.5 In Situ Stress and Permeability Measurements...... 16 3.6 Laboratory Tests and Result Interpretation...... 22 3.7 Borehole Geotechnical Interpretation...... 23 3.8 3D Geological Structural Interpretation and Model...... 26 3.9 3D Sedimentary Characterisation and Model...... 30 3.10 3D Geotechnical Characterisation and Model...... 39 3.11 Gas Characterisation and Model of Arrowfield and Bowfield Seams...... 42 3.12 Software for Simulation of Gas and Rock Behaviours...... 47 3.13 Implication of Research Findings to Date...... 52
3.14 Key technical issues and tasks...... 52
4 GEOLOGICAL AND GEOTECHNICAL SETTING OF THE NORTHERN HUNTER VALLEY...... 56
4.1 Introduction...... 56
4.2 Regional Geology...... 58 4.2.1 Stratigraphy ...... 59 4.2.2 Geological Structures ...... 61 4.2.3 Geological Structures - North Hunter Valley coalfield ...... 62 4.3 Coal Resources / Reserves...... 66 4.3.1 Introduction ...... 66 4.3.2 Economic Coal Seams ...... 68
2 4.3.3 Coal Seams Mined or Proposed to be Mined ...... 69 4.3.4 Coal Resources and Reserves ...... 71 4.3.5 Coal Quality ...... 72 4.3.6 Mine Development and Restrictions ...... 74 4.4 Coal Seam Gas...... 76 4.4.1 Overall Assessment ...... 76 4.5 Mine-Scale Coal Seam Gas...... 77 4.6 Summary...... 79 4.7 References...... 81
5 INNOVATIVE GAS RECOVERY TECHNOLOGY...... 82
5.1 Introduction...... 82 5.2 Techniques, Performance and Applicability...... 83 5.2.1 In-Seam Borehole ...... 83 5.2.2 Surface Wells ...... 84 5.2.3 In-Mine Cross-Measure Boreholes ...... 85 5.2.4 Surface Goaf Wells ...... 87 5.2.5 Purity and Variation of Captured Gas ...... 89 5.3 Application in the Hunter Valley coalfield...... 90 5.4 Summary...... 92
5.5 References...... 93
6 HUNTER VALLEY GAS UTILISATION OPTIONS...... 95
6.1 Introduction...... •...... 95 6.2 Regional & mine specific issues...... 96 6.2.1 The Wambo mine and the Upper Hunter Valley - context ...... 96 6.2.2 Applications for mine methane ...... 99 6.2.3 Gas quality ...... 99 6.3 Potential Utilisation Options ...... 99 6.3.1 Power Generation ...... 100 6.3.2 Other Uses...... 103 6.4 Discussion...... 106
6.5 References...... 107
7 GOVERNMENT POLICY AND REGULATION...... 108
7.1 Introduction...... 108 7.2 Domestic Climate Change Policy...... 108
3 7.2.1 The Australian Greenhouse Office ...... 109 7.3 Emissions Trading and Carbon Credits...... 110 7.3.1 Speculation in Carbon Credits ...... 111 7.4 New South Wales policy & regulation...... 111 7.4.1 Introduction ...... 111 7.4.2 Coal Seam Methane Extraction ...... 112 7.4.3 Mining Act 1992 No. 29 and Mining (General) Regulation 1992 No. 445 - New South Wales ...... 113 7.5 Discussion...... 114
7.6 References...... 115
8 AUTOMATIC BOREHOLE GEOTECHNICAL INTERPRETATION...... 116
8.1 Introduction...... 116 8.2 Geotechnical Characterisation from Geophysical Logs...... 117 8.2.1 Strata Classification ...... 117 8.2.2 Geophysical characterisation of the strata ...... 118 8.2.3 Geophysical characterisation ...... 124 8.3 Results of LogTrans Application to the control boreholes ...... 128 8.3.1 Geological Interpretation ...... 128 8.4 Geotechnical Interpretation...... 135
8.5 Conclusions...... 137 8.6 References...... 137
9 3D GEOLOGICAL STRUCTURES AND MODEL...... 138
9.1 Introduction...... 138 9.2 Characterisation of structures at the Wambo mine...... 139 9.2.1 NE-trending thrust faults ...... 140 9.2.2 NW-trending thrust faults - Redmanvale Fault ...... 144 9.2.3 NE-trending normal faults ...... 144 9.2.4 NS dykes and faults ...... 145 9.2.5 Cleat ...... 146 9.2.6 Joints and minor WNW faults ...... 148 9.2.7 Folds and rolls ...... 150 9.3 Updated 3D structural model...... 152
9.4 Conclusions and recommendations...... 153 9.5 References...... 154
10 3D SEDIMENTARY CHARACTERISATION AND MODEL...... 155 4 10.1 Introduction...... 155 10.2 General Geology...... 155
10.3 Methodology ...... 159 10.3.1 Data Manipulation ...... 159 10.3.2 Lithological Interpretation from Geophysical Logs and Lateral Correlation of Sandstone Units ...... 160 10.4 Results...... 167 10.4.1 Rock Types ...... 167 10.4.2 Distribution of Coal Seams and Sandstones ...... 170 10.5 Relationship of Sandstones to Structure...... 181 10.6 Zoning of Roof Conditions and Implications for Mining ...... 181 10.7 Relationship between Seam Gas, Structure And Sandstones...... 189 10.8 Sedimentary Environment ...... 196
10.9 Conclusions...... 197 10.10 References...... 199
10.11 Additional Figures...... 200
11 3D GEOTECHNICAL CHARACTERISATION AND MODEL...... 223
11.1 Introduction...... 223 11.2 Geotechnical Classification...... 223 11.2.1 Strength from Velocity ...... 224 11.2.2 LogTrans Analysis ...... 224 11.2.3 Classification Method ...... *...... 225 11.2.4 Cross-sections ...... 230 11.2.5 Sections between the Whybrow and Bowfield Seams ...... 232 11.2.6 Sections between Woodlands Hill and Bowfield Seams ...... 236 11.2.7 Three Dimensional Geotechnical Model...... 239 11.2.8 Roof conditions to the west and east of the NS dyke ...... 241 11.3 Geotechnical Properties ...... 242 11.4 Conclusions...... 244
12 GAS RESERVOIR CHARACTERISATION OF ARROWFIELD AND BOWFIELD SEAMS...... 245
12.1 Gas Sorption Isotherm ...... 245 12.1.1 Background ...... 246 12.1.2 Measuring Sorption Isotherm ...... 247 12.1.3 Sorption Isotherm of Arrowfield Seam ...... 248 5 12.1.4 Sorption Isotherm of Bowfield Seam ...... 251 12.2 Gas Content...... 254 12.2.1 Background ...... 255 12.2.2 Gas Content of Arrowfield Seam ...... 256 12.2.3 Gas Content of Bowfield Seam ...... 258 12.3 Gas Composition...... 261 12.3.1 Gas Composition of Arrowfield Seam...... 261 12.3.2 Gas Composition of Bowfield Seam ...... 263 12.4 Coal Seam Thickness ...... 266 12.4.1 Thickness of Arrowfield Seam ...... 266 12.4.2 Thickness of Bowfield Seam...... 267 12.5 Gas Diffusivity through CoalMatrix ...... 268 12.5.1 Background of Diffusion Theory ...... 268 12.5.2 Measuring/Estimating Gas Diffusivity...... 269 12.5.3 Sorption Time of Arrowfield and Bowfield Seams ...... 270 12.6 Reservoir And Desorption Pressure...... 270 12.6.1 Reservoir Pressure ...... 270 12.6.2 Desorption Pressure ...... 271 12.7 Seam Permeability...... 272 12.7.1 Permeability of Arrowfield and Bowfield Seams ...... 272 12.7.2 Factors Affecting Seam Permeability ...... 273 12.8 Relative Permeability...... 275 12.9 Gas Saturation...... 277 12.10 Summary...... v...... 278 12.10.1 Arrowfield Seam ...... 279 12.10.2 Bowfield Seam...... 281 12.11 Reference...... 283
13 3D GAS RESERVOIR MODEL...... 284
13.1 Model Geometry...... 284 13.2 Model Mathematics...... 285 13.2.1 Diffusional Flow Process in Coal Matrix (Pick’s Law)...... 286 13.2.2 Porous Flow Process in Cleats (Darcy's Law)...... 286 13.2.3 Sorption Mechanism (Langmuir Equation) ...... 286 13.3 Model Properties ...... 287 13.3.1 Gas Content ...... 287 13.3.2 Gas Composition ...... 289
6 13.3.3 Gas Adsorption Isotherm ...... 296 13.3.4 Seam Thickness...... 299 13.3.5 Reservoir Pressure ...... 300 13.3.6 Desorption Pressure ...... 301 13.3.7 Seam Permeability ...... 301 13.3.8 Relative Permeability ...... 302 13.3.9 Others...... 302
14 LABORATORY TESTS AND MATERIAL STRENGTH ANALYSIS...... 303
14.1 Introduction...... 303 14.2 Summary of Laboratory Test Results...... 304
14.3 Rock Strength ...... 311 14.4 CoalStrength ...... 314 14.4.1 Analysis of Laboratory Tests Results ...... 314 14.4.2 Coal Seam Brightness ...... 316 14.4.3 Mass Coal Strength Evaluation ...... 319 14.5 Slake Durability of the Seam Floors...... 321 14.6 Development of rock strength formula for the Wambo mine...... 322 14.6.1 Empirical formula of sonic velocity - rock strength ...... 322 14.7 Summary...... 326
14.8 References...... 327
15 IN SITU STRESS MEASUREMENTS...... 328
15.1 Rock Stress Measurements...... v...... 328 15.1.1 Data Selection ...... 328 15.1.2 Data analysis ...... 328 15.1.3 Discussion ...... 329 15.2 Coal Stress Measurements...... 335 15.2.1 Data and Analysis...... 335 15.2.2 Discussion ...... 335 15.3 Summary...... 336 15.4 References...... 336 15.5 Figures...... 337
16 GAS AND ROCK BEHAVIOUR SIMULATION SOFTWARE...... 352
16.1 Background...... 352 16.2 The trend in model development ...... 352
7 16.2.1 Single Porosity Model ...... 353 16.2.2 Double Porosity Model ...... 353 16.2.3 Hydraulic conductivity ...... 354 16.3 Practical Applications...... 355 16.4 Software for Simulation of Gas and Rock Behaviours...... 356 16.4.1 Empirical/Semi-Empirical Approach ...... 356 16.4.2 Flow simulators (Uncoupled with mechanical deformation) ...... 357 16.4.3 Coupled Mechanical - Flow Simulators ...... 358 16.5 Proposed Integrated Simulation Strategy...... 359
16.6 Summary...... 361 16.7 Bibliography ...... 362
17 DISCUSSION...... 364
17.1 Stress level at depths ...... 364 17.2 Permeabilities and Gas Contents of Arrowfield and Bowfield seams at Wambo mine ...... 365 17.3 Geological and geotechnical complexity...... 366 17.4 Recovery and utilisation technologies ...... 367
17.5 Key technical issues and tasks...... 367 17.6 References...... 369
APPENDICES
A HYDRAULIC FRACTURE SEDIMENT STRESS MEASUREMENT PROGRAM BORE HOLES WA 55, 58, 69 & 69R
B GAS SORPTION TESTING
C PERMEABILITY REPORT
D MULTIPHASE TEST REPORT
E RELATIVE PERMEABILITY AND RELATED PARAMETERS MODELLING
8 1 ACKNOWLEDGEMENTS
The authors wish to express their sincere gratitude to NEDO and JCOAL for providing a significant part of the funds for conducting this research work. Special thanks are due to Mr Haruhiko Yoshida and Mr Koji Tanaka of NEDO, Mr Toyoshi Takakuwa, Mr Mike Fujioka, Mr Kouichi Koizumi and Dr Gota Deguchi of JCOAL for their kind support.
The authors also wish to express their grateful thanks to Mr Norio Ishihara of Sumitomo Coal Mining Co Ltd for his valuable advice and support of the project.
The authors wish to record their gratitude to the management and staff of Wambo Mining Corporation Pty Ltd and the United Collieries for the significant in-kind contributions to the project. Special thanks must go Messrs. Phil McNamara, Stephen Smith and Bryan Atkins of Wambo Mining Corporation Pty Ltd, and Mr Ken Halverson of the United Collieries for their kind support and co-operation.
The authors would like to thank Exploration & Mining for the facilities and significant support provided in the conduct of the study, particularly in the areas of the rock testing and geophysical interpretation. The authors would like also to thank our following colleagues who contributed to various aspects of this work: Ms Michele Quigley, Dr Joan Esterle, Dr Rao Balusu, and Mr Michael Kelly at Exploration & Mining. Thanks must also go Dr Jim Enever of CSIRO Petroleum for his analysis of the in situ stress measurement data.
A number of other organizations have participated in this year ’s project work and we wish to express our sincere appreciation to:
• Messrs Jeff Edgoose and David Casey of Multiphase Technologies Pty Ltd for undertaking the in situ stress and permeability measurements at the Wambo mine;
• Dr Eugene Yurakov of GeoGas Pty Ltd for the relative permeability and related parameters modelling;
• Messrs John Howard and Michael Wade of Earth Data for providing the borehole data and site support, and
• Mr Paul Maconochie of GeoTek Solutions Pty Ltd for his kind assistance in reviewing this report.
9 2 INTRODUCTION
2.1 Project Background
Following the agreement in March 1999 between the ministers for MITI (Japan) and DISR (Australia) on co-operation in two new energy research projects, the New Energy and Industrial Technology Development Organisation (NEDO) of Japan, and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia have entered into an agreement to conduct a joint research project entitled “Predevelopment Studies for Mine Methane Management and Utilisation."
The objective of the project is to reduce the environmental impacts of mines by developing new recovery and utilisation technologies for fugitive greenhouse gases. The project is also to develop simulation techniques to predict methane gas flow during mining in order to select and plan the most suitable mining, gas management and utilisation strategies for new mining areas.
The project period is 5 years and commenced in October 1999.
The Wambo mine in the Hunter Valley, NSW has been selected as an experimental site. The site study is to focus on the deep seams that contain the majority of the future mining reserves. It is considered likely that the Wambo mine site will present conditions that are typical of many other areas in the Hunter Valley and also in other Australian coalfields.
Within the framework of the joint project, Exploration and Mining has entered into a research contract with the Japan Coal Energy Centre (JCOAL) and has now completed the second year project work plan, the results of which are presented in this report.
2.2 Scope of Work
The main components of the second year research work as specified in the contract and as were completed, are as follows:
• Data collation of:
o the northern Hunter Valley regional coal and gas resources;
o innovative gas recovery technologies and practice;
o gas utilisation options in relation to the conditions of the region and;
o government policy and regulation in relation to greenhouse issues.
• In situ stress and permeability measurements, and laboratory testing for rock and coal strengths;
10 • 3D site geological, geotechnical and gas characterisation and modelling of the Wambo mine; and
• A study of simulation software of gas and rock behaviour during mining.
11 3 SUMMARY
This report presents results of the second year research work carried out under JCOAL - Exploration & Mining JFY 2000 Contract, in relation to the project “Predevelopment Studies for Mine Methane Management and Utilisation ”.
The principal objective of the project is to research the recovery and utilisation technology of mining released methane gas in order to reduce environmental impacts, and also to develop simulation techniques to predict methane gas flow during mining.
The main tasks that have been completed are as follows:
• Data collation of:
o the northern Hunter Valley regional coal and gas resources;
o innovative gas recovery technologies and practice;
o gas utilisation options in relation to the conditions of the region and;
o government policy and regulation in relation to greenhouse issues;
• In situ stress and permeability measurements, and laboratory testing for rock and coal strengths;
• 3D site geological, geotechnical and gas characterisation and modelling of the Wambo mine; and
• A study of simulation software of gas and rock behaviour during mining.
The key findings of this year research work are summarised as follows:
3.1 Northern Hunter Valley Coal and Gas Resources
In 1998-99 raw production from twenty-two operating coal mines in the northern Hunter Valley coalfield amounted to around 77 million tonnes.
Measured in situ resource in the northern Hunter Valley coalfield is about 8,722 million tonnes, the recoverable reserve is about 3,900 million tonnes, and the marketable reserve is about 3,300 million tonnes. Significant coal resources of high quality exist to the west of Scone (Figure 3-1).
The northern Hunter Valley coalfield to the west of the Mount Ogilvie Fault covers approximately 530 km2, with in-place gas resources of 126.86 billion m3. The in-place
12 3.2 Gas Recovery Technologies
There are four major existing methane capture techniques currently used in coal mines. These techniques are in-seam boreholes, surface wells, in-mine cross-measure boreholes, and surface goaf wells.
Among these techniques, surface wells have not been widely used in Australian coal mining industry and the technique is primarily used by the coal bed methane industry and its technical and cost effectiveness in controlling gas emission from coal mines has yet to be proven.
The in-seam borehole technique has been extensively used for gas pre-drainage to control gas emission in roadway development and longwall operation, and to minimise the risk of outburst of coal and gas. However, its effectiveness depends on seam permeability. In general, the technique is successful if the working seam has a moderate to high permeability.
In-mine cross-measure borehole and surface goaf well techniques are widely used in underground coal mines to control the problem of high gas emission. The success of these techniques depends upon geology and longwall caving behaviour. In general the techniques are applied if there are overlying and/or underlying gassy seams contributing to gas emissions during longwall operations.
In the Hunter Valley coalfield, in-seam borehole and surface goaf well drainage have been successfully used in the South Bulga and Dartbrook mines. Particular success has been achieved in the Dartbrook mine where more than 50% of total gas emission has been captured using these techniques.
3.3 Gas Utilisation Technologies
Methane is a highly valued resource because it is free from sulphur and other contaminants that affect alternative fossil fuels. There are numerous applications for the utilisation of mine methane. However, given that the gas from the Wambo mine is predicted to contain up to 50% carbon dioxide, applications for direct utilisation such as integrating into natural gas pipeline grid are restricted. Without the added significant cost of gas cleanup, the gas could not be used in ‘pipeline ’ applications such as domestic consumption and some industrial processes. However, a review of potential applications and consultation with industry, conducted previously by CSIRO, has indicated that electricity generation and mine water management are two favoured options for mine methane utilisation. Both applications can successfully use seam gas of variable proportions of methane and carbon dioxide such as is likely to be produced from the Wambo mine.
14 Electricity generation is an attractive option as it can either be used locally or sold to the grid. The market and the infrastructure to market electricity already exist, and electricity generation is a major industry in the Hunter Valley region. Several suitable technologies exist to convert the gas into electricity: directly in gas reciprocating engines, as power station air, directly in gas turbines, or as part of a co-fired coal and gas fired turbine.
Mine water management is an issue for many mine sites and the use of methane to power water purifying processes would not only create a market for the methane supply but also supports the long term sustainability of coal mining. Water is an important and emotive issue in the Hunter Valley region, where competition for scarce water resources is increasing, and issues of water quality have recently seen the introduction of a salinity trading scheme. Water distillation utilizing mine methane is a technology already employed in Poland, and could be applied in Australia.
Other identified uses for seam gas in the Hunter Valley region include using it as a fuel supplement in a steel blast furnace or in industrial boilers, and for thermal drying of coal.
3.4 Government Policy and Regulation
Current Federal Government policy and regulation have no direct control on the exploration and extraction of coal seam methane (CSM) resources. However, Federal policy and regulations are likely to have a great impact on future CSM operations because of their potential importance to greenhouse initiatives and carbon trading schemes. State policy and regulation will also have a significant impact on exploration for, and exploitation of, CSM resources in NSW because of the lack of natural gas resources in NSW.
Australia ’s policy on greenhouse gas reduction is still being formulated. In particular, the Federal Government can influence future directions in energy consumption, through means of a carbon tax/trading scheme, and by setting mandatory renewable energy targets. It is possible that the Federal Government will implement some form of carbon trading scheme within the next 5 years, as part of the commitment to meet Australia ’s greenhouse gas targets established in the 1997 Kyoto Protocol. As fossil fuel industries are inherently carbon intensive and non-renewable, this will affect them significantly. Within this framework however, lie opportunities for derivative industries that have a lower greenhouse intensity than traditional fossil fuel activities. The exploitation of, and demand for gas resources throughout Australia is likely to increase substantially over the short to medium term as a logical first step in developing a less greenhouse intensive energy sector. The substantial coal seam methane resources in NSW are the State ’s only identified means of partially meeting projected gas demand from within its own borders.
15 3.5 In Situ Stress and Permeability Measurements
In situ stress measurements were carried out to determine the direction and magnitude of principal horizontal stresses in the key strata about the Arrowfield and Bowfield seams using the hydraulic fracture technique. The magnitude of the minimum effective stress in the Arrowfield and Bowfield seams was measured using the step rate test method. The results of these measurements indicate that stress conditions when mining the Arrowfield and Bowfield seams at depth will be significantly higher than those encountered in most of the currently operating mines in Australia. As a result, a relatively high degree of strata deformation and fracturing during mining can be expected, and significant ground control will be required.
The key results of the stress measurements are summarised as follows:
• The orientation of the major horizontal principal stresses measured from WA55 (324 m), WA58 (246 m), WA69 (227 m) and WA69R (541 m) ranges from the east-west direction to north-west, south-east direction. These measurements suggest that the stress field orientation at the Wambo mine is consistent with those measured regionally throughout the Hunter Valley. (Figure 3-2)
• The magnitude of the major horizontal principal stresses measured generally varied between the corresponding magnitude of the vertical (overburden) stress based on the depth of cover, and approximately twice this value. At a depth of about 500 m, the major principal horizontal stress is high, ranging from 23 MPa to 30 MPa which is more than twice the approximate vertical stress (12.5 MPa) (Figure 3-3).
• The magnitude of the minimum effective stress (effective stress) in the Arrowfield and Bowfield seams increases systematically with depth. From surface level to about 300 m depth, the value of the stress is approximately 50% of the corresponding overburden pressure. Below 300 m depth, the value of the minimum effective stress in the coal approaches the corresponding overburden pressure as indicated by the measurement (10.5 MPa) at 500 m (Figure 3-4). This trend with depth has been found throughout the eastern coal basins.
16 Orientation of Major Horizontal Stress All Boreholes
Orientation (degrees M)
No valid results from below 276.71 metres
Figure 3-2 Orientation of major horizontal stress.
17 • The tested coal samples from the Arrowfield and Bowfield seams were reasonably competent. The equivalent UCS of the laboratory size samples is estimated to be 23 MPa for the Arrowfield seam and 27 MPa for the Bowfield seam. The Arrowfield seam is slightly brighter and hence weaker than the Bowfield seam. The estimated mass UCS of the two seams is 4.3 MPa for the Arrowfield seam and 5.0 MPa for the Bowfield seam.
• The slake durability index for the floor rocks from both the Arrowfield and Bowfield seams are both about 85%. This value represents Medium to Medium-High durability. Generally competent floor conditions are expected but with local variation.
• Based on the current and previous test results, a rock strength - sonic velocity relationship has been developed for application at the Warn bo mine. This formula can be used with reasonable confidence given that it was developed from 183 tests of rock samples from the Wambo mine. The formula predicts well the UCS strength of the most rocks, particularly for velocities under 4500 m/s. The strength and velocity relationship has been used for the subsequent geotechnical characterisation and modelling work completed in this year.
3.7 Borehole Geotechnical Interpretation
A detailed study of geological and geophysical data from the five cored boreholes, WA48, WA50, WA51, WA55 and WA58 (Figure 3-9), was carried out to develop drill hole geotechnical classification and interpretation procedures to process all available borehole geophysical data in the area of the study. LogTrans, a geophysical interpretation computer program was successfully used to interpret the key strata units at the Wambo mine. The overall success rate of strata prediction for all five control boreholes was 80.4%. Figure 3-10 shows an example of a LogTrans interpretation. The results of this study indicate that this is an efficient method to interpret all other available drill holes in the study area for the subsequent 3D geotechnical characterisation and modelling of the project area.
23 Table 3-1 Legend of Key Strata Units at Wambo Mine Symbol Descriptions
CO Coal seam MS Mudstone ST Siltstone SS Massive sandstone SSG Massive sandstone with high gamma ray readings and low sonic velocities SSST Interbedded and interlaminated sandstone-siltstone CGSS Interbedded conglomerate sandstone SSCG Interbedded conglomerate sandstone with high gamma ray readings/low sonic velocity values MSSS Interlaminated mudstone and sandstone
Following the success of the LogTrans trial, all available borehole geophysical data was then interpreted for the subsequent 3D geotechnical characterisation and modelling.
3.8 3D Geological Structural Interpretation and Model
New structural observations were made and the structural analysis of the Wambo and United mines compiled in March 2000 (Guo et al, 2000) was updated. The new structure map and 3D geological structure model are shown in Figure 3-11 and Figure 3-12 respectively. The updated structural model provided an improved framework for future assessment of structural risks during underground mining at the Wambo mine.
26 • The NNE-trending normal faults are restricted to the northwestern portion of the lease areas. These faults are tight normal faults and coincide with the position of a roll in the Whybrow seam.
• Vertical dyke-filled structures mapped at the Whybrow seam level are likely to be vertically persistent into the Arrowfield seam level and below.
• Although dykes intersected in the Homestead mine were weak, some strong sections were intersected in the United workings, suggesting that the dykes may pose mining problems at the Arrowfield seam level.
• Cleat mapping has identified consistent NNW-trending face cleat and NE-trending butt cleat.
• All cleats and some early joints in the workings and in the boreholes are invariably filled with calcite. Further detailed characterisation of the cleat of the seams below the Whybrow seam could be gained from a detailed examination of the drill core.
• The cleats are overprinted by the normal and thrust faults.
• The orientations of mapped joints reflect the proximity of larger structures. Detailed joint mapping should be continued in all new developments, to serve as a warning tool for faults occurring ahead of the workings.
• Small normal faults that are parallel to the main joint set may be accommodation structures associated with rolls in the coal seam.
• Most folds and rolls in the Whybrow seam are controlled by large sand bodies in the floor, and not by tectonic folding.
Some of the above findings have been incorporated into the current 3D geological structural model. In particular the following modifications have been made to the model:
• The NE trending thrust faults to the east of the NS dyke have been re-modelled to remove the single postulated fault through borehole WA41. This single fault has been replaced with individual faults through holes WA41, WA37 and WA39. In addition an isolated fault is modelled through hole WA2 with the main thrust fault running south of WA2.
• The NE trending faults to the west of the NS dyke have been modelled.
29 The thick stacked sandstone packages formed around the zones (hinge lines) running in NE- SW, EW and NS orientations. Significant seam splitting occurs along these hinge lines. The spatial relationship between the local anticline, the position of split lines and thick sandstones suggests that much of the structure in the Bowfield and Arrowfield seams originated from differential compaction rather than tectonic deformation.
The development of these thick sandstones in the mine lease impacts on the goafing behaviour during mining. The interburden between the Arrowfield and Woodlands Hill seams has been divided into seven zones based on the proportion and thickness of sandstone units and the position of the coal seams above the Arrowfield seam. These zones can be correlated to overlying roof strength, with strength increasing from Zone 1 to Zone 7. Strong roof zones occur in an area covering much of Homestead mine and may cause some weighting problems. Weak roof zones associated with faulting and split zones occur in the eastern portion and north-western corner of the lease (Figure 3-14).
31 Table 3-2 Preliminary mechanical properties for geotechnical units. Unit Average Average Average Average UCS Density Young’s Cohesion Friction (MPa) kg/m 3 modulus (MPa) angle (°) (GPa) Unit 1 (strong) 2500 18 12.4 45 40-80 Unit 2 2500 14 8.7 40 30-45 Unit 3 2500 10 5.4 35 20-35 Unit 4 (weak) 2300 8 4.3 30 10-20 Unit 5 (coal) 1400 4 1.2 35 4.3-5.0
The 3D geotechnical model contains strata conditions and their spatial variation that will be required for detailed geotechnical analysis and mining simulation. The geotechnical model shows that the immediate roof condition of the Arrowfield seam is very different either side of the NS dyke, suggesting that mining induced caving characteristics would be quite different to the west and east of the dyke. To the east of the NS dyke the Woodlands Hill plies WHC and WHO come in close contact with the Arrowfield seam suggesting weak roof conditions. To the west of the NS dyke the Woodlands Hill plies are approximately 30-40 m above the Arrowfield seam and the height of the immediate cave zone should be influenced by the weak, intermediate and strong rock strata rather than the height of the first roof coal.
The 3D geotechnical block model will be used as input for a predictive strata behaviour and gas simulation work planned.
The geotechnical model should be refined as data being acquired while this report is being written becomes available including drilling data and seismic survey data.
3.11 Gas Characterisation and Model of Arrowfield and Bowfield Seams
Reservoir characterisation of the Arrowfield and Bowfield seams within the Wambo mining lease (study area) has been conducted and they are summarised in this section. The seams are characterised by
• Moderate to high gas content, gas content increasing with depth (up to 12 m3/t);
• Moderate permeability at around 230 m depth of cover, and low to very low permeability at and greater than 300 m depth. Permeability sharply decreasing with increasing effective stress (depth) and is greatly influenced by cleat infill (as low as 0.005 mD);
42 Carbon dioxide constituting a significant portion of seam gas, its portion increasing with depth (up to 50%) and;
• Arrowfield and Bowfield seams are closely spaced (about 10 to 30 m).
These major characteristics could have some significant implication and pose some major difficulties and challenges in mine development and operation, in the deep part of the Wambo mine (below 300 m depth of cover), in the context of mine gas control and gas drainage. Issues to be considered include:
• The need to substantially drain seam gas prior to mine development and operation in order to control gas emission and potential gas/coal outbursts because of high gas content.
• The impact of low permeability and high C02 on conventional gas pre-drainage. Innovative pre-drainage techniques need to be developed.
• Effective post-drainage would be required for the future mining to combat the gas emission from the overlying/underlying seam. This requires a clear understanding of gas flow mechanisms, a reliable prediction of gas emission during mining, and further development of current post-drainage techniques.
Arrowfield Seam
Adsorption Isotherms
• Three coal samples were taken from the Arrowfield seam in the boreholes WA55, WA58 and WA69R for adsorption isotherm measurement for both CH4 and C02.
• For CH4 adsorption isotherms (daf) of these three samples, Langmuir volumes are 23.07, 23.09 and 24.45 m3/t, Langmuir pressures are 2,314, 2,162 and 2,264 kPa.
• For C02 adsorption isotherms (daf) of these three samples, Langmuir volumes are 62.78, 62.01 and 59.50 m3/t, Langmuir pressures are 1,722, 1,676 and 1,972 kPa.
Gas Content
• Measured gas content of the Arrowfield seam ranged from 2.9 to 13.1 m3/t and most of the values were from 7.5 to 10.5 m3/t. The average gas content was 9.0 m3/t.
• Gas content consistently increased with depth to more than 6 m3/t at about 200 m. Gas content increased further in general with the cover depth greater than 200 m.
Gas Composition
• Gas in the Arrowfield seam is a mixture of CH4 and C02.
• CH4 percentage decreased as the cover depth increased. 43 • C02 percentage increased with the increase of cover depth.
• CH4/CO2 ratio varied from around 80/20 to about 60/40 as the cover depth increased from 200 to 600 m.
Seam Thickness
• The Arrowfield seam ranges from 2.75 to 4.52 m in thickness and the average is 3.61 m.
Sorption Time (Gas Diffusivitv)
• Sorption time values were derived from the desorption tests of three coal samples taken from the Arrowfield seam in the boreholes WA55, WA58 and WA69R. These values are 34, 40 and 8 days respectively.
Reservoir Pressure
• Reservoir pressures of the Arrowfield seam were measured in boreholes WA55, WA58 and WA69R. The reservoir pressures were 2,751, 1,926 and 4,556 kPa respectively.
• A linear relationship was found between the reservoir pressure and reservoir depth.
Desorption Pressure
• Desorption pressures of the Arrowfield seam were estimated from well tests and they were compared with those derived from the adsorption isotherm. The comparison indicates that in some cases there are some noticeable differences. It is revealed that the estimated desorption pressures from the well tests may not be accurate in these cases because desorption process may not be characterised by a definite pressure response.
• Desorption pressures from the coal samples taken from the boreholes WA55, WA58 and WA69R were 1,411, 1,271 and 1,507 kPa respectively.
Seam Permeability
• Well tests were conducted in the boreholes of WA55, WA58 and WA69R to measure the permeability of the Arrowfield seam.
• Permeabilities measured in boreholes WA55, WA58 and WA69R were 0.56-0.61, 8.35-8.38, and 0.005-0.014 mD respectively. These results are considered to show a moderately high level of variability
• The variability of the measured permeability is due mainly to effective stress and cleat infilling.
44 • A marked decrease in permeability is observed with increasing effective stress.
• The coal samples of the Arrowfield seam were visually checked and it was observed that coal cleats are very much filled by calcite in the sample taken from the borehole WA69R. This cleat infilling reduces coal permeability significantly. A combination of high effective stress and cleat infilling may explain the very low permeability of 0.005- 0.014 mD that was measured in the Arrowfield seam of the borehole WA69R.
Gas-Water Relative Permeability
• Relative permeability of the Arrowfield seam was estimated by curve matching the multiphase well test results using reservoir simulator.
• Irreversible water saturation was approximately 0.7.
Gas Saturation
• Gas saturation of the Arrowfield seam was analysed using gas contents, adsorption isotherms and the reservoir pressures. Results indicate that the Arrowfield seam is undersaturated. The degree of undersaturation ranged from 37 to 45%, retaining 63 to 55% of its maximum possible sorptive capacity.
Bowfield Seam
Adsorption Isotherms
• Three coal samples were taken from the Bowfield seam in the boreholes WA55, WA58 and WA69R for adsorption isotherm measurement for both CH4 and C02.
• For CH4 adsorption isotherms (daf) of these three samples, Langmuir volumes are 22.24, 24.58 and 23.15 m3/t, Langmuir pressures are 2,158, 2,240 and 2,304 kPa.
• For C02 adsorption isotherms (daf) of these three samples, Langmuir volumes are 58.20, 65.86 and 61.36 m3/t, Langmuir pressures are 1,606, 1,757 and 2,145 kPa.
Gas Content
• Gas content of the Bowfield seam ranges from 3.9 to 13.0 m3/t and most of the gas content values are between 7.5 to 11.0 m3/t. The average gas content is 9.1 m3/t.
• Gas content hovers between 4.1 to 13.0 m3/t once the cover depth is over 200 m and it does not increases with the cover depth.
Gas Composition
• Gas in the Bowfield seam is a mixture of CH4 and C02.
• CH4 percentage decreases as the cover depth increases.
45 • C02 percentage increases with the increase of cover depth.
• CH4/CO2 ratio varies from around 80/20 to about 50/50 as the cover depth increases from 200 to 600 m.
Seam Thickness
• Thickness of the Bowfield seam ranges from 2.87 to 4.31 m and the average thickness is 3.50 m.
Sorption Time (Gas Diffusivitv)
• The values of sorption time is derived from the desorption tests of three coal samples taken from the Bowfield seam in the boreholes WA55, WA58 and WA69R. These values are 21,32 and 18 days respectively.
Reservoir Pressure
• Reservoir pressures of the Bowfield seam are measured in the boreholes WA55 and WA58. The reservoir pressures are 2,917 and 2,088 kPa respectively.
• A linear relationship exists between the reservoir pressure and reservoir depth.
Desorption Pressure
• Desorption pressures of the Bowfield seam were estimated from well tests and they were compared with those derived from the adsorption isotherm. The comparison indicates that in some cases there are some noticeable differences. It is revealed that the estimated desorption pressures from the well tests may not be accurate in these cases because desorption process may not be characterised by a definite pressure response.
• Desorption pressures for the coal samples taken from the boreholes WA55, WA58 and WA69R were 1,177, 1,831 and 1,122 kPa respectively.
Seam Permeability
• Well tests were conducted in the boreholes of WA55and WA58 to measure the permeability of the Bowfield seam.
• Measured permeabilities in the boreholes WA55 and WA58 were 0.050-0.064 and 14.6-14.7 mD respectively. Results show quite a degree of variability in measured permeability for the Bowfield seam.
• A marked decrease in permeability is observed with increasing effective stress.
• If the measured permeabilities from the boreholes WA55 and WA58 are compared for both the Arrowfield and Bowfield seams, two very similar effective stress gradients 46 are apparent for each seam, suggesting a significant reduction in permeability with a only slight increase in effective stress.
Gas-Water Relative Permeability
• Relative permeability of the Bowfield seam was estimated by curve matching the multiphase well test results using reservoir simulator.
• Irreversible water saturation is around 0.7.
• Relative permeability is found to be the same for both the Arrowfield and Bowfield seams, indicating a similar behaviour in this regard.
Gas Saturation
• Gas saturation of the Bowfield seam was analysed using gas contents, adsorption isotherms and the reservoir pressures. Results indicate that the Bowfield seam is undersaturated. The degree of undersaturation ranges from 23 to 34%, retaining 77 to 66% of its maximum possible sorptive capacity.
3.12 Software for Simulation of Gas and Rock Behaviours
To achieve the aim of developing an efficient integrated simulation method capable of predicting the transient process of ground deformation and subsequent gas emission during mining, firstly a review of existing software and modelling methods was carried out.
Existing commercially available computer codes can be grouped into following three categories:
• Empirical/semi-empirical methods
• Flow simulators (uncoupled with mechanical deformation)
• Coupled Mechanical - Flow Simulators
Empirical/Semi-Empirical Approach
In these codes, gas emission into roadway excavation and on longwall faces and pillar panels is estimated using empirical or semi-empirical methods based upon historical measurements in mines.
Flow of gas into roadway excavation may be radial or linear depending upon the shape of the excavation. Field observations have shown that gas emission from a heading at a constant rate of advance can be approximated by an exponential function ( dV/dt = aVt ) or a power
47 function (V = At*). The constant a is dependent on local conditions and can be measured over a period of time for gas emission from a heading advancing at a fixed rate.
Gas emission from longwall faces and pillar panels comes from the seam being mined as well as from the surrounding medium, which may contain very large amounts of gas particularly in surrounding seams. Several empirical methods have been developed to estimate gas emission into mine workings when longwall systems are used. For emission of gas from the worked seam most methods are independent of advance rate and are based on the assumption that 100% of gas from the seam being mined is emitted. Emission from adjacent seams is estimated in the range of 0 to 100% depending upon the definition of emission zone and method used. The accuracy of these methods largely depends upon localised caving characteristics in relation to geological and geotechnical conditions.
These empirical/semi-empirical methods have been widely used all over the world for mine planning, gas control and ventilation design and will continue to play a significant role. However, there are some drawbacks using empirical/semi-empirical approach for gas emission estimation. These include: reliance on measurements made during mine development and operation, gas emission estimation is static, not real-time and dynamic and; consideration of caving characteristics is generally limited due to the lack of detailed modelling techniques of the extent and nature of relaxation and fractures of the surrounding strata. Further research and development is required to predict gas emission prior to mine development and operation, particularly, in a new mining environment where no previous experience or limited data are available.
Flow simulators (Uncoupled with mechanical deformation)
There are a number of coal seam gas simulators available. These simulators are distinguished by their underlying physics, boundary conditions, grids and other features. To illustrate the typical capabilities of currently available flow simulators, two commercially available flow simulators COALGAS and COMETS are briefly described in this section.
COALGAS is a three-dimensional, two-phase, dual-porosity reservoir simulator. It is designed to model flow of gas and water in coal seam gas reservoir and dual-porosity reservoir. COALGAS numerically models the processes that control the behaviour of coal seam gas reservoir: Darcy flow, Fickian diffusion and gas sorption (Langmuir isotherms). COALGAS models account for pressure dependent porosity and permeability, and saturation dependent capillary pressure and relative permeability. COALGAS cannot model binary gas sorption.
48 C0MET3 is a three-dimensional, two-phase, fully implicit, finite-difference fractured reservoir simulator. COMETS treats the release and transport mechanisms of desorption, diffusion and Darcy flow through a dual-porosity/single permeability network in coal seam (matrix and fractures). For gas-bearing shale, it models the process of the release and transport mechanisms by a triple porosity/dual permeability network. COMETS defines the non-linear relationship between free and adsorbed multi-component gas mixtures (methane-carbon dioxide and methane nitrogen) as a function of methane concentration using extended Langmuir isotherms. COMETS can model fault blocks, dipping reservoirs, layered systems, and stress-sensitive permeability and porosity. The model also accounts for saturation dependent capillary pressure and relative permeability.
In terms of its capabilities of simulating gas flow in various mining operations such as roadway development, gas drainage and longwall operation, COMETS is more comprehensive and realistic because of its underlying multi-component seam gas approach.
Coupled Mechanical - Flow Simulators
The literature on coupled mechanical flow simulators is vast but when dealing specifically with a coupled mechanical and multiphase flow problem, the number of commercially available codes is small. CSIRO have licenses for the Itasca codes UDEC and FLAG. Both of these codes create two-dimensional models. UDEC can model the coupled mechanical and single-phase flow problems for incompressible fluid. This code is well suited for simulating the flow through fissures and joints. FLAG can model coupled mechanical and two-phase flow problems. This code is based on the Biot ’s theory of poro-elasticity and handles two- phase flow in a single porosity medium. Importantly, the codes do not update the change in fluid flow properties associated with mechanical deformation, hence such a formulation needs to be added separately.
ABACUS is a general purpose three dimensional finite element package developed by Hibbit, Karlsson and Sorensen Inc that incorporates an option for conducting coupled mechanical and two phase fluid flow calculations. However, its two-phase calculation is simplified by assuming that the non-wetting phase (i.e. gas) does not contribute to pore pressure so that the pore pressure change is only generated by wetting fluid (i.e. water).
ELFEN is a three-dimensional package marketed by Rockfield Software Ltd, it also handles coupled mechanical and multiphase flow problems. It has only recently appeared in the market and hence, at this stage, it is hard to judge its applicability in solving real mining problems.
49 A group at the University of Swansea, UK has developed a three dimensional finite element model for simulating coupled mechanical and multiphase flow problems. Discussions with this group are now taking place to try to assess the features of the Swansea code particularly its capacity to handle material non-linearity (plasticity) and stage excavation problems; problem solution schemes and user-friendliness.
CSIRO has been developing a three-dimensional finite element computer code called “PartCoss ” for stress-strain analysis. This code has been developed especially for simulating the behaviour of bedded rock strata as are encountered in the coal mining environment. The code is based on Cosserat continuum theory, which describes the behaviour of bedded (discontinuum) rock strata in a continuum fashion.
Proposed integrated Simulation Strategy
To be able to predict strata behaviour and associated gas emission during mining, and to assess performance of gas control measures, a reliable integrated simulation method must have the capability to accurately determine:
• mining induced rock deformation, fractures and subsequent changes in reservoir pressure, permeability, and gas/fluid flow patterns, and
• gas emission as a result of mining, and effectiveness of gas drainage measures.
The review of existing software has indicated that none of them can fully simulate coupled mining induced mechanical and multi-phase problems. To achieve the aim of an efficient and integrated simulation, it will be necessary to:
• further develop the current CSIRO software PartCoss into a three-dimensional fully coupled mechanical and two-phase flow model that can provide a reliable prediction of mining induced changes in strata conditions, reservoir pressure and properties;
• develop an interface so that the reservoir property change can be imported to the existing commercial softwares such as COALGAS and COMET3 to simulate the production and control of gas and/or gases during mining as shown in Figure 3-24.
50 3.13 Implication of Research Findings to Date
The results of the research work have highlighted that future deep coal mining in the Hunter Valley coalfield will face significant economical, technical and environmental challenges. The challenges are a direct result of the deep, gassy, multi-seam environment characterised by low permeability, high in situ stress, and complex geology. The potential impact of these difficult conditions is that mining costs with current mining technology could sharply increase for a large amount of future deep coal reserves, in one of the largest coal producing areas in Australia. In extreme cases, these difficult conditions may render the recovery of some of these reserves uneconomic.
Complex geological and structural environment and high stress levels at greater depth will lead to greater levels of strata support and reinforcement, reduce roadway development rates, and cause an increase in the amount of coal sterilised within pillars.
The low in situ permeability measured at depth during the JFY 2000 field work, indicates that current pre-drainage methods with surface holes will probably not be viable particularly at cover depths greater than 300-500 m. Without effective surface drainage techniques, costly full scale in mine post-drainage system and associated infrastructure will be required. As there are a number of closely spaced coal seams at depth, the overlying and underlying coal seams are likely to release a significant amount of gas into the workings during mining. This will increase the drainage needs and costs of cross measure and in-seam drainage methods. The costs of goaf drainage with surface holes will increase significantly as the future mining depth will be at least twice the current depths.
There are also significant opportunities for integrated coal mining and gas utilisation operations in the region that could profit from both coal and mine gas production, and reduce the environmental impacts. To capture the opportunities, development and application of advanced technologies will be essential in gas drainage and utilisation, predictive simulation of ground condition and gas emission, and mining and gas drainage/control optimisation.
3.14 Key technical issues and tasks
Longwall mining in conditions such as those of the Arrowfield and Bowfield seams will present a new set of challenges. It is imperative that ground conditions and gas emission characteristics in this environment are accurately predicted, and cost-effective drainage technology is developed and applied to ensure coal mining remains economical.
These issues will also be critically important for other key coal mining areas in Australia, as future mining progressively goes deeper.
52 Given the importance and potential impact of the issues that this project is addressing on the future of coal production in Australia, it is recommended that, for the remainder of the project, the following tasks need to be carried out:
• Evaluation and development of new pre-drainage methods;
• Development of predictive mining and gas simulation methods;
• Post-drainage performance study of existing analogue sites;
• Completion of assessment and evaluation of key mining and gas issues at the Wambo mine site as an example and;
• Development and application of advanced utilisation technology to reduce the mining cost and environmental impacts.
The recommended project work for the next year is illustrated in Figure 3-25.
One of the key objectives of the next year work is to develop an integrated simulation method to predict strata behaviour and associated gas emission during mining, and to assess performance of gas control measures. Specifically, the integrated simulation method needs to accurately determine:
• Mining induced rock deformation, fractures and subsequent changes in reservoir pressure, permeability, and gas/fluid flow patterns and;
• Gas emission as a result of mining, and effectiveness of gas drainage measures.
Existing commercial software such as COALGAS and COMET3 are comprehensive in simulating gas desorption, diffusion and flow. However, no commercial software that has been reviewed fully couples gas emission processes with mining induced mechanical changes. To achieve the aim of an efficient and integrated simulation and take advantage of the existing commercial software, it will be necessary to:
• Further develop the current CSIRO software PartCoss into a three-dimensional fully coupled mechanical and two-phase flow model that can provide a reliable prediction of mining induced changes in strata conditions, reservoir pressure and properties and
• Develop an interface so that the reservoir property change can be exported to the existing commercial software such as COALGAS and COMET3 to simulate the production and control of gas and/or gases during mining.
While the above simulation methods are being developed, it will be essential to carry out geotechnical and gas investigations at analogue sites with similar conditions in Australia. The investigations should obtain information in relation to strata caving, fracturing and deformation characteristics, and gas emission and mechanisms during mining. The 53 simulation methods should then be validated and calibrated with the actual data from the analogue sites before being applied to the Arrowfield and Bowfield seams at the Wambo mine.
Additional gas and geotechnical data have been acquired while this report was being written. These data need to be analysed and the results need to incorporated into the gas and geological and geotechnical models.
Evaluation of the following new pre-drainage technologies and concepts needs to be carried out for their applicability:
• CSIRO's mechanical radial drilling from vertical holes;
• tight radius drilling with water jet and;
• longhole in-seam surface drilling.
Specific evaluation of potential utilisation methods needs to be undertaken in relation to use of drained gas and ventilation air with/without supplementary fuel.
54 4.2 Regional Geology
The Hunter Valley coalfield is bounded by the western margin of the Lochinvar Anticline on the eastern side, by the Hunter Thrust System on the northern side, and by the escarpment formed by the Early Triassic Narrabeen Group sediments on the southern and western sides (Figure 4-3).
STRUCTURAL FABRIC
HUNTER COALFIELD
MUSWELLBROOK
1 Mt. Cgih/ie Fault 2 St. Haliers Fault 3 Redmarvale Thrust Fault 4 Hetoden "Thrust Fault 5 Westbrook Thrust Fault
Figure 4-3 Regional structural fabric of the Hunter Valley coalfield, (after Lohe et al, 1992)
58 4.2.1 Stratigraphy
The Permian coal-bearing stratigraphy of the Hunter Valley coalfield has been reviewed by Sniffin and Beckett (1995). The material included here is largely summarised from their report. Three coal measure sequences occur in the Hunter Valley coalfield: the Early to Mid Permian Greta Coal Measures, the Late Permian Wittingham Coal Measures and the overlying Later Permian Wollombi Coal Measures (Figure 4-4).
I RECENT ALLUVIUM Qa f CAINOZOIC
TRn 1 HAWKES8URY SANDSTONE 1 GOSFORD SUBGROUP
MESOZOIC E | PATONGA CLAYSTONE CLIFTON GROUP TUGGERAH FORMATION SUBGROUP WIODEN BROOK CGL
GLEN GALLIC SUBGROUP WOLLOMBI DOYLES CREEK SUBGROUP COAL HORSESHOE CREEK SUBGROUP Psw SINGLETON APPLE TREE FLAT SUBGROUP MEASURES WATTS SANDSTONE DENMAN FORMATION
MT. LEONARD FORMATION SUPER JE WITTINGHAM ALTHORP FORMATION PLAINS MALABAR FORMATION MT. OGILVIE FORMATION Pswj A COAL sue MILBROOALE FORMATION GROUP MT. THORLEY FORMATION GROUP FAIRFORO FORMATION BURNAMWOOO FORMATION MEASURES ARCHERFIELO SANDSTONE
VANE BULGA FORMATION Pswv SUB FOY8ROOK FORMATION y GROUP SALTWATER CREEK FORMATION Pswc S < 2 MULBRING SILTSTONE Pmm 5 MAITLAND 2 1 MUREE SANDSTONE Pms GROUP •IPPIW
8RANXTON FORMATION Pmb
PAXTON FM GRETA ROWAN FORMATION Pgr KITCHENER FM COAL KURRI KURRI Pgk SKELETAR FORMATION MEASURES NEATH SS
FARLEY FM GYARRAN T RUTHERFORD FM ui GROUP VOLCANICS ALLANOALE FM
LOCHINVAR FM
Figure 4-4 Stratigraphy of the Hunter Valley coalfield, (after Beckett, 1988)
59 • Greta Coal Measures
The Greta Coal Measures crop out in two localities in the Hunter Valley: around the crest of the Muswellbrook Anticline and along the Lochinvar Anticline (Figure 4-3). In the Muswellbrook area, the Greta Coal Measures average more than 200 m in thickness and contain up to six economic coal seams. Close to the Lochinvar Anticline, the Greta Coal Measures are considerably thinner; they average 63 m in thickness in the type area near Greta. There are several coal seams, the most important being the Greta seam which is up to 11 m in thickness near Cessnock. Although the Greta Coal Measures are known to be continuous, they are only known in detail around the Muswellbrook and Lochinvar Anticlines.
• Wittingham Coal Measures
The boundary between the Maitland Group and the overlying Late Permian Wittingham Coal Measures is transitional (Britten, 1972; Stuntz, 1972). The Wittingham Coal Measures are typically 1200 m thick and have been subdivided into two important coal bearing subgroups, the lower Vane Subgroup and the overlying Jerrys Plains Subgroup. The boundaries of each group are recognized by distinctive coal-barren marker units: the Saltwater Creek Formation, the Archerfield Sandstone, and the Denman Formation.
The Vane Subgroup, overlying the Saltwater Creek Formation, has been subdivided into two formations, the Foybrook Formation and the overlying Bulga Formation. The Foybrook Formation is a coal-bearing sequence containing up to six economically important coal seams. The overlying Bulga Formation consists of widespread laminated siltstones with abundant burrows and pyritic phases (Britten, 1975). The Bulga Formation is overlain by the Archerfield Sandstone which is a widespread massive sandstone and a regionally significant marker horizon. The Archerfield Sandstone forms the floor of one of the most important coal seams in the Hunter Valley, the Bayswater seam of the Jerrys Plains Subgroup.
The Jerrys Plains Subgroup includes a complete cycle of terrestrial coal measure sedimentation up to 800 m thick. Fifteen economically important coal seams occur within the subgroup, all of which are currently mined at some locality in the Hunter Valley coalfield. The Jerrys Plains Subgroup has been divided into nine formations:
• Wollombi Coal Measures
The lowest unit of the Wollombi Coal Measures is a coal-barren formation named the Watts Sandstone. This unit forms a continuous marker horizon and separates the underlying Wittingham Coal Measures from the rest of the overlying Wollombi
60 succession. It crops out and forms a distinctive cliff line in the western part of the coalfield area.
The Wollombi Coal Measures have been subdivided into four subgroups. In ascending order these are the Apple Tree Flat, Horsehole Creek, Doyles Creek, and Glen Gallic Subgroup. These subgroups contain numerous thin, banded coal seams that split and merge and locally are up to two metres thick. Tuffaceous claystones are characteristic throughout the succession, thus adding to the banded nature of the coal.
4.2.2 Geological Structures
The Hunter Valley coalfield region is part of the north eastern structural sub-division of the Sydney Basin. The northeastern boundary of the Sydney Basin, is defined by the Hunter Thrust System. The structural margin of the basin is considered to be located in about the same position as the thrust system.
Two major fault systems define the commercial boundaries of the coalfield: the Hunter-Mooki Thrust Fault to the east and the Mount Ogilvie Fault to the west.
The regional structure of the Hunter Valley coalfield is characterised by the following features (Figure 4-3) (Lohe, et al, 1992):
• The broadly NW-SE trending Hunter Thrust system. It consists of a series of long-angle thrusts along which Carboniferous rocks of the Southern New England Fold Belt have been thrust to the southwest over the Permian rocks of the northern Sydney Basin. The Permian basinal sequence has also been affected by a series of large-scale thrust faults which can be interpreted as splays off the main Hunter Thrust. They include the Aberdeen, Hebden, and West Brook Faults, which run broadly parallel to the main thrust system.
• A series of broadly north-south trending anticlines or domes, and intervening synclines, with associated meridional faults and monoclines. The Lochinvar and Muswellbrook Anticlines are the most prominent of these structures. Between them, from west to east, are the Bayswater Syncline, the Camberwell Anticline, the Rix’s Creek Syncline, the Loder Anticline, the Darlington and Sedgefield Anticlines, and the Belford Dome. The Calool Syncline occurs to the west of the Muswellbrook Anticline (see Figure 4-3 and Figure 4-5).
Some of these features, e.g. the Lochinvar and Muswellbrook Anticlines, were synsedimentary structures (e.g. Sniffin, 1988) which were active during the Permian, and to varying degrees into the Triassic. They have had a major influence on the outcrop geometry of the Permian coal-bearing sequences of the Hunter Valley coalfield.
61 • Meridional faults, such as the Mt Ogilvie and St Heliers Faults, and other fault systems which occur on the flanks of many of the regional anticlines or domes, for example the Mt Thorley Monocline structure. The meridional faults are interpreted as basement structures which in some cases were active from Permian to Triassic times and possibly into the Tertiary, and essentially led to the formation of basement-controlled fault blocks such as the Lochinvar Anticlines (Rawlings and Moelle, 1982).
• Meridional trending thrust structures such as the Aberdeen Thrust and the northern extension of the Hunter-Mooki Thrust System.
• Northwesterly trending fault structures, such as those along the southern and southwestern margin of the coalfield (e.g. faults south and west of the southern closure of the Muswellbrook Anticline).
• Northeasterly trending fault systems; the most obvious of which are those south of the Muswellbrook Anticline, and those defining the horst-and-graben system of the normal faults of the Ravensworth Fault Zone, which runs through the central region of the Valley from Swamp Creek Mine in the north and Hunter Valley No.1 Mine in the south. Dykes are commonly orientated parallel to this fault direction.
• East-west orientated fault structures, such as those cutting across the coalfield in the Mt Arthur to Lake Liddell region.
4.2.3 Geological Structures - North Hunter Valley coalfield
The major structural features in the Northern Hunter Valley coalfield include the Muswellbrook Anticline, Hebden Thrust, Bayswater and Rix’s Creek Synclines, and the Camberwell Anticline (Figure 4-3). These structural features have been comprehensively reviewed by McLennan and Lohe (1992). There has been little published since then. The following material included here has largely been summarised from their report.
• The Muswellbrook Anticline
The Muswellbrook Anticline is the major structural feature of the North Hunter Valley coalfield. The fold axis has a broadly southerly trend for the most part but trends to the southeast around the southern closure. It can be traced for a distance of approximately 28 km from Muswellbrook in the north to just north of the Hunter River in the south. The oldest rocks exposed in the core of the anticline are elements of the Early Permian Gyarran Volcanics, which are overlain by the Greta Coal Measures.
The Muswellbrook Anticline is a doubly plunging structure, but the dominant regional plunge is to the south. It is an asymmetrical fold structure, with generally gentler dips on
62 the eastern limb. However, in the northern area around Muswellbrook, the geometry of the anticline changes owing to oversteepening of the eastern limb by the Aberdeen Thrust, and here the western limb is the more gently dipping limb. The anticline is locally complexly faulted by both normal and thrust faults. As well, the geometry of the structure is locally affected by smaller-scale flextures, the most important of which is the Western Anticline, a fault-induced anticlinal flexture on the western limb of the Muswellbrook Anticline.
Significant meridional normal faulting has affected both the western and eastern limbs of the anticline, and has probably played an important role in the evolution of the anticline. Further to the south, the western limb of the anticline is cut by a series of broadly north- northwest trending normal faults, the largest of which have displacement of up to 200 metres. The crest of the fold is also marked by locally extensive normal faulting.
At its northern end, the Muswellbrook Anticline forms the foot-wall of the Aberdeen Thrust. Structure-contour maps of the Greta Coal Measures in this area indicate that the northern extension of the anticline plunges sharply under the thrust, and may actually terminate close to the edge of that structure. The distribution and character of thrust faulting changes markedly down and across the anticlinal axis, a factor which has implications for coal-mining techniques and mining conditions.
In the northern area, thrust faulting has not only ridden up the eastern limb of the anticline, but has also significantly affected the coal-bearing sequence on the western limb, which is also in the immediate foot-wall area of the Aberdeen Thrust. The locally tight, asymmetrical form of the anticlinal hinge in this area is probably due to the compression and overthrusting. Further south, thrusting is mainly restricted to the coal bearing section on the eastern limb of the anticline. There appears to be only minor evidence of thrusting in the sequence on the western limb that was related to regional compression.
• Other Major Structures
Other major structures in the North Hunter Valley coalfield include the Hebden Thrust, which is a northwesterly trending foot-wall splay of the main Hunter Thrust system, Bayswater and Rix’s Creek Synclines, and the Camberwell Anticline. Normal and reverse fault systems, and igneous dykes have been recorded in the region. Most of the dykes are broadly parallel to, and commonly associated with, north-easterly trending fault systems.
In general, apart from the Muswellbrook Anticline, the structural geology of the region is dominated by four major features:
63 3- Lake Liddell .
v—
River
Figure 4-5 Other major structural features in North Hunter Valley coalfield. (Lohe et al, 1992)
1. Low-amplitude northerly and northwesterly trending regional-scale fold structures
The main fold structures are the Bayswater and Rix’s Creek Synclines and the Camberwell Anticline. These broad, open, non-cylindrical folds with sub-horizontal fold axes essentially determine the outcrop pattern and regional distribution of the coal measures in this region. It is considered that there are two main types of fold structure within this region. They are: a) drape folds related to fault-bounded basement blocks and; b) “thin-skinned ” thrust-related folds. This division of fold type is based on the variation in orientation and character of the fold systems.
2. A northeast-trending, horst-and-araben normal fault-block zone (Ravensworth Fault Zone)
The Ravensworth Fault Zone is the most significant northeasterly trending fault system in the North Hunter Valley coalfield. It is a weakly curved, relatively broad
64 zone of relatively high-angle, conjugate normal faults. It traverses the Hunter Valley coalfield from the footwall region of the Hebden Thrust in the northeast, to the Hunter River region in the southwest. The main fault zone ranges from 200 m to 450 m wide, and essentially consists of an array of normal faults which define a series of horst- and-graben fault blocks. Faults are commonly orientated at a slight angle to the regional strike of the overall fault zone. Individual faults are hinged and discontinuous along strike, with recorded strike lengths of larger faults commonly ranging from 60 m to more than 250 metres. Recorded throws commonly range between 0.3 m and 4.5 metres.
3. Northwesterly trending normal faults
Major northwesterly trending normal faults have been identified mainly around the southeasterly trending closure of the Muswellbrook Anticline. The faults are sub- parallell and locally branch and coalesce to form wedge-shaped horst-and-graben fault blocks. The northwesterly trending normal faults in this area are generally large- scale, hinged structures that have throws are commonly less than 30 m but range to 50 metres. There are indications that some of the faults have undergone a component of strike-slip deformation. Faulting dies out to the southeast. It also decreases to the north away from the main hinge zone.
4. Thrust and hiah-anale reverse faults
Thrust and high-angle reverse faults are not a common structural feature in the North Hunter Valley coalfield. The most significant thrust structure is the regional-scale Hebden Thrust. Minor thrusts, reverse faults and horizons of bedding-parallel shear have also been detected in the mines of the region. The Hebden Thrust is a northwest trending, southwest directed thrust, which is a footwall splay off the main Hunter Thrust System. The thrust plane is a low-angle structure, with a dip of 15-20° to the northwest, and consists of a 1 m wide shear zone, with relatively sharp bounding surfaces. The coal-measure sequence within the immediate footwall of the thrust has been dragged up into the overlying fault plane and folded as a results of the overthrusting. The beds are locally overturned in the area adjacent to the fault plane.
65 4.3 Coal Resources / Reserves
4.3.1 Introduction
The Hunter Valley coalfield is the largest coal producing area in New South Wales. In 1998- 99 raw production from the Hunter Valley coalfield amounted to 84 Mt, a 6% increase on the previous financial year. This increase in production was due to a number of new mines and production increases at existing operations.
The coal resources of the Hunter Valley coalfield extend from the western flank of the Kulnura-Lochinvar Anticlines north-westerly around the Muswellbrook Anticline to Denman and Merriwa.
As described in Section 4.2, there are three coal measures in the Hunter Valley coalfield, namely: the Early to Mid Permian Greta Coal Measures, the Late Permian Wittingham Coal Measures and the overlying Later Permian Wollombi Coal Measures.
Wollombi Coal Measures
Wollombi Coal Measures are the upper most coal measures in the Hunter Valley coalfield. The Measures are characterised by a high proportion of coarse channel deposits. Rapid channel migration has eroded laterally equivalent coal swamps. Seam splitting and erosion is common and makes regional coal seam correlation difficult. With a general lack of lateral continuity of coal seams and the rather limited extent and depth of coal (i.e. thickness of cover) prior to dipping beneath areas of National Park, the Wollombi Coal Measures are an unattractive target for coal exploration in all but a few isolated areas. Resource potential exists in the far south of the Hunter Valley coalfield near Broke and in the area north of Denman, where the coal seams are spatially closer to the economic upper seams of the Jerrys Plains Subgroup.
No existing mine in the Hunter Valley coalfield is recovering coal from the coal seams in the Wollombi Coal Measures.
Wittingham Coal Measures
Wittingham Coal Measures underlie the Wollombi Coal Measure, and are, conversely, quite extensive and contain several thick coal seams distributed throughout the total sequence - which is typically 1200 m thick.
The target Wittingham Coal Measures of the Hunter Valley coalfield form a north-west to south-east band approximately 10 km to 15 km wide by 80 km in length roughly parallel to the Hunter-Mooki Thrust. To the south, the coal measures strike north-east. To the north, around the Calool Syncline, the strike and dip swing to the south-west and south- 66 east, respectively, along the eastern flank of the Mount Ogilvie Fault. Further to the north, on the western side of the Mount Ogilvie Fault, the seams tend to dip to the north. The Mount Ogilvie Fault represents the boundary between two distinctly different structural settings.
On the western side of the fault system there is a marked change in both structure and depth to the target coal seams. The dip changes from predominantly south to north, across the Mount Ogilvie Fault. There is very little information on seam characteristics to the west of the Mount Ogilvie Fault, although thicknesses of 20 m to 30 m are indicated. If laterally consistent, this would significantly increase the potential resource of the Hunter Valley coalfield adding 650 km2 of prospective land.
Two major coal-bearing subgroups have been identified within the Wittingham Coal Measures, the uppermost Jerrys Plains Subgroup and the basal Vane Subgroup. Within these subgroups there is a thickness of up to 60 m thickness of total net clean coal.
Coal bearing intervals of up to 300 m in thickness span the Mount Thorley and Burnamwood Formations, and commonly contain a coal thickness of 30 m to 40 m total net clean coal. The basal Foybrook Formation, lowermost unit of the Vane Subgroup, has a thickness of up to 300 m within the total Wittingham Coal Measures sequence. Net coal thickness averages 30 metres.
Most coal mines in the Hunter Valley coalfield are mining the coal seams in the Wittingham Coal Measures.
Greta Coal Measures
Greta Coal Measures are the oldest coal measures in the Hunter Valley coalfield, and represents the first episode of constructive delta deposition in the area. The coal measures crop out in two localities in the Hunter Valley: around the crest of the Muswellbrook Anticline and along the Lochinvar Anticline. In the Muswellbrook area the Greta Coal Measures average more than 200 m in thickness and contain up to six economic coal seams. Although the Greta Coal Measures are known to be continuous, they are only known in detail around the Muswellbrook and Lochinvar Anticlines.
There are only three operating mines in the Hunter Valley coalfield extracting coal from the Greta Coal Measures.
The Hunter Valley coalfield has by far the thickest net coal sequence in the Sydney Basin. There are numerous coal seams greater than 2 m thick, with seams such as the Warkworth, Mt Arthur, and Bayswater, consistently around 4 m in thickness, and as thick as 6 m in places. These provide attractive individual targets in addition to closely spaced (or stacked) overall thick coaly sequences. 67 4.3.2 Economic Coal Seams
The Greta Coal Measures and the Wittingham Coal Measures (Foybrook Formation and Jerrys Plains Subgroup) are the major sources of commercial coal in the Hunter Valley coalfield. Economically important coal seams in these coal measures are shown in Figure 4-6.
The Greta Coal Measures occur at depth of more than 600 m over most of the area, and it is only where the coal measures are associated with major structural features, such as the Muswellbrook Anticline, that the sequence occurs at reasonable mining depths. Seam thicknesses tend to be greater (and the seams show less splitting) in the Muswellbrook area. In the south the coal seams are commonly split and separated by persistent non-coal intervals, and many of the seams are affected by igneous intrusion. Seam continuity is also often disrupted by small scale faulting.
The coal seams of the Foybrook Formation are characterised by erratic splitting, which has resulted in the adoption of various nomenclatures. The coal seams are best developed in the vicinity of the Muswellbrook Anticline, with the thickest seam being the Liddell seam which reaches a maximum of 14 m in the Foybrook area. Towards the south, multiple splits develop in most of the seams and the seams tend to thin and deteriorate. Igneous intrusions are less common in the Greta Coal Measures than the Wittingham Coal Measures.
The coal seams of the Jerry Plains Subgroup have the highest economic potential in the Hunter Valley coalfield. This is partly due to their superior thickness and quality, but more importantly it is because the coal subcrops extensively and is at shallow depths over a large area.
The Bayswater seam is the lowest seam in the Jerrys Plains sequence, and is an important source of thermal coal. It varies in thickness from 1 m to 14 m, generally thickening towards the south, and is distinctive because of its general dull character and high inertinite content.
The Broonie through to Warkworth seams are brighter coals, but are subject to extensive splitting and coal coalescence. These features have hindered regional seam correlation in the coalfield. The total thickness of the interval ranges from 100 m to 300 m, with cumulative coal thickness ranging from 12 m to 50 metres. The thickness of the section generally increase away from the major anticlinal structures, and this thickening usually coincides with an increase in the total coal content of the sequence.
The Bowfield through to Blakefield seams are also extensively split, but several of the upper seams in this interval contain distinctive stone bands which assists in correlation. The individual seams vary in thickness from 0.3 m to greater than 4 m over most of the area.
68 The upper sequence of seams (Whynot through to Whybrow seams) range in aggregate coal thickness from 4 m to 17 m to the south of the Muswellbrook Anticline. These seams are usually split, and are characterised by association with several distinctive tuffaceous claystone marker beds.
4.3.3 Coal Seams Mined or Proposed to be Mined
Although there are about sixty coal seams within the three coal measures in the Hunter Valley coalfield, only some thirty of these seams are mined or proposed to be mined. The seams are listed in Figure 4-6.
It should be noted that the coal seam nomenclature applied in Figure 4-1 and Figure 4-6 is in accordance with the current stratigraphy ratified by the Standing Committee on Coalfield Geology of New South Wales which is not necessarily the same as the stratigraphy used by the companies. The variations to the current stratigraphy and other features related to the seam nomenclature for individual mines and exploration areas are explained in detail by Beckett (1999).
69 The numbers shown in blue colour are the estimated from the available data
The measured in situ resource is about 10,433 Mt, recoverable reserve is about 4,180 Mt and the marketable reserve is about 3,486 Mt.
4.3.5 Coal Quality
The quality of the coal in the Hunter Valley coalfield varies considerably from seam to seam, within seams, and between the different formations and coal measures sequences across the coalfield. Trying to establish patterns of variation is complicated because of correlation difficulties on a regional scale due to the large numbers of seams, the complex lateral variation and splitting of the seams, and historical nomenclature resulting from geographical isolation of the original colliery holdings.
The economic coal deposits in the coalfield can be grouped into three areas: the northern area where the Greta Coal Measures are mined, the central area of Foybrook Formation coal belonging to Vane Subgroup and the southern group of deposits belonging to the Jerrys Plains Subgroup. Up to 30 seams and seam splits are mined and a range of different coal products is produced.
Up to six seams and their splits are mined from the Greta Coal Measures along the core of the Muswellbrook Anticline in the north of the coalfield. Most of this coal is marketed either low to medium ash, medium volatile, thermal quality coal for export or as medium to high ash, medium volatile thermal coal for the domestic market. An export quality semi-soft coking coal is also produced by washing and blending product from several seams from the coal measures. The organic sulphur content of the Greta coals tends to be high which impairs the marketability of these coals. Indicative coal quality of the coals is shown in Table 4-2.
Table 4-2 Indicative quality for coals from Greta Coal Measures in the northern area of the Hunter Valley coalfield. Specifications Raw Coal Thermal Coking Moiture total (a.r.) % 7.0-9.0 7.0-9.5 9.0 Mositure (a.d.b.) % 2.0-3.0 2.5-3.0 2.0 Ash % 11.0-18.0 5.0-16.0 10.0-11.0 Volatile % 30.0-36.5 31.0-35.0 34.0-35.0 Sulphur % 0.7-1.7 0.7-1.2 0.4 Phosporus % 0.01-0.1 0.01-0.1 0.04 Specific energy (kcal/kg) 6200-7000 6450-6750 6780-6950 C.S.N. 1-3 1-3 1-5 H.G.I 35-50 40-50 35-50 Def. temp. °C 1300 1300-1375 1160 Flow temp. °C 1400 1430-1500 1370
72 The coal seams of the Foybrook Formation crop out along the flanks of the Muswellbrook Anticline in the central area of the coalfield. Up to 10 seams occur in the sequence, with the middle four (Barrett, Liddell, Arties and Pikes Gully) and their splits currently being mined. Foybrook Formation coals are primarily sold as thermal quality coals, both to the export market and for local consumption by power stations. Several commercial blends of low ash, medium volatile, high fluidity coking coals are produced to supply the export markets. Indicative coal quality of the coals is shown in Table 4-3.
Table 4-3 Indicative qualityfor coals from Vane Subgroup - Foybrook Formation in the central area of the Hunter Valley coalfield. Specifications Raw Coal Thermal Coking Moiture total (a.r.) % 7.0-10.0 7.0-9.0 Mositure (a.d.b.) % 1.8-3.5 1.8-3.1 Ash % 7.0-42.0 4.9-26.1 6.4-13.7 Volatile % 33.0-37.0 30.4-41.3 36.0-39.4 Sulphur % 0.4-0.8 0.4-0.8 Phosporus % 0.006-0.05 0.006-0.07 Specific energy 5710-7265 6920-8120 (kcal/kg) C.S.N. 1-8 1-4 2.5-7.0 H.G.I 45-58 41-53 Def. temp. °C 1270-1500 1180-1580 Flow temp. °C 1340-1600 1360-1580
The southern group of deposits embraces the seams of the Jerry Plains Subgroup. Fifteen named seams occur in this sequence, and every seam of the subgroup is currently mined at some location in the Hunter Valley coalfield. Most of the coal is marketed as low to medium ash, medium volatile, low sulphur thermal coal for export and as low ash, medium volatile soft coking coal. Several collieries mine only the lower seams of the subgroup (Bayswater and Ravensworth seams) to supply thermal coal for domestic power consumption. Indicative coal quality of the coals is shown in Table 4-4.
73 Table 4-4 Indicative qualityof coal from Jerrys Plain Subgroup in the southern area of the Hunter Valley coalfield. Specifications Raw Coal Thermal Coking Moiture total (a.r.) % 8-11 8-10 8-10 Mositure (a.d.b.) % 2-3 2.5-35 2.0-3.0 Ash % 8-30 8.5-14.0 7.0-8.5 Volatile % 25-36 29-35 31-36 Sulphur % 0.3-1.0 0.4-1.0 0.4-0.6 Phosporus % 0.02-0.4 0.01-0.03 0.01 Specific energy (kcal/kg) 5070-7410 6600-7050 7140-7410 C.S.N. 4-5 H.G.I 50-60 48-55 50-60 Def. temp. °C 1300-1550 1200-1300 Flow temp. °C 1250+ 1500-1600+ 1400-1600
4.3.6 Mine Development and Restrictions
There are a number of potential new projects and expansions of existing operations in the Hunter Valley coalfield (Table 4-5).
Table 4-5 Potential new projects and expansion of existing operations in the Hunter Valley coalfield. Expansion of Existing Mine type Coal type Operations
Carrington Opencut Coking/Thermal Cheshunt Opencut Coking/Thermal Lemington South Opencut Coking/Thermal Ravensworth West Opencut Thermal
Potential New Projects
Glendell Opencut Thermal/Coking Kayuga Opencut Thermal Mt Arthur North Opencut Thermal Mt Pleasant Opencut Thermal Nardell Underground Coking/Thermal Raversworth East Opencut Thermal Saddlers Creek Opencut/Underground Thermal Sandy Creek Underground Thermal
The Bengalla mine construction was completed and the first coal shipped in April 1999. The Bengalla open cut will produce about 1 Mt in its first year and is expected to increase its annual output to 5 Mt by 2003, depending on market conditions.
74 Trial underground mining at Glennies Creek commenced in May 1999 from the highwall of the adjacent Camberwell open cut. The trial mining will determine whether a full-scale longwall mine can be developed, or whether continuous miners will be used.
The Mount Pleasant deposit contains export and domestic quality coal amenable to open cut and underground mining. A Commission of Inquiry was held into the development during 1998-99. The Commissioner recommended to the Minister for Urban Affairs and Planning in June 1999 that the project be approved, subject to a range of conditions, to ensure impacts are within acceptable levels.
Rio Tinto (NSW) has plans for extensions to their Hunter Valley operations at Carrington to the west and Cheshunt to the south. These extensions will allow production levels from the Hunter Valley operation to be maintained at current levels.
Liddell Coal Operations Pty Ltd has approval to develop an open cut mine at Glendell with saleable production of 2.7 Mtpa. Development of the mine will commence when market conditions allow. The Nardell project, an underground development with a likely capacity of 2 Mtpa, could commence production in late 2000.
Peabody Resources plans to develop a new open cut to the west of their current Ravensworth operation, for which development consent has been granted. A Planning Focus was held for the Mount Arthur North Project operated by Coal Operations Australia Limited (COAL) and an EIS was completed in late 1999. COAL has been awarded a contract to supply 3 Mtpa to Macquarie Generation for five years from 2003.
Development consent for the Sandy Creek underground mine was granted to Muswellbrook Coal Company in April 1999. Development consents have been granted for extensions to mining operations at Lemington and Howick.
The Hunter Valley coalfield does not contain significant areas of urban development. However, the major regional centres, particularly Singleton and Muswellbrook and the surrounding smaller settlements, do have a significant impact on potential coal development in the region.
The Wollemi and Yengo National Parks overlie the western boundary of the coalfield.
Prime agricultural land associated with the Hunter River floodplain and the protection of associated groundwater resources within the alluvial deposits, affects a significant percentage of the undeveloped coal resources in the region.
Significant coal resources of high quality exist to the west of Scone. It is estimated that there are sufficient resources in that area for an additional one or two mines. While there is some
75 potential for limited open cut operations, it is expected that these developments will principally be underground operations.
4.4 Coal Seam Gas
There is little quantitative information available on in situ coal seam gas (CSG) and its characteristics in the Hunter Valley coalfield except for some data from a few operating underground coal mines in the area. This section gives a very brief description of coal seam gas in the Hunter Valley coalfield generally as well as at the mine-scale.
4.4.1 Overall Assessment
Seam gases have been found in the coal seams in the Hunter Valley coalfield, typical examples are Dartbrook mine located in the north-western end of the field and South Bulga located in the south-eastern part of the coalfield. There is little seam gas data available for the coal seams located in the central part of the Hunter Valley coalfield, because most of the mining operations in the area are working in shallow coal seams, less than 200 m below the surface, and are using open cut methods. However, indications are that more gas is stored in the coal seam as the depth of the cover increases. In Wambo and United mines, for example, gas (3-4 m3/t) has been found in the coal seam at depths of 230-240 m and gas content increases as the cover depth increases (Atkins et al, 1998).
In the Hunter Valley coalfield, the gas content is found to range from nil to 12 m3 per tonne of coal in the coal seams currently being worked. A coal sample taken from 650 m below the surface in South Bulga indicated a gas content of 32 m3/t. Seam gas is shown to be predominantly methane (more than 90% CH4j). In some localised areas such as in Dartbrook mine, carbon dioxide is the predominant gas with C02/CH4 ratios ranging between 90:10 and 60:40 (Hayward, 1998). The high level of C02 is associated with igneous activity that occurred underneath the working seam. High proportions of C02 are also encountered in some part of Ellalong mine. Generally, though methane is the dominant seam gas in the Hunter Valley coalfield.
Factors that effect the amount of in situ coal seam gas include mainly the rank and type of coal, depth of the cover, thickness of coal seam, adsorption/desorption characteristics, geological structures, cleat network, and cleat infilling.
In general, the rank across the Hunter Valley coalfield decreases north from the Calool Syncline. The higher rank portion of the Coalfield is coincident with the occurrence of the thickest coal. Although at the low end of the optimal rank window, coal rank in the Hunter Valley coalfield does increase with depth and gas contents of 7 m3/t to 8 m3/t have been
76 recorded. The coal seam methane, coupled with a very thick coal sequence, are indicative of a considerable potential resource.
It is important to note that a large portion of the Hunter Valley coalfield is at sufficient depth to have significant adsorbed gas. Given the rank and measured gas contents, there is no evidence to suggest that significant degassing of the seams has taken place, other than adjacent to localised igneous intrusive activity and associated with localised faulting. The coals can therefore be considered to be at or close to full gas saturation (Brown et al, 1996).
The Hunter Valley coalfield is broadly in compression, but there are possible areas of tension on both sides of the Mount Ogilvie Fault in which there could have been development of enhanced permeability. There are no other known large-scale structural features, such as anticlines, likely to provide extensional targets.
As reported by Brown et al (1996), the Northern Hunter Valley coalfield to the west of the Mount Ogilvie Fault covers approximately 530 km2, with in-place gas resources of 126.86 billion m3. The Southern Hunter Valley coalfield covers 172 km2 between the Wollemi National Park and the Mount Thorley Monocline, with 83.96 billion m3 of in-place resources. The Southern Hunter Valley coalfield has proportionately more in-place gas because of a thicker coal sequence along the western edge of the Southern Hunter Valley coalfield. It should be noted that the in-place gas resources quoted above are those contained in the coal seams at depths in the 250 m to 850 m range and exclude those areas occupied by National Parks, colliery holdings, and urban development.
4.5 Mine-Scale Coal Seam Gas
Of the 25 operating coal mines in the Hunter Valley coalfield, 19 mines are open cut and the remaining 6 mines are underground. In open cut mines, seam gas is vented directly into the atmosphere during exposing and mining of the coal seams. Monitoring or measuring of seam gas in open cut mines is not practised, so there is no coal seam gas data available. Among the six underground coal mines in the Hunter Valley coalfield, Dartbrook and South Bulga mine are taking some measures to deal with seam gas in their operations, and the Wambo mine is actively assessing its coal seam gas potential.
• Dartbrook
Dartbrook mine encompasses a lease area of 14 km2 and is located in the Upper Hunter Valley. All the current mining operations are in the Wynn Upper seam. The working section is between 190 m and 410 m below the surface and between 4.0 and 4.5m in thickness. The working sections, floor and immediate roof average 20 m in thickness and contain 5 to 11 m3/t of gas with C02/CH4 ratios ranging between 90/10 to 60/40.
77 Working seam gas characteristics are as follows:
Working seam: Wynn Upper seam
Average depth: 210-380 m from the surface
Thickness of the working seam: 20 m (from which 4.5 m is mined)
Height of longwall extraction: 4.0 to 4.5 m (Wynn Upper A seam)
Seam gases: C02/CH4 = 90/10 to 60/40
Virgin seam gas content: 5 to 11 m3/t
Seam gas pressure: 0.9 to 1.7 MPa
Seam permeability: 0.2 to 1.7 mD, KVer/K hor = 0.1
Gas pre-drainage and goaf drainage are practised to combat the problem of high gas emissions in both development and longwall operation.
• South Bulga
South Bulga is located in the Southern Hunter Valley coalfield, some 18 km south of Singleton. Current operation is in the Lower Whybrow seam. Mining method is longwall operation and the mining height is approximately 2.6 metres. The coal seam contains 3 to
4 m3/t of gas with CH4/CO2 ratio around 90/10. Although the seam does not contain much gas, the permeability of the seam is extremely high (around 30 mD) and gas pre-drainage has to be practised to combat the problem of high gas emissions in mine development.
• Wambo
The Wambo mine is located in the central part of the Hunter Valley coalfield, some 16 km west of Singleton. Current mine operations are in the Whybrow seam. Recent drilling indicates that a considerable coal resource exist in the Arrowfield seam which is below the current seam. A conceptual mine plan has development of the Arrowfield seam starting in the year 2003. The greatest constraint to Arrowfield seam development is the issue of coal seam gas from the seam as well as the Bowfield seam which is 20-40 m below the Arrowfield seam.
Coal seams within the Wambo mine lease dip towards the south-west. The thickness of both Arrowfield seam and Bowfield seam is around 3 to 4 m and the cover depth of the Arrowfield seam ranges from 150 m to 580 metres. The Bowfield seam is about 20 to 40 metres beneath Arrowfield seam. Over the last several years the Wambo mine has been conducting gas desorption tests for several coal seams, in particular Arrowfield seam and Bowfield seam within its mine lease.
78 Results from the tests show that gas content can reach as high as 8 m3/t once the cover depth reaches about 200 metres. This is the case for both Arrowfield and Bowfield seams. Gas content of around 10 m3/t is observed at the cover depth of between 200 to 300 metres. Seam gas consists of 60-90% CH4 and 10-40% C02.
4.6 Summary
The main objective of the review and assessment described in the previous four sections is to provide a better understanding of the coal and seam gas resources in the Hunter Valley coalfield, with particular reference to the North (Upper) Hunter Valley coalfield.
The review of the stratigraphy and structural geology in the North Hunter Valley coalfield indicates that:
• Three coal measure sequences occur in the Hunter Valley coalfield: the Early to Mid Permian Greta Coal Measures, the Late Permian Wittingham Coal Measures and the overlying Later Permian Wollombi Coal Measures.
• The Greta Coal Measures and Wittingham Coal Measures are the major sources of commercial coal in the Hunter Valley coalfield. Six seams from the Greta Coal Measures and twenty seams from the Wittingham Coal Measures are mined or proposed to be mined in the Hunter Valley coalfield.
• Major structural features in the North Hunter Valley coalfield include the Muswellbrook Anticline, Hebden Thrust, Bayswater and Rix’s Creek Synclines, and the Camberwell Anticline. These structural features in the North Hunter Valley coalfield have been very well identified and documented, although there might be some scope in identifying and fine-tuning the mine-scale localised structures.
The review of the coal resources reveals that:
• In 1998-99 raw production from twenty-five operating coal mines in the Hunter Valley coalfield amounted to 84 million tonnes while the production from twenty-two operating coal mines in the Northern Hunter Valley coalfield was about 77 million tonnes.
• In the Hunter Valley coalfield, measured in situ resource is about 10,433 million tonnes, the recoverable reserve is about 4,180 million tonnes, and the marketable reserve is about 3,486 million tonnes. In the Northern Hunter Valley coalfield, measured in situ resource is about 8,722 million tonnes, the recoverable reserve is about 3,900 million tonnes, and the marketable reserve is about 3,300 million tonnes. Significant coal resources of high quality exist to the west of Scone, and it is estimated that there are sufficient resources in that area for an additional one or two mines.
79 The study of coal seam gas shows that:
• The North Hunter Valley coalfield to the west of the Mount Ogilvie Fault covers approximately 530 km2, with in-place gas resources of contains 126.86 billion m3. It should be noted that the in-place gas resources quoted above are those contained in the coal seams at depths in the 250 to 850 m range and exclude those areas occupied by the National Parks colliery holdings, and urban development.
• No data on seam gas is available for most operating coal mines in the Hunter Valley coalfield except Dartbrook, South Bulga and Wambo mines.
• In Dartbrook, gas content in Wynn Upper seam ranges from 5 to 11 m3/t with C02/CH4 ratios ranging from 90/10 to 60/40.
• In South Bulga, gas content in the Lower Whybrow seam ranges from 3 to 4 m3/t with C02/CH4 ratios about 10/90.
• In the Wambo mine, gas content in Arrowfield and Bowfield seams is around 8 m3/t when the cover depth reaches about 200 metres. With the cover depth of between 200 to 300 m, gas content of around 10 m3/t is observed. Seam gas consists of 60-90% CH4 and 10- 40% C02.
80 4.7 References
Atkins B, Nambo H, Hanna 0, Coverington M, and Kelly M, 1998. In situ gas measurement and simulation of gas flows for mine ventilation purposes - Arrowfield seam - Wambo mine. In: Proceedings of Australia-Japan Technology Exchange Workshop in Coal Mining ’98, Brisbane, Australia, November 10 & 11, 1998, 9p.
Beckett J, 1988. The Hunter Valley coalfield - Notes to accompany the 1:100,000 geological map. NSW Geological Survey Report GS 1988/051, NSW Department of Mineral Resources, 94p.
Beckett J, 1999. Coal seam nomenclature application in the Hunter Valley coalfield. In: Bulletin of the Coalfield Geology Council of New South Wales, Bulletin No.1, May 1999, pp. 103-112.
Brown K, Casey DA, Enever JR, Facer RA, and Wright K, 1996. New South Wales Coal Seam Methane Potential. Coal and Petroleum Branch, NSW Department of Mineral Resources, Petroleum Bulletin 2, 96pp.
Britten RA, 1972. A review of the stratigraphy of the Singleton Coal Measures and its significance to coal geology and mining in the Hunter Valley region of New South Wales. In: Proceedings of Annual Conference, AusIMM, pp 11-22.
Britten RA, 1975. Singleton-Muswellbrook district. In: Traves DM & King D (Editors): Economic Geology of Australia and Papua New Guinea - 2: Coal. AusIMM, Monograph No 6, pp191-205.
Hayward J, 1998. Dartbrook mine - a case study. In: Proceedings of 1st Australasian Coal Operators Conference (COAL 98). Wollongong, Australia, February 18-20, 1998, pp224-238.
Lohe EM, McLennan TPT, Sullivan TD, Soole KP, and Mallett CW, 1992. Sydney Basin - Geological Structure and Mining Conditions, Assessment for Mining Planning. External Report (New Series) No 20, CSIRO Division of Geomechanics, 1992.
NSW Department of Mineral Resources, 1999. 1999 NSW Coal Industry Profile, 271 p.
Rawlings CD, and Moelle KHR, 1982. The Lochinvar structure: not just an anticline. In: Proceedings of 16>h Newcastle Symposium on Advance Study Sydney Basin, pp25-26.
Sniffin MJ, and Beckett J, 1995. Hunter Valley coalfield. In: Ward CR, Harrington HJ, Mallett CW, and Beeston JW (Editors) Geology of Australian Coal Basins. Geological Society of Australia Incorporated Coal Geology Group, Special Publication No.1, 1995, pp. 177-195.
Stuntz J, 1972. The subsurface distribution of the ‘Upper Coal Measures ’, Sydney Basin, New South Wales. Annual Conference, Australasian Institute of Mining and Metallurgy, Newcastle, pp1-9.
81 5 INNOVATIVE GAS RECOVERY TECHNOLOGY
5.1 Introduction
This chapter examines the main techniques used for capturing coal seam gas prior to, during or after mining. The detailed objectives of this chapter are to describe briefly:
• main gas capture techniques;
• performance and applicability of the techniques and;
• application of the techniques in the Hunter Valley coalfield.
The techniques are conventionally classified as:
• Pre-drainage techniques (prior to mining) and
• Post-drainage techniques (during or after mining).
Pre-drainage techniques
Pre-drainage techniques involve removing methane from coal seams prior to mining. They are applied predominantly to the seam to be worked. The objective of pre-drainage is to remove as much methane as possible from the strata likely to be disturbed by mining before coal extraction commences. A continuous and open cleat fracture network within the coal appears to be a pre-requisite for satisfactory rate of recovery of methane from virgin coal seams. There are two principal techniques of pre-drainage: in-seam boreholes (in-mine pre- drainage) and vertical wells (pre-drainage from the surface boreholes).
Both techniques can be implemented anywhere from six months to several years prior to the commencement of active mining operations, depending upon the amount of degasification required and various geologic factors such as methane content and permeability.
Post-drainage techniques
Post-drainage involves intercepting methane released by mining disturbance before it can enter a mine airway. The techniques involve obtaining access to the zone of disturbance above and also sometimes below, the worked seam. Access is gained by drilling from the underground roadways, drilling from the surface, driving roadways into the disturbed zone or exploiting old workings which lie within the disturbed zone.
The objective of post-drainage, irrespective of the method of access, is to maximise the rate of gas removal from the underground mining districts and hence minimise the gas flow into the mine airways to ensure the requisite coal production targets can be safely attained. Two main techniques are in-mine boreholes (in-mine post drainage) and vertical goaf wells (surface goaf drainage). 82 These techniques include:
• Horizontal boreholes;
• Vertical wells;
• In-mine boreholes; and
• Vertical goaf wells.
5.2 Techniques, Performance and Applicability
5.2.1 In-Seam Borehole
Technique Descriptions
In-seam boreholes have been used extensively for methane pre-drainage (Figure 5-1). In seam boreholes are drilled to produce methane prior to mining. These boreholes can be drilled in two ways: 1) into a longwall panel, or 2) into mine development areas prior to the preparation of panels for mining. In the first case, in-seam boreholes are drilled across the width of a developed longwall panel and typically will produce gas for a period of several months until they are mined through. In the second case, much longer boreholes can be drilled into the coal reserves from the main headings and drain gas for several years in advance of mining. The arrangement of the boreholes can be parallel to the face or in fan formation (Figure 5-1). The principal variables to be considered when designing in-seam pre drainage systems are borehole location, borehole length, borehole spacing, collection system capacity, and the time available for drainage.
Figure 5-1 In-seam boreholes. In-seam boreholes are connected to an in-mine piping system often operated under negative pressure to remove gas. Depending on the permeability of the coal and the suction applied, the holes are cased to a length of 10 to 20 metres, sealed, and connected to the drainage
83 range. In-seam boreholes maybe drilled using top drive rotary drilling or down-hole motor drilling. The latter is becoming more popular because it is possible to drill longer boreholes and achieve more accurate positioning of the borehole.
Performance and Applicability
Production volumes vary with local geology (particularly gas content and coal permeability) and borehole length, and typically have ranged from 700 to 5,000 m3/day. The maximum reported production from a mine-scale, multiple in-seam horizontal boreholes, configuration is 200,000 m3/d of methane has been achieved (Trevis and Finfinger, 1986; Baker et al, 1986; Kline et al, 1987). Useful production lifetimes are reported to be from six months to several years. The production lifetime is limited by how far in advance the mine is developed. Typical gas quality and flowrates using this technique are:
Gas quality, high quality up to 90% CH4, normally 60 to 90% CH4. Depending on specific arrangements, gas may be diluted with ventilation air.
Gas flow rate: up to 100 I/s per hole, 700 - 5,000 m3/d per hole and 200,000 m3/d per mine has been achieved
Theoretically, any seam can be pre-drained using this technique if sufficient boreholes are drilled and there is no time limit for gas extraction. Practical limitations of panel access ahead of mining usually means that the technique is generally only applied to seams with a moderate to high permeability.
5.2.2 Surface Wells
Pre-drainage using vertical wells and hydraulic fracturing techniques are primarily used in the coalbed methane industry (Palmer et al, 1992) where the primary aim is gas recovery from virgin coal seams independently of mining. Surface wells have not been widely used by the coal mining industry probably because of; non-uniformity of drainage; possible mechanical damage to roof and floor and high cost.
Pre-drainage may be a very effective method of reducing the methane content of coal seams and could consequently reduce the emissions from mining operations. Recovery rates of up to 70 percent over a 10-year period have been documented using this technique. Gas quality is high (over 90 percent methane) because the methane is not diluted with ventilation air. Production rates depend on reservoir and geological factors, the success of hydraulic stimulation, coal rank and well spacing. Typical gas quality and flowrate using this technique are as follows:
Gas quality high quality normally above 95% CH4
84 Gas flow rate: production rate ranging from 10,000 to 300,000 m3/d/well have been reported, (Murray, 1996). Wells run for 2 to 10 years
A new technique to pre-drain seam gas from the surface has been developed and tested by the Centre for Mining Equipment and Technology in Australia. Using water jets, in-seam holes are drilled into the coal seam from a vertical hole intersection. A major trial at the site of a new mine was carried out in 1999 but results are still confidential to the mine.
5.2.3 In-Mine Cross-Measure Boreholes
Technique Descriptions
This technology consists of drilling boreholes from the mine workings into the unmined areas of the coal seam and surrounding rock. There are two types of in-mine boreholes: cross- measure boreholes and long horizontal boreholes. Both types of boreholes are drilled in the zone above or below the working seams (Figure 5-2).
Cross-measure boreholes are angled above and, in some instances, below the goaf close to the coalface, these boreholes are typically tens to hundreds of metres in length. The boreholes are connected to an in-mine vacuum piping system, through which recovered methane is transported out of the mine. The largest methane flows usually arise from the roof a few metres behind the coalface and reasonable flows persist generally for a distance up to 30 m behind the face. Methane drainage from the floor is not always successful and dewatering aids may be required to assist gas flows. Development of a de-stressed zone in the floor may lag behind the coalface by tens of metres. Consequently there is a significant delay before gas flows are detected.
vertical goaf wells
cross-measure boreholes cross-measure boreholes (drilled ahead of the coalface) (drilled over the goaf)
Figure 5-2 In-mine cross-measure boreholes and vertical goaf wells.
85 Performance and Applicability
Experience shows that the effectiveness of methane drainage performance of cross measures boreholes varies with mining techniques and geology. Typically 30-70% of the total gas released in an individual longwall district can be captured, with methane concentrations usually in the range of 35-70% and flows (in terms of pure methane) generally of around 100-700 I/s per hole. Methane capture on advancing longwall face is usually around 50-70%, whereas on retreat coalface it tends to be lower, commonly 30-50%. Where the predominant coal seam methane sources lie in the floor strata rather than in the roof, high gas captures may not be consistently obtained but methane purities are often relatively high. Typical gas quality and flowrate using this technique are as follows:
Gas quality. 20 to 80% CH4
Gas flowrate: 500 to 6,000 m3/day per hole, up to 60,000 m3/day per longwall district
Modern guided long borehole drilling techniques have been used to capture the methane above or below the worked seam. A borehole started from the worked seam can be guided through an arc to run parallel with the workings at a selected horizon above or below the workings. Three or more boreholes (a few hundreds of metres long) are normally required to achieve a reasonable gas capture, and also allow for borehole damage as the longwall face retreats. This technique has produced some mixed results. An attempt to use this technique in the United Kingdom failed due to drilling difficulties resulting from swelling of mudstone and borehole instability (Bennett, 1994). However, successful application has been reported in the Sydney Basin, NSW, Australia, where an inclined borehole was drilled from a roadway in the worked seam (Bull! seam) to intercept and follow the Balgownie seam located around 10 m below the Bull seam (Greedy et al, 1997). Kratis and Li (1995) also demonstrated the technique in the United States when a series of 89mm diameter boreholes were drilled over the goaf of two longwall panels at the Cambria Slope No. 33 mine, Pennsylvania. Major problems that were reported were water accumulations in dips within boreholes, ventilation air ingress diluting the gas in one borehole and the loss of a directional drill.
Similar to the cross-measure boreholes, long boreholes drilled along the longwall panel into the goaf and are operated under negative pressure to maximise gas production (Figure 5-3). In the process however, mine air is also drawn into the goaf area and ultimately into the gas stream. The quality and quantity of gas recovered will vary greatly depending on such factors as local geology, gas content, length and position of the borehole, and the efficiency of the recovery system. Previous experience indicates that gas production rate of 500 to 6000 m3/day for boreholes of 200 to 300 metres in length and the gas purity range from 20 to 80 percent (Balusu et al, 1998).
86 >s\%\s »s\%\s\s\s\s'» s» y-% • s ■ s ■% ■;
IlllllilllilSjglllL (a)
Goaf gas plant
(b)
S u r f a c e SS'SrS/SSSSSSSSSSSSSSSSSSSSSSSSSSSVSSA 'ss'sA *71777777?JSJSSSJL1JSSS/S/SS/SS7S/SS/SSJSSJ
Figure 5-4 A typical vertical goaf well for gas capture a) Plan view of longwall with a single surfacegoaf hole
b) Section (over longwall) from coal to the surface
c) Details of a typical goaf drainage hole.
Performance and Applicability
The gas quality is similar to that of the in-mine borehole technique, although it may be easier to produce high quality methane using surface goaf wells. As with in-mine boreholes, surface goaf wells are usually operated under negative pressure to obtain reasonable gas flow rates, and in the process may draw mine air into the goaf which dilutes the recovered methane.
88 Through careful management of mine ventilation, adjustment of the vacuum pressure and goaf sealing operations, it is possible to maintain a higher and more consistent gas quality. One mine in Alabama, US, produces gas with over 95 percent methane from the goaf wells (Dixon, 1989).
Surface goaf wells alone may recover 30 to 40 percent of the methane contained in the goaf area (Diamond, 1994). Typical production figures are 2800 m3 per day, but are highly dependent on site-specific factors and can up to 30,000 m3 per day (Baker et al, 1988; Balusu et al, 1999). One mining operation in Alabama, U.S., recovered 849,000 m3 per day from 80 surface goaf wells (Dixon, 1987) Typical gas quality and flowrates using this technique are:
Gas quality: 20 to 80% CH4, up to 90% CH4 in a totally sealed goaf
Gas flowrate: 2,800 to 30,000 m3/d/well, up to 849,000 m3/d/mine
5.2.5 Purity and Variation of Captured Gas
Methane percentage in the gas captured using drainage techniques depends upon many factors such as:
• Gas composition in the coal seams and surrounding strata.
Though methane is the predominant seam gas in most coal seams, there are some cases where seam gas consists of substantial carbon dioxide, such as in Tahmoor and Dartbrook mines in Australia.
• Methane capture technique. Pre-drainage techniques normally yield higher quality gas than post drainage techniques, because the methane is not diluted with ventilation air. Of the four techniques, the surface well technique produces the highest gas quality (above 90% methane).
• Site-specific geological and mining parameters such as caving characteristics, mining layout, mine ventilation design, and goaf sealing sequences.
The total amount of methane that may be captured using methane drainage techniques depends on site-specific conditions the most important of which are:
• Gas content of the coal seams and surrounding strata;
• Permeability and diffusion of the coal seams and surrounding strata;
• Drainage time;
• Amount of negative pressure applied and;
89 • Other variables of the geologic and drainage systems.
Variability of gas purity is related to sources of gas in a mine and the factors that affect each of the sources. The most important factors are:
• Desorption of gas from coal throughout the body of the mine into the mine ventilation air. This increases with the expansion of the mine, decreases with the time that areas are exposed and is subject to atmospheric pressure fluctuations.
• The rate of mining, caving mechanisms, percentage of seam extracted and atmospheric pressure effects.
• The drilling schedule because in seam drainage declines with time for each drainage hole.
5.3 Application in the Hunter Valley coalfield
Gas drainage is practised in two underground coal mines in the Hunter Valley coalfield, namely Dartbrook mine and South Bulga mine. While both pre-drainage and post-drainage are used in Dartbrook mine, only in-seam pre-drainage is used in South Bulga mine.
South Bulga is located in the south part of the Hunter Valley coalfield, some 18 km south of Singleton. Current operations are mining in the Lower Whybrow seam using a longwall system with a working height of between 4.0 and 4.5 metres. The coal seam contains 3 to 4 m3/t of gas with CHVC02 ratio around 90/10. Although the seam does not contain much gas, the permeability of the seam is extremely high (around 30 mD) and in-seam borehole technique has to be used to combat the problem of high gas emissions during mine development. A set of parallel in-seam boreholes is drilled across the longwall panel. Hole spacing is around 50 to 70 metres. Gas flow is mainly governed by in-place gas (gas content) rather than permeability. Overall high seam permeability has enabled the in-seam borehole technique to be successfully applied in South Bulga mine.
Dartbrook mine encompasses a lease area of 14 km2 and is located in the Upper Hunter Valley. All the current mining operations are in the Wynn Upper seam. The working section is between 190 to 410 m below the surface and between 4.0 and 4.5 m in thickness. The coal seams of working section, floor and immediate roof average 20 m in thickness and contain 5 to 11 m3/t of gas with C02/CH4 ratios ranging between 90/10 to 60/40. The in-seam borehole technique and vertical goaf well methods are used to control high gas emission during roadway development and longwall operation. A general layout of pre- and post- drainage is shown in Figure 5-5 (McLean, 2000). Currently, the in-seam borehole technique and vertical goaf well technique capture 20% and 30% respectively of total gas emission.
90 5.4 Summary
There are four major existing methane capture techniques currently used in coal mines. These techniques are in-seam boreholes, surface wells, in-mine cross-measure boreholes, and surface goaf wells. The general description and their performance and applicability of the main methane capture techniques have been given in this chapter.
Among these techniques, surface wells have not been widely used in coal mining industry. Surface wells are primarily developed used by the coalbed methane industry and their effectiveness and economics to control gas emission has yet to be proven in coal mine operations in Australia.
In-seam borehole techniques have been extensively used for gas pre-drainage to control gas emission in roadway development and longwall operation, and to minimize the risk of outbursts of coal and gas. However its effectiveness is dependent on seam permeability. In general, the technique can be successfully applied if the working seam has a moderate to high permeability, ie greater than 3 mD.
In-mine cross-measure borehole and surface goaf well techniques are widely used in underground coal mines to control the problem of high gas emission. The success of these techniques depends upon geology and longwall caving behaviour, however in general the techniques are applied if there are overlying/underlying gassy seams contributing to gas emission during longwall operations.
In the Hunter Valley coalfield, in-seam borehole technique and surface goaf well technique have been successfully used, particularly in Dartbrook mine where more than 50% of total gas emission has been captured using these techniques.
92 5.5 References
Baker EC, Garcia F, and Cervik J, 1988. Cost Comparison of gob Hole and Cross-Measure Borehole Systems to Control Methane in Gobs, U.S. Bureau of Mines, Report of Investigation, Rl 9151.
Baker EC, Gray III RH, and Finfinger GL, 1986. Economic Evaluation of Horizontal Borehole Drilling for Methane Drainage from Coalbeds. U.S. Bureau of Mines Information Circular, IC 9080.
Balusu R, Xue S, and Mallett C, 1998. Mine Gas Control - First year project report. CSIRO Exploration and Mining Report 490C, Australia, March 1998, 287p.
Balusu R, Xue S, Wendt M, and Mallet C, 1999. Mine Gas Control - Second year project report. CSIRO Exploration and Mining Report 607C, Australia, March 1999, 238p.
Bennett SC, 1994. Improved safety and control of methane gas in UK coal mines. In: Prospects for clean coal technology, proceedings for Nottingham Coal Conference. Nottingham, UK, Department of Trade and Industry, ETSU, Harwell, Oxfordshire, 10p.
Greedy DP, Saghafi A, and Lama R, 1997. Gas Control in Underground Coal Mining. IEA Coal Research, IEACR/91, April 1997, 119p.
Diamond WP, 1994. Methane Control for Underground Coal Mines. Information Circular 9395, Pittsburgh, PA, USA, US Department of the Interior, Bureau of Mines (Pittsburgh Research Centre), 51 p.
Dixon CA, 1987. Coalbed methane - a miner's viewpoint. In: Proceedings of the 1987 Coalbed Methane Symposium. Tuscaloosa, AL, USA.
Dixon CA, 1989. Maintaining Pipeline Quality Methane from Gob Wells. Pittsburgh Coalbed Methane Forum, April 4, 1989, 7p.
Garcia F, and Cervik J, 1988. Review of Membrane Technology for Methane Recovery In: Mining Operations. U.S. Bureau of Mines Information Circular, IC 9174, 6p.
Kline RJ, Mokwa LP, and Blankenship PW, 1987. Island Creek Corporation ’s experience with methane degasification. In: Proceedings of the 1987 Coalbed Methane Symposium, Tuscaloosa, AL, USA., pp.279-284.
Kravits SJ, and Li J, 1995. Innovative in-mine gas recovery techniques implemented by Resource Enterprises. In: International Symposium-cum-Workshop on Management Control of High Gas Emission and Outbursts in Underground Coal Mines. Wollongong, NSW, Australia, March 1995, pp523-532.
Lama RD, 1986. Increasing Efficiency of Gas Drainage. End of Grant Report, NERDDP Project No 574, Department of Primary Industries. 462pp 93 McLean D, 2000. Gas management in Dartbrook mine. Personal Communication, 2000.
Murray DK, 1996. Coalbed methane in the USA: analogues for worldwide development. In: Coalbed Methane and Coal Geology. Geological Society Special Publication No. 109, London, pp1-12.
Palmer ID, Mavor MJ, Seidle JP, Spitler JL, and Volz RF, 1992. Openhole cavity completion in coalbed methane wells. SPE 24906, Procedures of the Annual Technical Meeting of SPE, 2 Volume, Washington DC, USA, October 1992.
Rich J, and Azinger K, 1999. Review of the Appin Tower coalbed methane power project after 2.5 years of operation. In: ACMER Workshop on Practical Implications of Greenhouse Emissions Management for the Minerals and Energy Industries. Brisbane, Australia, 7p.
Trevits MA, and Finfinger GL, 1986. Results given of studies concerning methane extraction from coalbeds. Mining Engineering, August 1986, pp.805-808.
USEPA (United States Environmental Protection Agency), 1990. Methane Emissions from Coal Mining: Issues and Opportunities for Reduction. Office of Air and Radiation, Report 9ANR-445, Washington, DC, USA, EPA/400/9-90/008.
USEPA (United States Environmental Protection Agency), 1993. Options for Reducing Methane Emissions Internationally. Volume 1: International Opportunities for Reducing Methane Emissions: Chapter 4: Coal Mining. Report to Congress. U.S. Environmental Protection Agency/Office of Air and Radiation, EPA-430-R-93-006, 38p.
USEPA (United States Environmental Protection Agency), 1999. White Paper: Guidebook on Coalbed Methane Drainage for Underground Coal Mines. USEPA Report, April 1999, 46p.
Zhou SN, China University of Mining and Technology, China, Personal Communication, 1998.
94 6 HUNTER VALLEY GAS UTILISATION OPTIONS
6.1 introduction
Methane is a highly valued resource because it is free from sulphur and other contaminants which affect alternative fossil fuels. There are a wide variety of uses for methane which have been developed over time varying from chemical processes such as carbon black and liquid fuel production to electricity production with gas turbines and fuel cells. Most of these technologies have been investigated for use with mine gas and some have been tried, however, almost all are uneconomical. The major challenges are the variability of the supply and economy of scale. Many of the technologies that are economically viable using a large natural gas supply are not viable when scaled down to match the methane production from a mine or are not feasible because of the high variability of the fuel source.
The nature of conventional coal seam methane (CSM) operations results in gas with a high degree of purity, generally in excess 95% methane. Gas of this quality, supplied in a fashion that is sufficiently constant, finds ready acceptance for a variety of industrial or domestic applications, and can generally be placed into a pipeline system with little or no beneficiation. Two commercial CSM operations exist in Australia. These are located in the Bowen Basin in Queensland, and together supply over 5% of that State ’s annual gas production. Production from these regions is growing steadily and annual production will likely exceed 22 petajoules by mid-2001 .
Coal Mine Methane (CMM) differs from CSM is several ways. Principally, CMM is the result of coal extraction activities from underground coal mines releasing contained methane. This methane is a waste product and safety hazard. In addition, CMM comes in distinct streams of varying quality: pre-and post-drainage gas, which usually contains 50% - 95% methane by volume, and mine ventilation air, which generally contains between 0.1 and 1% methane by volume.
Whilst CSM and CMM have some distinct differences, there are also many similarities in the way the methane can be treated or utilized. For the purposes of this report, technologies and techniques for gas utilisation from both CSM and CMM will be discussed together.
There are currently two successful and environmentally beneficial CMM operations in Australia. These are located at BHP’s Appin and Tower Collieries and Shell Coal ’s Central Colliery. In New South Wales, the Appin and Tower Collieries produce electric power by using drained coal mine methane to fuel 94 one-megawatt reciprocating engines. In addition, the 54 engines located at the Appin site also consume 15-25% mine ventilation air as combustion air.
95 The German Creek Central Colliery in Queensland recently installed a methane flare that combusts goaf drainage gas at a rate of approximately 1 kg/s. The flare technology was adapted from that utilised at sewage treatment plants. Plans are under way to extend the operation to drainage gas as well, which would increase the methane volume burnt by a further 20%.
In order to choose the best option for a particular mine the flexibility and efficiency of each of the main processes has to be taken into account as well as the capital and operating costs and the value of the electricity produced.
6.2 Regional & mine specific issues
6.2.1 The Wambo mine and the Upper Hunter Valley - context
The Wambo mine is located in the Upper Hunter Valley, some 16km west of the town of Singleton. The Upper Hunter has a combined population in excess of 51,000 people, centred primarily in Singleton (-12,000), Muswellbrook (-11,000) and Scone (-4,000). Mining, together with agriculture, forestry and retail industries are the largest employers. In addition to coal mining and the associated power production, the area is well known for its cropping, grazing, vineyards, tourism and forestry industries (DUAP, 1997).
Macquarie Generation operates the Liddell (2000 MW) and Bayswater (2640 MW) coal fired power stations, located between Singleton and Muswellbrook (see Figure 6-1), and fed from locally sourced coal. The combined generating capacity of these stations can contribute up to 42% of New South Wales ’ electricity supply (DMR, 2000). Numerous 330 kV transmission lines connect these two stations to the State ’s grid.
The Upper Hunter is not currently connected to a major gas pipeline network. However, the NSW Government is implementing a series of initiatives in the area of energy supply and efficiency which could impact on the dominance of the coal fired power generation industry, including (as described in Agnew, 1999):
• Comprehensive reform of the electricity market, which includes:
o The requirement of electricity retailers to submit plans for reducing greenhouse gases;
o The establishment of the Sustainable Energy Development Authority (SEDA), which has developed a number of initiatives to promote energy efficiency. This includes the Green Power Accreditation program, which allows individual consumers to choose to receive electricity generated from renewable sources.
96 Encouraging the burning of natural gas. Initiatives in this area include:
o Initiatives for the establishment of gas fired cogeneration powers plants, which have resulted in a number of such plants being built or proposed.
o The removal of barriers to trade in natural gas, which has given third parties access to existing pipelines
o The promotion of new gas pipeline construction, thereby expanding the interstate reticulation system and diversity in supply
• Encouraging alternative energy technologies which has resulted in the following:
o Construction of the 128 MW Redbank Power Station near Singleton, due for completion in 2001. This fluidised bed combustion power station will be fuelled by washery reject from an adjacent coal mine. The possibility exists for other similar facilities adjacent to coal mines.
o Other alternative technologies include: wind power, solar energy facilities, and the use of hot dry rock, exploration for which is currently under way in the Hunter Valley.
97 6.2.2 Applications for mine methane
There are numerous applications for the utilisation of mine methane. However, a survey of potential applications and consultation with industry, conducted previously by CSIRO (see Wendt et al, 2000), indicated electricity generation and mine water management as two favoured options for mine methane utilisation.
Electricity generation is an attractive option as it can be used locally or sold to the grid. The market and the infrastructure to market electricity already exist, and are prominent industries in the Hunter Valley.
Mine water management is an issue for many mine sites and using methane to power the cleaning of water provides not only a market for the methane but also long term sustainability of the coal mining operation. This is a particular issue in the Hunter Valley, where competition for scarce water resources is increasing, and issues of water quality have recently seen the introduction of a salinity trading scheme. If water production capacity exceeds the mine site needs, excess water can easily be transported and sold to local consumers. Because of the ease of water storage, water can be cleaned at a rate which is dependent only on the gas flow rate.
6.2.3 Gas quality
Data provided here suggests that the gas composition at the Wambo mine varies significantly with depth and between potential target seams. Average gas composition in the Arrowfield seam is 69% CH4:31% C02; in the Bowfield seam 64% CH4:36% C02. Depth/composition relationships average an 80:20 CH4:C02 split at 200 m depth, to 50:50 CH4:C02 at 800 m. Clearly this is well outside pipeline-quality gas, and the treatment costs to bring the gas up to pipeline quality would be significant. Gas extracted from these seams would not resemble traditional CSM operations, but the gas is of sufficient quality that it could be used for specific applications.
6.3 Potential Utilisation Options
Given that the gas from the Wambo mine is predicted to contain up to 50% carbon dioxide, applications for direct utilisation are constrained. Without the significant cost of gas cleanup, the gas cannot be use in ‘pipeline ’ applications such as domestic consumption and for industrial applications. To be economic utilisation technologies have to be customised to a mine ’s gas supply characteristics, site logistics, and local markets for products.
The following section, based largely on Wendt et al, 2000, but tailored to Wambo ’s situation, I discusses potential mine methane utilisation options for the Wambo mine.
99 6.3.1 Power Generation
Power generation using the gas available from the Wambo mine is perhaps the most obvious utilisation option, as electricity is an in-demand resource and a readily saleable commodity. In Wambo ’s case, the close location of the mine to one of the state ’s major electricity distribution systems is an added advantage. Electricity generation has been the favoured option so far by coal mine methane (CMM) usage projects (notably at BHP’s Appin/Tower collieries) because:
• It is easy to sell because of a large deregulated market;
• The infrastructure for transport is in place;
• It can displace the mine's purchase of electricity;
• There is a range of commercial as well as promising new technologies in this field;
• There is a further increase in the net greenhouse gas reduction by displacing coal- fired power and;
• There is a reduction in SOx, NOx, C02 and particulates compared to coal.
Ways in which gas from the Wambo mine could be converted into electricity are outlined in the following sections.
6.3.1.1 Gas Reciprocating Engines Australian power producer Energy Developments Ltd (EDL) installed reciprocating engine based power plants utilising drainage gas at two coal mines in Appin and Tower, New South Wales, Australia. The Broken Hill Proprietary Ltd (BMP) Steel Division owns both mines.
The Appin project commenced in 1995 with 54 generating sets converting 92 kt/year of coal seam gas into electricity. The power plant exports electricity to the state grid and provides the mine with a reliable source of standby power (Azinger 1998). Each of the generating sets is rated to produce 1030 kW of continuous power and consists of a 16-cylinder spark ignition internal combustion reciprocating engine coupled to a generator. Power from blocks of units (up to 20 generator sets) is connected to a single 25 MVA transformer, where power is stepped up to 66 kV prior to connection to the local utility grid. Most of the power generated is exported and only a fraction is used by the mine.
The power plant is connected to the grid in a way that allows it to maintain power supply to the mine in isolation from the grid. The plant is automatically disconnected from the grid in the case of any grid malfunction.
The plant is operated as a base load station 24 hours a day, with the quantity of power exported dependent on the availability of coalbed methane. During peak demand periods
100 natural gas from the pipeline is used to top up the fuel supply so the plant can run at full capacity. After extraction from the mine, the drainage gas is delivered to the power plant at pressure of about 20 kPa. It is subsequently filtered to 1 micron to remove dust particles and delivered to the engines at 10 kPa. The engines ’ fuel control system caters for fuel quality which can vary instantaneously from 40% CH4 to 100% CH4. The volume of fuel going to the engines is controlled by a proprietary control system developed by EDL, which allows rapid changes of fuel quality without affecting the engines ’ performance.
The modularity of the EDL plant makes this concept particularly attractive. First, it makes it easier to install at the minesite as the generator units are easily transportable and installation can be done in stages. It also makes decommissioning easier and allows generating sets to be transferred to another site. None of this would be possible with one large gas fired power plant. Second, the output of the plant can be varied by partial shutdown, depending on drainage gas supply and/or energy demand, without compromising the efficiency. However, the small size of this plant makes the per-unit cost and maintenance cost high. Without a special user contract for the sale of the electricity it is unlikely that such a plant could be economically viable.
6.3.1.2 Power Station Air The Wambo mine is located within 20 km of two large conventional coal-fired power stations, Liddell (2000 MW capacity) and Bayswater (2640 MW capacity). This means it may be possible for methane to be fed directly into the power station boilers as a fuel supplement to the coal, through displacing some of the air already being injected. Given the percentage of methane present in the Wambo mines gas, it would be necessary to significantly dilute the gas with air so that the methane content in the air upon entering the boiler was around 1 % by volume. This is required for safety reasons, and as a mechanism to allow the power station to maintain a consistent fuel input to the boilers.
6.3.1.3 Gas Turbines Current CMM utilisation schemes have significant variability in flow rate which leads to operational difficulties. Conventional gas turbines have a low efficiency if not run at the design fuel flow rate. A series of smaller gas turbines could be used which could be switched on and off as gas flow rate varies, however, the cost per kW and low efficiency of small turbines makes this uneconomic.
Gas turbines are a well-established technology used for power generation. Gas turbines are a very flexible in their application, and are available in a wide range of power ratings from micro-turbines of about 20 kW to large units reaching 300 MW. They also have very short start-up and shutdown times and can be installed as banks of independent units, which
101 allows for variation in fuel supply and facilitates maintenance. Their efficiency ranges from 25% for micro-turbines to 43% for larger units.
Conventional gas turbines are internally fired (open cycle) and can burn CMM if the concentration of methane exceeds 40%. Two gas turbine systems have already been tested in Australia for the burning of CMM. West Cliff colliery used a 10 MW turbine to generate electricity with a gasometer to even out the fluctuations in supply. At Appin colliery a 16.8 MW Mitsui SB-90 turbine was operated between 1988 and 1995.
Gas turbines can operate in both conventional internally and externally fired arrangement. For externally fired (closed cycle) turbines heat is added to the turbine by the recuperator only which is modified to take higher than normal temperatures. Although closed cycle turbines are more expensive and difficult to construct than open cycle turbines they have many advantages when considering their use with CMM. These include:
• No combustion occurs inside the turbine and hence cleaning of the fuel gas is minimised.
• Higher efficiency inert gases such as helium or carbon dioxide can be used inside the turbine.
• No oxidation, erosion or carbonation of the turbine occurs which leads to longer turbine life.
• No pressurisation of the fuel is required at the combustor runs at atmospheric pressure.
• The gas can be burnt at any rate or concentration and stored in a latent heat storage device prior to use.
• Supplementary fuels such as coal or coarse reject can be used.
6.3.1.4 Coal and Gas Turbine To even out the variability in supply of CMM, locally available coal or waste coal could be used. An alternative to using steam plant is the use of externally fired gas turbines fired on a combination of coal and gas. Coal burning turbines were built in the late 1950 ’s and 60’s in Germany and Russia.
Two Australian companies, Thermal Technologies and Liquatech Turbines are presently forming a joint venture to build a 230 kW externally fired turbine using heat from waste coal. Although waste coal can be burnt in a number of combustion systems, a new system is currently under trial which not only can convert the waste coal, mine ventilation air and drainage gas efficiently into electricity but turns the reject ash into a valuable by-product with sufficient value to be transported away form the minesite. 102 A trial program is presently underway for a 150 kg/hr of waste coal system with coal waste initially taken from stockpiles in the Ipswich area in Queensland. The new system uses a rotating kiln attached to an indirect-fired turbine generating electricity from the waste flue gas. The coarse reject coal waste is ground to a target size, approximately 6-8 mm, before entering the kiln. The carbon is burnt to generate heat while the remaining ash is converted into nodules ideal for the manufacture of lightweight building materials. The product can be either open of closed celled in structure and has a variety of uses from hydroponic pumice to a substitute for gravel. Limestone may also be added to convert any sulphur dioxide to calcium sulphate. All waste methane from pre- and post-drainage can be converted to energy in the kiln, with coal flow rate varied to balance the methane input.
6.3.2 Other Uses
6.3.2.1 Desalination & Waste Water Treatment In current usage of CMM, fluctuations in gas supply are dealt with by using multiple small units and supplementary fuel such a natural gas. Most utilisation options are not practical without some form of storage system or supplementary fuel to even out supply. From a mine ’s perspective the most favourable solution is to produce a commodity at a rate that is dictated by the normal operation of the drainage facilities. From a marketing perspective the value of a commodity is enhanced if the minesite can generate excess saleable quantities that are readily transportable to the local community.
Mine water management is an issue for most mine sites and the use of waste methane to power the cleaning of this water is a solution not only for the mitigation of the methane but also for long term sustainability of the coal mining operation. Because of the ease of water storage, water can be cleaned at a rate which is dependent only on the gas drainage rate. A methane powered desalination system could solve site water supply and disposal issues. If water production capacity exceeds the mine site needs, excess water can easily be transported and sold to local consumers or discharged of site. If excess gas is available, highly mineralised bore water could also be purified, the dried salts could be sold or disposed of underground.
A demonstration plant at Morcinek Mine in Poland has been purifying water using CMM for the past four years, albeit using a very inefficient technique (Tait, 1998). Highly efficient distillation processes have resulted in 70 to 100 kW of distillation for 1 kW of power input. This translates to 120 litres of distilled water per kW of electrical power input (Patrick, 1999). The drained gas can fuel conventional gas engines, similar to the ones presently used at the Appin/Tower colliery, to drive conventional vapour compression distillation of water at high efficiency either through direct drive or via electrical machinery. The distillation system uses
103 the latent heat of the clean condensing water to provide the heat for evaporation of the dirty water. One disadvantage of this use is that it is capital intensive. Although the application is novel, the technology is entirely conventional.
The recycling of mine waste water removes the need to purchase clean water from authorities at a cost of up to $1000 per megalitre (ML). It also reduces the need to provide evaporation dams for waste water disposal. Most collieries are not allowed to release water off the lease and must find on site disposal facilities. Taking into account the variability in power requirements would mean that a 10 MW supply of gas would provide approximately 9 ML of distillation per day or over 3000 ML/year which meets most coal mine requirements.
Development of an efficient system to allow adjustment of the distillation rate to match gas flow rate will be required. An alternative hybrid system could be used which included power generation. The gas engines could be used to generate electricity, the base load electricity supply could use on-site or sold and excess electricity used to drive the distillation plants.
An alternative would be to use waste heat from a power generation system to provide desalination. A modified version of vapour compression known as Thermal Vapour Compression uses steam generated from waste heat to drive a small impulse turbine to in turn drive the vapour compression fan. Other systems are also available that use waste heat directly.
6.3.2.2 Industrial Boilers For many years mines in China, the Czech Republic, Poland, Russia and Ukraine have co fired CMM with coal in boilers to produce heat or electricity. The use of medium concentration CMM in coal-fired boilers as a supplementary fuel has the following advantages (Pratapas & Chan 1998):
• Reduction by non-selective reduction of NOx;
• Reduction of emissions of S02, C02 and CH4;
• Reduced operating and maintenance costs;
• Reduced particulate emissions;
• Improved ash quality allowing the sale of ash in some circumstances;
• Higher efficiency because of improved carbon burnout;
• Higher efficiency through lower excess air requirements;
• Easier start up;
• Increased short-term peaking capacity and;
104 • Methane can be selectively injected into various parts of a boiler to prevent slag build up.
The use of CMM can also result in lower capital investment costs for a new plant, which can result in a more rapid return on investment. Conversion of an existing boiler is more difficult because of the lower radiation emission of a CMM produced plume. Additional convectively heated sections may need to be installed or the boiler derated.
6.3.2.3 Thermal Coal Drying The maximum allowable coal product moisture content is specified in the contracts of coal producers. Excess moisture needs to be removed from wet coal to lower transport costs and to reduce the extra energy consumed in the power station boiler evaporate it. Although burning coal is a common heat source for coal drying, some overseas mines such as the Pniowek Mine in Poland, the Buchanan mine in Virginia, USA and the Severnaya Mine in Russia use CMM for the job (Miller 1998).
The use of CMM instead of coal has a number of advantages:
• Medium quality drainage gas is useable without extensive cleaning;
• A better heat distribution is possible which results in lower grate maintenance costs;
• Corrosion is reduced in wet areas due to reduction in H2S04 from coal firing;
• Reduction in NOx, SOx and C02 and fine particulates and;
• Allows the sale of the coal that would have been burnt.
Coal is generally dried in a two-part process. A mechanical centrifuge can dry the coal to approximately 12% moisture content. However, coal contracts usually specify values of 7- 8% and the remainder of the drying can be achieved via thermal drying. At the power station, hot flue gas is passed through the mill during grinding to further dry the coal from ~8% to ~3%. There are several drier types such as a rotary direct drier, a fluidised direct drier, a flash drier or an indirect fired drier.
The economics of drying with CMM are dictated by the value of the alternative uses for the gas, the cost of coal displaced and the amount of moisture removal required. Conversion of a drier from coal to gas firing is a relatively simple process. However, the most efficient use of gas is in a cogeneration system. Here the CMM is used to produce electricity and the waste heat is used to dry the coal.
105 6.4 Discussion
The most obvious and probably lowest risk choice for the exploitation of gas from the Wambo mine is for electricity production. This is due to the nature and quality of the gas, the readily saleable nature of electricity, and because of the proximity of the mine to existing electricity distribution infrastructure. Electricity could be produced by one of several techniques: combustion as a supplementary fuel in power station boilers; in a gas turbine, either internally or externally fired, or in a turbine co-fired with waste coal. Each of these techniques have specific economic or technical risks, though these should not be overstated. One factor likely to increase the usefulness of the gas from the Wambo mine is consistency of supply. If the gas supply can be held at a relatively constant level and with minimal variation in composition and quality, the gas could find ready application in the technologies described.
It would be possible to upgrade the gas from the Wambo mine to pipeline quality, although at a cost. Whilst this option may be feasible if a gas pipeline were nearby, the Hunter Valley is not currently connected to a significant gas pipeline reticulation system, making the option less attractive than direct electricity production.
Other attractive options for using the gas are waste water treatment and thermal coal drying. The former is an attractive prospect, as all mine sites in the Hunter Valley are facing significant pressure on their use of water, and water quality is becoming a major issue for stakeholders throughout the Valley. Demand for and cost of this limited resource will only continue to grow in the future. Therefore a system that treats mine water, either from underground or from coal preparation facilities would be of great benefit to the mine, reducing the dependency on imported water. If the water treatment is of sufficient quality, and excess exists, the possibility of fresh water sales to local consumers exists, as does the capacity to return clean water to local river systems.
The use of gas as an energy source in a cogeneration system, for electricity production and thermal drying of coal could conceivably have two major benefits: by helping the mine meet contract requirements for coal moisture content, or by using heat to dry out wet tailings, which can then be disposed of more easily or even be marketed as a product in themselves.
Other options, such as the use of the gas in industrial boilers or blast furnaces, whilst feasible, are more dependent on external factors and relevant industries locating to nearby areas.
106 6.5 References
Agnew D, (compiler) 1999. Strategic Study of the Northern NSW Coalfields, Produced by Minerals Consultative Committee, published by NSW Department of Mineral Resources. 80p.
Armstrong M, (compiler) 2000. 2000 New South Wales Coal Industry Profile, Published by NSW Department of Mineral Resources, 279p.
Azinger KL, 1998. Review of the Appin Tower Coal Bed Methane Power project After 2.5 Years of Operations.
Brown K, Casey DA, Enever DR, Facer RA, and Wright K, 1996. New South Wales coal seam methane potential, Published by NSW Department of Mineral Resources, viii + 96p
DUAP 1997. Upper Hunter Cumulative Impact Study and Action Strategy, Published by NSW Department of Urban Affairs and Planning. 153p.
Miller I, 1998. Use of Coal Mine Methane in Coal Dryers United States EPA Coalbed Methane Outreach Program Options Series.
Patrick KL, 1999. New Evaporator Technology Cuts Mill Effluent Flows, Process Water Makeup Pulp and Paper online, news.PulDandDaperonline.com/case-studies/1998Q/28- Z36.html
Pratapas JM, Chan I, 1998. Cofiring Coal Mine Methane in Coal-Fired Utility and Industrial Boilers USEPA Coalbed Methane Outreach Program Options Series.
Smith D, Obenchain WA, 1997. Natural Gas Injection in Blast Furnaces, Gas Research Institute.
Tait, JH, 1998. Coal Mine Methane use in Brine Water Treatment US EPA Coalbed Methane Outreach Program Options Series.
Wendt MN, Mallett C, Lapszewicz J, Xue S, Foulds G, Mark R, Sharma S, Dannell R, Worrall R, and Balusu R, 2000. Methane Capture and Utilisation Final Report, Unpublished Report, ACARP Project C8058, CSIRO Exploration & Mining Report 723R. 107p.
107 7 GOVERNMENT POLICY AND REGULATION
7.1 Introduction
Current Federal Government policy and regulation have no direct control on the exploration and extraction of coal seam methane (CSM) resources. However, Federal policy and regulations are likely to have a great impact on future CSM operations because of their potential importance to greenhouse initiatives and carbon trading schemes. State policy and regulation will also have a significant impact on access to CSM resources in NSW because of the lack of natural gas resources in NSW.
7.2 Domestic Climate Change Policy
Under the 1997 Kyoto Protocol Australia agreed to assigned amounts of "aggregate anthropogenic carbon dioxide equivalent emissions of greenhouse gases" over the period 2008 to 2012 (Article 3 of the Protocol). Australia ’s commitment is 108% of the 1990 level by 2008 to 2012. If ratified, the Kyoto Agreement is likely to initiate marked and sustained changes in global energy demand, fuel mix and electricity generating efficiency, with both direct and indirect consequences for the fossil fuel industry in Australia (Environment, Economics & Ethics, 2000).
Australia has taken a number of steps in response to the targets proposed in the Kyoto Protocol. To date the approach has been characterised by a voluntary framework with a focus on ‘no regrets ’ measures. More recent policy pronouncements would extend industry ’s commitment further, well beyond the voluntary, ‘no regrets ’ framework of previous initiatives (Environment, Economics & Ethics, 2000). •
Key measures, which may impact upon the operations of Australian fossil fuel producers include the introduction of a domestic emissions trading scheme and the introduction of a mandatory renewable energy target (Environment, Economics & Ethics, 2000). These measures will be discussed further in this chapter.
Australia ’s domestic climate change policy is characterised by three distinct strategies: regulatory measures, accompanied by voluntary initiatives and augmented by the use of market based mechanisms and incentives (Environment, Economics & Ethics, 2000).
Regulatory measures focus upon the electricity and transport industries, with components such as:
• Mandatory renewable energy targets;
• Efficiency standards for power generation;
108 • Automotive industry environment strategy;
• National carbon accounting system and;
• Energy performance codes.
Market based mechanisms comprise a range of incentives intended to increase the uptake of renewable technologies, as well as the possible introduction of a domestic emissions trading scheme, such as:
• Domestic emissions trading scheme;
• Renewable energy showcase grant;
• Renewable energy equity fund;
• Renewable energy commercialisation program and;
• Alternative fuels conversion program.
Voluntary initiatives are centred upon the existing Greenhouse Challenge Program and a number of strategies from the National Greenhouse Strategy, such as:
• National greenhouse strategy;
• Greenhouse challenge;
• Bush for greenhouse;
• Industry energy efficiency benchmarking and;
• Photovoltaics on residential buildings.
7.2.1 The Australian Greenhouse Office
The Australian Greenhouse Office (AGO) was established in 1998 as a separate agency within the environment portfolio to provide a whole of government approach to greenhouse matters, and to deliver the Commonwealth Government ’s $180 million climate change package, Safeguarding the Future: Australia ’s Response to Climate Change. Importantly, the AGO is the agency responsible for the coordination of domestic climate change policy and the delivery of Commonwealth programs such as the Greenhouse Gas Abatement Program, Greenhouse Challenge, National Carbon Accounting System and the National Greenhouse Strategy, and provides a central point of contact for stakeholder groups (http://www.greenhouse.gov.au/ago/ ).
The Greenhouse Gas Abatement Program (GGAP) is a major Commonwealth Government initiative to assist Australia in meeting its commitments under the Kyoto Protocol. The objective of GGAP is to reduce Australia ’s net greenhouse gas emissions by supporting activities that are likely to result in substantial emission reductions or substantial sink enhancement, particularly in the first commitment period under the Kyoto Protocol (2008- 109 2012). $400 million has been allocated to the Program between 2000-01 to 2003-04 (http://www.greenhouse.gov.au/ggap/ ).
GGAP is targeting opportunities for large-scale, cost-effective and sustained abatement across the economy. Literature states that GGAP will only support projects that will result in quantifiable and additional abatement, not expected to occur in the absence of GGAP funding. Priority will be given to projects that will deliver abatement exceeding 250,000 tonnes of carbon dioxide equivalents (C02-e) per annum. Projects that do not meet this threshold but meet other criteria to a high degree may be selected (http://www.greenhouse.gov.au/ggap/ ).
Projects funded under GGAP are expected to provide complementary benefits, for example opportunities for rural and regional Australia, ecologically sustainable development, employment growth, the use of new technologies and innovative processes, and non government investment. In this sense, the utilisation of low carbon-intensity gas (relative to coal) in the Hunter Valley may have some potential for Government programs of this nature.
7.3 Emissions Trading and Carbon Credits
A key feature of the Kyoto Protocol is the agreement on the use of various ’Kyoto mechanisms ’ to achieve country commitments. One of the Kyoto mechanisms is emissions trading (Article 17). However, the details of a potential trading system have yet to be negotiated at an international level.
(http://www.greenhouse.gov.au/pubs/factsheets/fs_emissions.html ).
The Australian Government has sought policy advice from the AGO on the feasibility of the introduction of a national emissions trading system in meeting Australia ’s commitments under the Kyoto Protocol.
The Government has not made a decision on the establishment of a national emissions trading system and the potential incorporation of carbon credits in such a system. The AGO strategy for developing advice to Government involves consultation with key experts, State and Territory Governments, peak industry organisations and the general public.
Nevertheless, the introduction of a domestic emission trading regime has the potential to be the most significant outcome of the Kyoto Protocol. The potential benefits of an emissions trading mechanism and its role in minimising the cost of complying with targets proposed at Kyoto are well established (Environment, Economics & Ethics, 2000).
An emissions trading system, if introduced, would be based on a permit that authorised the holder to emit a specified amount of greenhouse gas. In essence, tradeable permits are private and transferable property rights, the possession of which allows an individual to emit 110 up to a prescribed amount of greenhouse gas emissions. The number of permits issued to individual parties reflects the desired level of greenhouse gases that can be emitted over a given period. Because only a limited number of permits are issued they attract a value. To meet the necessary emission target, parties must either purchase existing permits or reduce their emissions. Those parties that are able to reduce their emissions at relatively low cost will do so through the implementation of emission abatement technologies and sell any excess emission permits. Conversely, those parties that have relatively high abatement costs will purchase tradeable permits in order to meet their respective emissions targets in preference to undertaking expensive abatement measures (Environment, Economics & Ethics, 2000).
In the context of Australia ’s greenhouse reduction strategy, a consensus is emerging at the Federal and State Government level that emissions trading will likely be a core element of the Governments response to its obligations under the Kyoto Protocol, and that a pilot scheme could be in place by 2005 (Environment, Economics & Ethics, 2000).
Depending upon the nature of any future emission trading regime, the cost of carbon abatement will likely fall in the range of $5/t-C02-e to $50/t-CO2-e, with a current best estimate guess of $10/t-CO2-e (Australian dollars). On the basis of the initial trading scenario being set at $10/t-CO2-e and that the adjustment task required by Australia to comply with the Kyoto Protocol targets in 2010 is 79 Mt-C02-e, the maximum potential value of any future domestic emissions trading market would be in the order of $790 million per annum (Environment, Economics & Ethics, 2000).
7.3.1 Speculation in Carbon Credits
Speculation is occurring in carbon credits. Traders are taking significant risks given that the Government has not established any formal mechanisms for recognising or crediting their trades internationally. At present, globally traded carbon has a low value, usually under $1.50 per tonne (Australian dollars).
7.4 New South Wales policy & regulation
7.4.1 Introduction
The NSW Government has been relatively pro-active in tackling the greenhouse issue. One of the lead agencies tasked with managing this is the Sustainable Energy Development Authority (SEDA), which in 1997 launched the Green Power Accreditation Program to accredit electricity retailers ’ green power products. These include power generated from sources such as biomass co-firing, landfill gas generation, agricultural wastes, forestry 111 wastes, hydro-electric, wind power and solar thermal sources. Unfortunately for coal seam methane operations and for coal mine waste gas utilisation, documentation from the authority states specifically “Coal Mine Waste Gas and Coal Seam Methane... cannot be considered under the definition of the Accreditation Program” (NGPAP 2000, p. 15), as it is still a fossil fuel.
7.4.2 Coal Seam Methane Extraction
The following general conditions are relevant to the exploration for, and extraction of oil and gas in NSW. They are set out in detail in the Petroleum (Onshore) Act 1991 No.84 and Regulations, and in the New South Wales Mining Act 1992 No.29 and Regulations. Much of the following is available from the NSW Department of Mineral Resources publication ‘New South Wales Coal Seam Methane Potential ’ (Agnew 1996)
7.4.2.1 Petroleum (Onshore) Act 1991 No. 84 and Petroleum (Onshore) Regulation No. 435 - New South Wales The following is a summary of legislative matters relevant to coal seam methane in NSW.
The Petroleum (Onshore) Act 1991 defines a range of parameters and procedures for the exploration and production of petroleum onshore in NSW:
• Mineral oil and gas is owned by the Government of New South Wales.
• Three titles are relevant to the exploration for, and evaluation of oil and gas, including coal seam methane:
o Exploration Licence
o Assessment Lease
o Special Prospecting Authority
• A Production Lease is the title which permits the extraction and sale of oil and gas.
• An Exploration Licence for petroleum can be granted over private or Government- owned land exempted from such grant.
• The size of an Exploration Licence ranges from:
o Maximum - 140 contiguous blocks to
o Minimum - 1 block (Note: 1 block is 5 minutes of latitude and 5 minutes of longitude on edges)
• The term of the Exploration Licence is initially for a period up to six years, with renewals of up to 75% of the area for successive periods.
112 • Special Prospecting Authorities can be of any size agreed by the Government and may be granted for a term not exceeding 12 months.
• Assessment leases of not greater than 4 blocks may be granted for a term not exceeding 6 years in order to carry out resource evaluation.
• Production Leases of not greater than four blocks may be granted for a term not exceeding 21 years.
• Before production operations can commence, development consent may be required under the Environmental Planning and Assessment Act 1979, along with approvals from local Government Bodies, as appropriate.
• Compensation agreements are required with the owners of surface and/or agricultural rights over land required for exploration and/or production.
• Royalties:
o A minimum State royalty of 10% of well head value is payable (less treatment costs where these apply). Provision is made for the government to set the royalty at a lower rate in special circumstances.
o Provisions exist for a royalty holiday to exist for five years from the start of production. The royalty then commences in year 6 at 6%, increasing by 1 percentage point each year to a maximum of 10% in year 10.
Where a coal lease is granted, any oil and gas within the boundaries of the lease may be owned by the proprietor of that coal lease provided that the proprietor has made a successful application for the inclusion of these commodities within the coal lease.
• As in all legislation, best industry practice is required during exploration and production operations.
• Best industry practice is also required to ensure protection of the environment.
7.4.3 Mining Act 1992 No. 29 and Mining (General) Regulation 1992 No. 445 - New South Wales
In certain cases, the holder of a Mining Lease can obtain the rights to coal seam gas under the provisions of the Mining Act.
• A Mining Lease is granted for periods of up to 21 years.
The holder of a Mining Lease for coal which includes the rights to petroleum may explore for petroleum products. In order to produce petroleum products the holder may be required to obtain a Production Lease for petroleum over the same area as the Mining Lease for coal.
113 • The holder of a Mining Lease for coal may apply for the inclusion of petroleum (oil and gas) in the Mining Lease and such inclusion may be granted subject to the provisions of the Act.
• If the Mining Lease for coal to which an application for the inclusion of petroleum relates is subject to an existing Petroleum Exploration Licence (“PEL"), then the application for such inclusion cannot be granted until the PEL expires, or is renewed in a form which removes the rights to petroleum from the mining lease for coal, or the holder of such Licence agrees to the inclusion of petroleum within the Mining Lease.
It is the policy of the Department of Mineral Resources that:
• Colliery holdings are excised from petroleum exploration licences.
• Coal gas drainage operations in colliery holdings be conducted under the provisions of the Mining Act.
7.5 Discussion
Federal policy and regulation are likely to have the greatest impact on future CSM operations as they relate to greenhouse initiatives and carbon trading schemes. State policy and regulation will have the greatest impact on access to CSM resources in NSW because of the lack of natural gas resources in NSW.
Direction on Australia ’s commitment to greenhouse gas reduction is still at this stage being formulated. In this sense, it is difficult to predict what the exact implications of greenhouse policy and regulation will be on CSM operations through Australia. In general, it is possible that the Federal Government will implement, some form of carbon trading scheme over the next 5 years, and has already set an ambitious goal for increasing the percentage of renewable energy sources contributing to Australia ’s overall energy consumption.
In this scenario, fossil fuel producers are at a disadvantage compared to renewable energy producers. However, this is not a blanket disadvantage. Indeed, the outlook for the utilisation of gas in Australia ’s energy picture is very positive. The less carbon intensive nature of gas, as compared to oil and especially coal, means many energy producers, Governments, and consumers see gas as a logical step in reducing greenhouse emissions, on the path to ever increasing use of renewable energy sources. In addition, in the short to medium term, renewable energy simply will not be available in sufficient quantity to meet demand for greenhouse friendly energy sources. Coal fired power stations are able to utilise gas directly or in conjunction with coal, lowering greenhouse emissions whilst still utilising the massive investment tied up in coal-fired infrastructure.
114 NSW is the only State in Australia to not produce at least some of it own gas. Strategically, NSW is very interested in securing at least some of its gas requirements from within the State, as it currently imports all its gas from interstate, mostly from South Australia (Agnew 1996). Research has indicated very large coal seam methane resources in the Wollongong, Newcastle and Hunter Valley coalfields. It seems increasingly likely that exploitation of these resources will occur, partially to reduce the reliance on coal based generation of electricity, partially to reduce greenhouse gas emissions, and partially to meet growing demand for pipeline gas.
7.6 References
Agnew D, (compiler) 1999. Strategic Study of the Northern NSW Coalfields, xx+80 pp. Produced by Minerals Consultative Committee, published by NSW Department of Mineral Resources.
Australian Greenhouse Office Website: http://www.areenhouse.gov.au http://www.qreenhouse.aov.au/pubs/factsheets/fs emissions.html http://www.qreenhouse.qov.au/aqo/safequardinq.html http://www.qreenhouse.qov.au/aqo/
Environment, Economics & Ethics, 2000. Climate Change Policy: Implications for the Australian Coal Industry. ACARP Report no. C8002
NGPAP, 2000. National Green Power Accreditation Program Accreditation Document. Sustainable Energy Development Authority, NSW.
NSW Sustainable Energy Development Authority: http://www.seda.nsw.qov.au/ http://www.greenpower.com.au/
115 8.2 Geotechnical Characterisation from Geophysical Logs
LogTrans is a computer program for automatically identifying the geophysical signatures of lithological and geotechnical units first developed by CSIRO through the CMTE in an AMIRA project (Fullagar et al, 1999). It assumes that different rock types can be discriminated by their different petrophysical signatures. Borehole geotechnical interpretation using LogTrans involves the following key steps:
1. Detailed analysis of key control cored holes where geotechnical logs, core photographs, rock strength testing results are available to set up the classification criterion for the various units.
2. Statistical characterisation of geophysical signatures of these strata units using LogTrans.
3. Automatic interpretation of geophysical logs from other open holes for the geotechnical units using the identified geophysical signatures.
Once the statistical training of LogTrans is completed, the analysis of logs using this fast and consistent method allows a large amount of the geophysical data to be quickly processed. More detailed discussion of the method can be found in Fullagar et al (1999).
8.2.1 Strata Classification
The first step in the log interpretation is to create a classification suitable for the geotechnical assessment in the study area. Sandstone, siltstone, mudstone and conglomerate are the main rock types to be identified in the Wambo mine area. Some of these rocks are interbedded (sandstone / siltstone and sandstone / conglomerate) or interlaminated (sandstone / mudstone). There are other materials such as siderite (SD), tuff (TF), and cement (CM) that only form a very small portion of the total stratigraphic column and so they are not included in the classification at this stage. The lithological description is essentially based on the logging of the drill core by the site geologist. The key strata are listed in Table
8 -1.
117 Table 8-1 Key strata atthe Wambo mine Symbol Descriptions CO Coal seam MS Mudstone
ST Siltstone SS Massive sandstone
SSG Massive sandstone with high gamma ray readings and low sonic velocities SSST Interbedded and interlaminated sandstone-siltstone
CGSS Interbedded conglomerate sandstone Interbedded conglomerate sandstone with high gamma ray readings/low sonic SSCG velocity values MSSS Interlaminated mudstone and sandstone
8.2.2 Geophysical characterisation of the strata
The second step of the LogTrans interpretation is to statistically characterize each rock type in Table 8-1 using geophysical logs, i.e. to compute the geophysical signatures for each rock type. From all the logs, it was found that the density (CODE), gamma ray (GRDE) and sonic (VL4F) logs were the most useful. These logs are commonly available at mine sites.
8.2.2.1 Quality check of the geophysical loos The success of LogTrans for geophysical log interpretation is largely dependent on the quality of the input data. Simple histogram analysis and statistical attribute analysis helps to diagnose potential problems in the data so that corrections can be made if necessary. Table 8-2 summarises the statistics of the density log from the five control boreholes used in this project. It is evident that the density logs are quite consistent for all boreholes. This is confirmed by the density histograms for all the control holes which are shown in Figure 8-2 and the stratigraphic distribution of the density for the three boreholes WA48, WA50 and WA51 shown in Figure 8-3.
118 issst
lu 2.0 -
Figure 8-9 Medians and range of density for the strata classes for the control data.
8.2.3.2 Gamma Rav (GRDE) The median values and the range of natural y for the strata classes are shown in Figure 8-10. In general, classes CO and TF have medians of 52.5 API and 209.5 API, respectively, sitting in two extremes and can be distinguished from the surrounding rocks by using the gamma log. CGSS and SS have similar gamma ray readings of 100 API. SSCG, SSG, MS and SSST have close value of 120 API while ST and MSSS have a value around 130 API. From this plot, it can be seen that the rocks can be divided into five classes at most based on gamma ray values.
125 MSSS
Figure 8-10 Medians and range of natural yfor the strata classes for the control data.
d.2.3.3 Velocity (VL4R The statistics of the sonic velocity logs and inferred UCS (derived from the velocity) are displayed in Figure 8-11 and Figure 8-12. Except for ST, SSCG and SSG, the classes are well separated from each other based on their median velocities. Therefore, the velocity log will generally be a good parameter to use for rock recognition. However, TF has a relatively large spread of values that span those of MS’ and some of ST. From these graphs, the strata can be grouped into four groups according to their rock strengths: coal seam, soft rock unit (TF and MS), intermediate strong rock unit (ST, SSG and SSCG) and strong rock unit (MSSS, SSST, SS and CGSS). These rock units are marked in different colors in Figure 8-11 and Figure 8-12.
126 8.3 Results of LogTrans Application to the control boreholes
8.3.1 Geological Interpretation
The geophysical characteristics obtained in the previous steps can be used to convert the geophysical logs into a geological log. The density (CODE), gamma ray (GRDE) and sonic velocity (VL4F, or DCS derived from VL4F) logs are used to interpret the geological strata listed in Table 8-1. The statistical data as shown in Figure 8-9 to Figure 8-11 was applied to the control holes to produce a graphical geological log. The results are presented in Figure 8-13 to Figure 8-17. The overall success rate 1 of the interpretation for all five holes was 80.4% and the success rates of individual borehole are listed in Table 8-5. During the interpretation, SSG, CGSS and SSCG were not interpreted due to its geophysical similarity to ST and SS. TF was not interpreted due to large spreads of values for each of the parameters.
Table 8-5 Success rates of LogTrans interpretation to the control boreholes. Borehole Overall Success Rate (%)
WA48 85.75
WA50 76.40
WA51 80.10
WA55 80.83 WA58 81.60
All Holes 80.40
In Figure 8-13 to Figure 8-17, the first column is the geological log from the mine. The second column is the geological rock classifications from the first column with reconciliation of the geophysical logs.
The third column is a graphical log of the geological rock types as interpreted by LogTrans using the geophysical logs in the rest of the columns. The following procedure was followed to get the results shown in the third column: 1) the LogTrans interpretation of the geological strata was obtained based on the density, gamma ray and the DCS (derived from the sonic log) logs with a 2 metre median filter applied to the logs; 2) the LogTrans interpretation of the
1 The success rate is calculated by the number of correctly interpreted classes divided by the total number of classes.
128 coal seams was obtained based on the density log without any filter applied; 3) the results from the two previous interpretations were combined results to get the third column of the final interpretation.
In general, the interpreted strata are well matched with the reconciled strata and with more details revealed. All the key coal measures are recognized and the strong sandstone units (SS/CGSS) have been correctly interpreted, but the thicknesses of some sandstone units have been overestimated compared with the original logging by a geologist. These estimations are geophysically viable although they may not be consistent with the original geological rock classifications.
However, it is worth noting that the first 50 m of WA50 and the first 100 m for WA55 and WA58 have been interpreted as being of lower strength than was logged by geologist. The main reason is that the sonic velocities in these zones are relatively low and gamma readings are relatively high. It is possible that these parts of the boreholes are more highly weathered and may have a higher clay content. To check the correctness of the geophysical logs, the sonic velocities from the borehole log were compared with the UCS measurements from the laboratory for the same depth samples for the borehole WA58. The results are shown in Figure 8-18. A strong correlation between the sonic velocity and the UCS is observed. The rock closer to the surface in the shallow part of the borehole was confirmed as being of lower strength than at depth. A similar correlation is observed for the borehole WA55. These results provide additional confidence in the LogTrans interpretations.
129 8.5 Conclusions
Existing borehole data is very important for establishing a geotechnical model for longwall coal mining, often though the data is not available in a form that is helpful in characterising a site. LogTrans, an automated geophysical interpretation software tool, has been successfully used to identify key geotechnical strata at the Wambo mine. The overall success rate of strata identification for all five control holes was 80.4%. It is now expected that a realistic 3D geotechnical model can be developed once all the boreholes in the study have been interpreted using LogTrans. Such a model would provide a basis for further study of the geotechnical issues in the area.
During the LogTrans interpretations, it is important to make sure that the input data (geological and geophysical logs) are consistent from one borehole to another and of good quality. Simple histogram analysis and statistical attribute calculations of each log from different boreholes can easily reveal potential problems and make corresponding corrections before interpretation as demonstrated in this exercise. Rock classes with large spreads of parameter values indicate either inaccurate class boundary allocation or unusually high geological variability. For these cases attempts should be made to reduce the ambiguity by checking both the geological information and geophysical logs. If the large spread in data is found to be true then the class(es) based on the parameter having the large spread should be excluded from the interpretation. An example of such a class is the TF (tuff) material that occurs in the Wambo mine lease.
8.6 References
Fullagar PK, Zhou B, and Fallon GN, 1999. Automated interpretation of geophysical borehole logs for orebody delineation and grade estimation. Mineral Resources Engineering, 8, 269- 284.
Guo H, Mallett C, Xue S, Kahraman H, Boland J, Sliwa R, Zhou B, Boole P, Poulsen BA, Harbers C, and Maconochie AP, 2000. Predevelopment Studies for Mine Methane Management and Utilisation, CSIRO Exploration and Mining Report 699C, 2000
137 9 3D GEOLOGICAL STRUCTURES AND MODEL
9.1 Introduction
This section presents new geological observations and updates the structural analysis of the Wambo and United mines that was compiled in March 2000 (Guo et al, 2000). It improves the framework for future assessment of structural risks during underground mining at the Wambo mine.
In addition to the data compiled for the initial report, which was based on the Wambo mine archives, this update includes (see Figure 9-1):
• New joint and cleat mapping at the Wollemi mine;
• Observations from mine visits to selected sites in the United and Homestead mines;
• Results of examination of drill core photographs for joint development and;
• Close examination of the sedimentological model to establish links between fault position and interburden distribution.
The aim of this section is to provide a complete and up-to-date description and characterisation of structures at the Wambo mine (including all data presented previously). The focus is on structures that may cause problems during mining rather than on the regional structural context and structural evolution, which were discussed in the earlier report.
138 • The Redmanvale thrust fault, an inferred regional-scale structure and;
• Small-scale structures including cleats and joints that are important for the description of geotechnical and gas reservoir characterisation of the coals and interburden. Both of these were mapped at the Wollemi mine as part of this study.
9.2.1 NE-trending thrust faults
A low angle thrust fault system, consisting of several overlapping fault segments, has been mapped in the Whybrow seam at the Homestead mine, and in the Wambo seam at the Wambo mine. The fault system trends ENE, and dips to the NNW at 20-30°. It has a throw of 7-8 m within the Wambo mine workings (which terminated against this structure). The throw of the thrust decreases to 2-3 m and then dies out within the Homestead mine against a dyke (Figure 9-2). This suggests that the dyke existed when the thrust fault propagated through the sequence. Minor movement on the dyke would have accommodated the strain at the tip of the thrust fault.
The strike length of at least 2.5 km and the penetration of the thrust fault through the Whybrow and Wambo seams, suggests that the fault system penetrates into the Arrowfield seam.
The fault system was visited in the Homestead main headings (Figure 9-1), where it is very well exposed. The fault plane contains a zone of crushed coal and a clay plug (suggesting ground water inflow). The fault is associated with minor warping of the rocks in the immediate footwall. Boreholes WA2 and WA3 intersected fracture zones that are interpreted as thrust fault segments (Figure 9-3a).
A further thrust fault was identified in the geophysical drill hole log of WA39, to the southeast of the mapped fault (Figure 9-3b). in this hole five seams of the Glen Munro seam are repeated with a total throw of 6m. This repetition occurred above the cored section of the hole and cannot be confirmed with photographs. As the fault is only found in one borehole, its trend is interpreted by correlation with the mapped fault to the north.
Fracturing suggesting faulting occurs in WA37 and WA41, however there is no evidence of repetition, a criterion required to interpret thrust faults. Strong oxidation is associated with the fractures in WA37, suggesting that the structures may be open to ground water circulation.
The size and indicated frequency of these structures make them the most important structural hazard in the exploration area east of the dyke. Results of a seismic survey recorded in late 2000 across these structures, was not available for this report, but should be integrated as soon as it becomes available.
140 9.2.2 NW-trending thrust faults - Redmanvale Fault
The Redmanvale Fault is a regional-scale thrust fault mapped on the Hunter Valley coalfield 1:100 000 scale map (DMR, 1993). It consists of several NW trending en-echelon fault segments of which the southernmost one is inferred to transect the southwestern Wambo lease (Figure 9-2).
Harrison (1999) described the Redmanvale Fault at the Wambo mine as a large reverse fault with a throw of -20 m down-to-the-south, suggesting a dip to the northeast. However, the structural floor to the Whybrow seam (Minex model supplied by mine) shows no noticeable steepening or bending near the Redmanvale fault, although there are closely spaced drill holes on either side of the inferred structure. If the fault exists it may intersect the sequence at lower levels, which are not controlled by exploration drilling.
There is some suggestion that minor structures in the Homestead and Wollemi workings are associated with the Redmanvale fault. Several joints mapped in the development drives of the Homestead mine (see Figure 9-8 below), and two small faults mapped in maingate (see Figure 9-6 below) are parallel to the inferred structure.
Therefore only weak indications exist within the exploration database of the Wambo and United mines for the presence of the Redmanvale fault in this area.
Two thrust faults with a similar orientation and dip as the Redmanvale fault were mapped from United exploration drilling in the northeast of the leases. However, recent drilling and reinterpretation of the data has thrown doubt on the existence of these structures (K Halverson, personal comment).
9.2.3 NE-trending normal faults
A several kilometres wide zone of steep normal faulting cuts through the Wambo and United leases in a northeasterly direction (Figure 9-2). The zone of faulting is documented by underground mapping in the United mine and several 2D-seismic lines with complementary close-spaced drilling to the northwest of the Homestead mine.
The fault zone is mapped in greatest detail in the United workings. Here individual faults are short (-200 m) with throws ranging from 5 cm to >2 m. They occur in high strain zones (200- 400 m wide) of parallel and overlapping faults, separated by zones (800-1500 m wide) of less dense faulting (Figure 9-2). The seismic survey shows a very similar pattern with zones of very disturbed seismic reflectors separated by wider zones of faulted, but clear reflectors (Figure 9-5). The seismic lines also show that the faults generally have normal throws down-
144 along the fault is 1-1.5 m with a down-to-the-east sense of displacement. No faults with this orientation have been mapped elsewhere in the lease areas.
The dyke was inspected in the southern Homestead mine workings. It is strongly altered and weathered to clay. Where the clay breaks from the walls of the dyke, it exposes a major joint with straight, smooth walls. Only some minor joints parallel to the dyke were observed in the host rock close to the walls.
Although the dykes in the Homestead mine were weak, some strong sections were intersected in the United underground workings, and posed a problem during mining. Strength of the dykes is generally related to alteration during intrusion. Hence stronger sections may be expected where the dyke is thicker, such as at the United mine where it posed a mining problem.
Dykes need to tap deep into the crust to bring molten material close to the surface. All the major dykes mapped at the Whybrow seam level are likely to be vertically persistent into the Arrowfield seam level and below.
9.2.5 Cleat
No information on cleats was found in the mine records during the earlier stage of this project. Therefore cleat was mapped during the December 2000 Wollemi mine visit. Representative cleats from the maingates of longwalls 10,11 and 12 were mapped (Figure 9-6). The results show that the cleat orientation is very consistent across this mine. Face cleat trends west-northwest to northwest (123°) and butt cleat trends northeast (200°). Spot checks of cleat orientations in the United and Homestead mines were consistent with the mapped cleats in the Wollemi mine. The cleat orientation is also parallel to the dominant joint orientation mapped at Wollemi.
Cleats and rare early joints are filled by calcite everywhere in the mine. An inspection of photographs of the 51 cored holes shows that all other coal seams including the Arrowfield seam are also filled with calcite (Figure 9-7). No calcite fill was observed associated with the thrust or normal faulting, both of which clearly overprint the cleats, suggesting that the cleats formed very early in the structural history of the coal measures. Further detailed characterisation of the cleat of the seams below the Whybrow seam should be gained from a more detailed examination of the drill core.
The ubiquitous calcite fill on cleats and joints has strong implications for the permeability and reservoir characteristics of the coal seams. These are discussed in detail in section 12.
146 9.4 Conclusions and recommendations
• A thrust fault with 7-8 m throw occurs to the east of the Homestead mine, and several further thrust faults are interpreted from drill hole data in the eastern part of the lease.
• All thrust faulting appears to be restricted to the area east of the dyke.
• There is no apparent relationship between the distribution of sandbodies in the interburden and the position of the thrust faults.
• The strike length, the known vertical penetration and the lack of suitable coal seam sliding plane, suggests that the thrust systems penetrate through the sequence into the Arrowfield seam.
• The NE-trending thrust faults form the main structural hazard in the exploration area east of the dyke. The results of the recent 2D seismic survey would be able to identify these structures.
• Only weak indications exist within the exploration database of the Wambo and United mines for the presence of the Redmanvale thrust fault in this area.
• The NNE-trending normal faults are restricted to the northwestern portion of the lease areas. These faults are tight normal faults and coincide with the position of a roll in the Whybrow seam.
• Vertical dyke-filled structures mapped at the Whybrow seam level are likely to be vertically persistent into the Arrowfield seam level and below.
• Although dykes intersected in the Homestead mine were soft, some hard sections were intersected in the United workings, suggesting that the dykes may pose a mining hazard at the Arrowfields seam level.
• Cleat mapping has identified consistent NNW-trending face cleat and NE-trending butt cleat.
• All cleats and some early joints in the workings and in the boreholes are invariably filled with calcite. Further detailed characterisation of the cleat of the seams below the Whybrow seam should be gained from a detailed examination of the drill core.
• The cleats are overprinted by the normal and thrust faults.
• The orientations of mapped joints reflect the proximity of larger structures. Detailed joint mapping should be continued in all new developments, to serve as an advanced warning tool for faults occurring ahead of the workings.
153 • Small normal faults that are parallel to the main joint set may be accommodation structures associated with rolls in the coal seam.
• Most folds and rolls in the Whybrow seam are controlled by large sandbodies in the floor, and not by tectonic folding.
9.5 References
Department of Mineral Resources, NSW, 1993. Hunter Valley coalfield Regional Geology, 1:100000. Geological Series Sheet 9033.
Harrison PF, 1999. Geology Report Wambo colliery (Underground - lower sequence). NSW Coal Compensation Board, No. RGB 199/29.
Guo H, Mallett C, Xue S, Kahraman H, Boland J, Sliwa R, Zhou B, Soole P, Poulsen BA, Harbers C, and Maconochie AP, 2000. Predevelopment Studies for Mine Methane Management and Utilisation, CSIRO Exploration and Mining Report 699C, 2000
154 10 3D SEDIMENTARY CHARACTERISATION AND MODEL
10.1 Introduction
This section presents an updated sedimentary model for the Wambo mine study area. Previous sedimentary modelling compiled in March 2000 (Guo et al) had only employed lithological descriptions from the WA series boreholes and was developed for the interburden between Arrowfield seam ply AFA and Woodlands Hill seam ply WHD3D. This section presents an updated sedimentary model that:
• Characterises the rock units occurring within the coal seam interburden above the mining interval of the Bowfield/Arrowfield seam that impact on goafing behaviour;
• Examines the relationship between coal seam splitting patterns, the distribution of sandstone units and structure that provide a geological framework in which to investigate the geomechanical behaviour of the goaf during coal extraction and;
• Determines the relationship between seam gas contents, gas composition and geological framework that will aid in risk assessment for mine planning.
The complex seam splitting patterns and rapid variations in interburden thickness between major coal seams hinder the identification of faults. Although the seam splitting patterns are well known for the Wambo mine, the character of the interburden for the entire section had not yet been extensively mapped nor modelled.
For this purpose, the sandstone units occurring between the Bowfield/Arrowfield and Whybrow seams were characterised using geophysical logs, borehole photographs and borehole records.
10.2 General Geology
The general geology and stratigraphy of the Late Permian Wittingham Coal Measures was described in a previous report by Guo et al, 2000. A summary of the stratigraphy and coal seam nomenclature used for the Wambo mine is presented in Table 10-1.
Two coal seams, Bowfield and Arrowfield were identified as suitable for longwall mining in the Wambo area. One of the main characteristics of the Wambo mine is the presence of a NS striking dyke running through the central portion of the lease. Seam structure is not highly variable in the Wambo lease. Structure contours of the floor of the working section (ply AFC) show that the working seams overlie a broad SW plunging and striking anticline 155 seams is significantly reduced in the east. Gas contents are significantly higher in this area. This leads to added issues of goaf gas control and drainage to ensure safe mining practices.
The Arrowfield and Woodlands Hill seams also show complex splitting patterns. As the thickness of the splits increases in the lease, the interburden becomes dominated by a series of stacked sandstone units each varying from 1 to 20 m thick. The close relationship between the distribution of thick sandstone bodies and the coalescence coal seams, especially within the Woodlands Hill plies may suggest that subsidence was compaction related, rather than structurally controlled. According to Sniffin and Beckett (1995), these sandstone bodies were deposited by south to southwestward flowing distributary channel systems in the Jerrys Plains Subgroup of the Wittingham Coal Measures (Figure 10-2).
The objective in creating this sedimentary model was to better define these sandstone units and their distribution within the Wambo lease in the overburden of the working Bowfield/Arrowfield seam to asses the impact on mining.
157 Table 10-1 A summary of the seam stratigraphy and seam split nomenclature. Subgroup Formations Seam Splits (coded by Wambo Mine)
Names
Coal Glen Gallic Subgroup
Doyles Creek Subgroup Horseshoe Creek Subgroup Apple Tree Flat Subgroup
Measures Watts Sandstone Wollombi Denman Formation Mt Leonard Whybrow WWA WWA2 WWB WWC1 WWC2 WWC3 WWD1 WWD2 Malabar Redbank RCA, RGB, RCD, RCE Ck Wambo WRA, WRB, WRC, Rider c Wambo WMA, WMB, £ iS Whynot WTA, WTB, WTC 3 Q_ <3 Blakefield BLA1, BLA2, BLA3 8 £: 10.3 Methodology 10.3.1 Data Manipulation The Wambo mine through Earth Data provided the borehole geophysical and lithological records as well as previously written reports on the mine site geology. Of the 1000 borehole records received, only recently drilled WA series boreholes and some of the UG series boreholes from United Colliery located within the study area contained information on the complete sequence from the Bowfield/Arrowfield to Whybrow/Monkey Place Creek Tuff stratigraphic interval. The list of the data files supplied by the Wambo mine and United Colliery is given in Table 10-2. The ASCII text files for the lithological descriptions observed by the logging geologist from each borehole were produced in ProLog Software. In addition, the geophysical LAS files 159 from these boreholes were loaded into Paradigm-GeologG package to display wireline log information as well as the ASCII lithology text files. Table 10-2 Data files provided by Wambo mine and United Colliery. Mine Prolog files Modelling and other (Borehole Lithology Files) mine related files (lease, surface, structure etc files) Wambo Walfnal, Wa2fnalr, Wa3, Wa4fnal, BATS.GM3 WaSfnal, WaBfnal, Wa7f, Wa8f, Wa9, FAULTS.GM3 Wa9r, Wa10, Wa10r, Wall, Wa12, JUN99.B31 Wa13, Wa14, Wa15, Wa16, Wa17, JUN99.B32 JUN99.B33 Wa18, Wa19, Wa20, Wa21 ,Wa22, JUN99.B34 JUN99.B35 Wa22r, Wa23, Wa24, Wa25, Wa26, NOV98STR.GRD Wa26, Wa26r, Wa27, Wa28, Wa30, WA.PICK Wa30, Wa31, Wa32, Wa33, Wa34, WA.SURV Wa35, Wa36, Wa37, Wa38, Wa39, Wa40, Wa40r, Wa41 , Wa42, Wa43, Wa44, Wa45, Rwa46, Wa47, Wa48, Wa49, Wa50, Wa51 , Wa52, Wa53, Wa55, Wa56, Wa57, Wa58, Wa61, Wa62, Wa63, Wa64, Wa65, Wa69r United UG01*. UG02. UG03. UG04. UG05. 1999ucbores.mdb UG06. UG07. UG08. UG09. UG10. UC BHLEAS.DXF UG11. UG12. UG13. UG14. UG15. UC STRUCT.DXF UG16. UG17. UG18. UG19. UG20. UCPL_WH.DXF UG21. UG22. UG23. UG24. UG25. Contours.dxf UG26 Contours. 1st 0RTH01.tif 0RTH02.tif * The boreholes underlined were used in modelling exercise 10.3.2 Lithological Interpretation from Geophysical Logs and Lateral Correlation of Sandstone Units Although colour, texture and sedimentary structures are visible in core, lithology interpretation from geophysical logs must, by necessity, be simplified to basic rock units. In this report, only the sandstone units were picked for the modelling purpose. Identifying a geophysical signature for the sandstone required the use of gamma ray, long spaced density, and sonic logs. For each borehole, the mean gamma ray reading was accepted as the limit value between sandstone and siltstone/finer units in general. However this was calibrated with other geophysical logs such as sonic and caliper as well as the original lithological description and borehole photographs. 160 A number of north-south and east-west oriented cross sections were created from the processed borehole records in Geolog6 software. These were constructed along a line where the maximum number of boreholes lies within the cross section corridor. The cross sections were used to manually pick the sandstone units from the computer screen across the study area. Once correlated, the major sandstone units within the coal seam interburdens flanking the working and overlying seams (Bowfield/Arrowfield to Whybrow) were coded within the framework of the existing mine coding system. Only the massive and laterally continuous sandstone units were correlated for input to the model as these, similar to coal seams, are easy to identify and hence update and modify in the model as subsequent drilling results become available. In places, some of these sandstone units may contain interbeds of finer grained units, or conglomerates. These fine and coarse grained beds were not laterally persistent enough for correlation and were therefore not differentiated from the sandstone. In doing this work, it was observed that some of the seam picks from Woodlands Hill seam were miscorrelated. Adjustments were made based on the continuous signatures from the neighbouring boreholes. The list of the re-correlated seams from these boreholes is given in Table 10-3. The seam nomenclature at United Collieries is different from the Wambo mines and some of the plies did not correspond to the Wambo seam naming convention. Seam picks from these boreholes were readjusted based on the Wambo mines nomenclature. A distinctive unit, the Monkey Place Creek Tuff, above Whybrow seam was correlated for the first time in the lease area. This is a tuff dominated, interbedded unit with sandstone and coal layers. Codes generally follow the seam sequence, e.g. AW means the sequence between Arrowfield and Woodlands Hill seams whilst GB means the sequence between Glen Munro and Blakefield seams. Intra seam sequences were coded by using the two letters of the seams e.g. AF means the sandstone units occurring within Arrowfield seam plies. The coding system of these units is given in Table 10-4. The seam interburdens within the Bowfield/Arrowfield to Whybrow are generally dominated by thick sandstone bodies. From the core descriptions and photographs, these sandstones vary from conglomeratic to crossbedded and ripple laminated sandstones with abundant carbonaceous laminae. Sandstone body geometry is complex and laterally variable. Based on the geophysics, only massive to bedded sandstone-dominated units and distinctive 161 breaks in the gamma signature (mudstones/carbonaceous mudstones/siltstones) (Figure 11-3) could be reliably correlated for any distance. Within each successive interburden interval, the capping coal was used as a datum for correlation of sandstones. By adjusting the logs to this datum the pre-compactional or depositional geometry of the units could be accounted for. Hence, correlations of units worked on a “best fit” scenario. Table 10-3 Re-picked seam picks in WA series boreholes. Borehole No. Seam Pick Comments WA14 WHA4 Shifted up to WHA3 level WA30 WRB Shifted up to near roof level WA39 WHD3A Shifted to WHD3B level WA41 WHD3A, WHD3B, WHD3C Shifted upwards WA42 WHD3B Shifted to WHD3C level WA44 WHD3A and WHD3C Added to the seam list WA45 WRA, WRB and WRC Added to the seam list WA47 WTB, WTA, WMA, WRA, WRB, Added to the seam list RCC, RCD, WWC WA52 WHC1, WHC2, WHC3 Merged as WHC1 WA55 WHD3A, WHD3C Added to the seam list WA56 WHD3A, WHD3C, WRC Added to the seam list WA57 WHD5, WHD4, WHD3 Deleted from the list WHD3C, WHD3B, WHD3C, Added to the seam list WHA4 Shifted upwards WHA3, WHB1 WA58 GMA1A, GMA2A Added into GMA WA61 Picks were changed WA62 WWA-WWD Re-picked RCA-RCD Added to the seam list WA63 Picks above Woodlands Hill seam were changed WA64 Picks above Woodlands Hill seam • were changed WA69R WRA, WRB, RCA-RCD, WWD Added to the seam list 162 10A Results 10.4.1 Rock Types The following rock types were observed in core photographs and hand specimens, and examples are presented in Figure 10-4: • Coal -typically black in core photographs and dull in nature -intact in general but could be fragmented in some sections -well developed cleats filled with calcite -low density response in the geophysical signatures • Mudstone/Si Itstone -near black to dark grey, massive to laminated with carbonaceous lamellae -dark to light grey with siltstone stringers -sideritic bands up to 50 cm -dark to light grey with siltstone and sandstone beds < 5 cm -high gamma signature in the geophysical logs • Interbedded siltstone/sandstone -dark to light grey, beds >5 cm -common parallel and cross lamination -coaly and carbonaceous fragments on the lamination surfaces -sideritic bands up to 50 cm -intermediate gamma response in the geophysical signatures • Sandstone -dark to light grey, fine to medium grained with abundant carbonaceous and mudstone laminae, flat to ripple laminated -light grey, medium to coarse grained with minor carbonaceous laminae, flat to ripple laminated -light grey, medium to coarse grained, cross bedded to massive -silica cement 167 -low to very low gamma response in the geophysical signatures • Conglomerate -light grey, fine to coarse pebble conglomerate, poor to moderate sorting -strong silica cement with occasional carbonate cement -light grey with coal spars and/or mud rip up clasts -low to intermediate gamma response in the geophysical signatures • Tuff -white to buff in colour with no visible structure -weathered in some photographs and hand specimens -high gamma response in the geophysical signatures Beneath the coal seams, mudstones, siltstones and some sandstones contained root bioturbation structures and coal stringers. Minor burrowing and water escape structures were also observed in the interbedded rock types and fine-grained sandstones, as were soft sediment deformation (slumping) and synsedimentary microfaulting. These environmental indicators are not identifiable in the geophysical signatures and, except where fine grained units are severely root penetrated and slickensided, do not have geotechnical significance. Hence, there was no attempt to map their distribution within the sedimentary model. 168 ripple cross lamination, are also thin where there are abundant carbonaceous and siltstone laminae. As the sandstones become coarser grained, bedding thickness also increases, with some beds attaining 4 m thickness without a fine grained parting plane. Where sandstone units are thick (10 to 15 m), they can contain conglomerates up to 5 m thick. The most commonly observed feature in the cores is the “fining upward ” sequence. In the study area, the firing upward sequence is characterised at the base by a sharp to erosively based crossbedded sandstone (with or without conglomerate) that becomes finer grained upwards and composed at the top mainly by siltstones and mudstones. In general, the thickest (5 to 20 m) sandstone units contain one or more of these sequences. Thinner sandstone units may also display this trend or the inverse, a “coarsening upward ” sequence, or no visible trend. The internal architecture and lateral variability of the sandstones within the study interval were too complex and drilling data to sparse to attempt division into individual facies. The fining and coarsening upwards sequences are consistent with the previous interpretations of the formation of the sandstone bodies in the Jerrys Plains Subgroup, of which the study interval is a part, as distributary channel systems with accessory lobate mouth bars or splays. 10.4.2 Distribution of Coal Seams and Sandstones The results of the coal seam and sandstone thickness distribution are presented at the end of this chapter as a series of thickness maps (Figure A10-22 to Figure A10-35) and cross sections (Figures B1-B9) generated from the sedimentary model. Sandstone unit geometries are defined by their thickness and aerial extent and commonly occur either as linear belts (channelised flow) or as bifurcating lobes or sheets (unconfined flow). Confidence in the isopach patterns is limited in areas where drilling has been sparse. The section from the Bowfield/Arrowfield (BF/AF) to Whybrow (WW) seams consists of a series of interburden packages that become progressively thinner up section and offset in their thickness distributions. An EW trending “depositional hingeline ” occurs between N1391000 and N1393000 gridding lines whilst broad secondary NE-SW and NS hingelines develop across and eastern margin of the lease. Significant seam splitting occurs along these hingelines (see Figure 10-5 to Figure 10-9). Sandstone units stack either side of these hingelines. Each interburden consists of a series of vertically stacked sandstone bodies each separated by siltstones/mudstones. Laterally, these sandstones pinch out or interfinger with interbedded to laminated rock units. Vertically, the stacked sandstone units in each interburden package become thinner up sequence. Each interval is capped by a coal seam that forms the base of the next interval. A simplified schematic of these relationships is 170 presented in Figure 10-6 and detailed model sections are presented in Figure B10-36 to Figure B10-44. The most widespread sandstone development occurred at the following stratigraphic intervals: 1. Between the Bowfield and Arrowfield seams (BA series) 2. Within the Arrowfield seam plies (AF series) 3. Between the Arrowfield and Woodlands Hill seams (AW series) 4. Within the Woodlands Hill seam plies (WH, WX, WY, WZ series) 5. Between the Woodlands Hill and Glen Munro seams (WG series) 6. Between the Blakefield and Whynot seams (WB series) 7. Within the Whynot seam plies (WT series) 8. Between the Whynot and Wambo seams (WN series) 9. Within the Wambo seam plies (WM, WA series) 10. Between the Wambo Rider and Redbank Creek seams (WR series) 11. Within the Whybrow seam plies (WW series) 12. And finally, between the Whybrow seam and Monkey Place Creek tuff (SS series) Sandstone units within these intervals vary from 500 to 4000 m wide, with thickness ranging from 1 to 20 m. A brief description of the coal seam thickness distribution and coded sandstones up to Glen Munro seam within the lease area is given in the following sections. This is the expected goafing zone for Arrowfield seam. 171 10.4.2.1 Thickness Distribution of Bowfield Seam (BFA-WKG1 The Bowfield seam is subdivided into nine plies (BFA to WKG). The thickest seam development lies in the east of NS oriented dyke, and in plan view it is oriented to NE-SW (Figure A10-22). 10.4.2.2 Thickness Distribution of BA Sandstone Series (BA 10-30) The thickest part of the BA series sandstone develops in the NW corner of the lease and reaches up to 50 metres. Individual units can range between 0.5-12 m (Table 10-4). The orientation of these sandstones is NE to SW in the northern half of the lease and it turns to SE in the southern half (Figure A10-23). 10.4.2.3 Thickness Distribution of AF Sandstone Series (AF 10-50) This series of sandstones develop with a channel belt shape within the Arrowfield seam between ply AFD and ply AFC in the south-eastern corner of the lease (Figure A10-24).The composite thickness can reach up to 45 m whilst the individual units range from 0.5m to 27 m (Table 10-4). 10.4.2.4 Thickness Distribution of Arrowfield Seam: Plies AFC to AFA A few centimetre thick siltstone or tuff layers can part these three plies in places. The thickest seam development (>3.5 m) occurs in the eastern part of the lease with a NS alignment (Figure A10-25). 10.4.2.5 Thickness Distribution of AW Sandstone Series (AW 10-60) AW series of sandstones occurs between the Arrowfield and Woodlands Hill seams and thickest portion develops in a broad “T” splay form, throat of which is NE-SW oriented whilst its “t-bar ” is NW-SE aligned (Figure A10-26). The AW sandstone series will have the most significant impact on goafing behaviour in the Wambo mine as it may behave as a relatively strong, massive unit during caving. The individual unit thickness ranges from 0.1 to 20 m (Table 10-4) whereas the composite interburden thickness attains >50 metres. 10.4.2.6 Thickness Distribution of WH Sandstone Series (WH 50-80) This series of sandstones occurs within the Woodlands Hill seam between the plies WHD3A and WHD1 (Figure A10-27) The interburden thickness is greatest, 42 m in the north-east 179 corner and its geometry in plan view is similar to AW series sandstone with a narrower shape. 10.4.2.7 Thickness Distribution of WX Sandstone Series (WX 10-30) This sandstone series developed only to the north of the lease. The general geometry is narrow channel in plan view with EW orientation (Figure A10-28). Individual unit thickness ranges from 0.9 to 12 m (Table 10-4), and interburden thickness attains more than 30 metres. 10.4.2.8 Thickness Distribution of WY Sandstone Series (WY 10-40) This series develops in the south eastern corner of the lease with a narrow EW trending belt (Figure A10-29). The individual unit thickness ranges from 0.4 to 20 m (Table 10-4), and interburden thickness attains more than 30 metres. 10.4.2.9 Thickness Distribution of Woodland Hill Seam: Plies WHB5-WHB3 These plies form an extensive blanket seam across the Wambo lease (Figure A10-30) with some parting layers between them. Composite ply thickness varies from 0.2 m to 3 m (Table 10-4) and the thickest development occurs in a broad NE-SW oriented strip in the centre of the lease. 10.4.2.10 Thickness Distribution of WZ Sandstone Series (WZ 10-30) This series develops in the western part of the lease in splay shape in plan view (Figure A10-31). The individual unit thickness ranges from 0.3 to 13.4 m (Table 10-4), and interburden thickness attains more than 30 metres. 10.4.2.11 Thickness Distribution of WZ Sandstone Series (WZ 40-51) The Woodland Hill seam ply WHB1 seam, caps the thick sandstone package described above and the WHB1 ply forms an extensive blanket across the lease. The WZ40-51 series of sandstones overlies this ply and develops in a splay shape similar to AW series sandstones that formed earlier (Figure A10-32). The individual unit thickness ranges from 0.3 to 23.8 m (Table 4), and interburden thickness attains more than 35 metres. 10.4.2.12 Thickness Distribution of WZ Sandstone Series (WZ 55-701 This sandstone series developed only to the north of the lease. The general geometry is splay in plan view with NW-SE orientation (Figure A10-33). Individual unit thickness ranges from 0.6 to 16.3 m (Table 10-4), and interburden thickness attains more than 35 metres. 10.4.2.13 Thickness Distribution of WG Sandstone Series (WG 10-30) An EW trending channel belt within this sandstone series develops between N1391000 and N1393000 gridding lines in the Wambo lease (Figure A10-34) Individual unit thickness ranges from 0.6 to 16.3 m (Table 10-4), and interburden thickness reaches up to 30 metres. 180 10.5 Relationship of Sandstones to Structure General structural trends in the Wambo lease are best exhibited by the elevation contour on the floor (AFC) of the proposed working section. It appears that the interval between Arrowfield and Woodlands Hill seams was relatively unaffected by any major structural disruption. The main feature within the mine layout is a broad local anticline that strikes and plunges SW. Its broad crest occurs across the lease and it steepens on its western flank whilst on its eastern flank it shallows into a syncline and a subsequent anticline towards the southeast (Figure 10-1). Mapped faults generally strike NE and NW and NE-W thrusting has also been observed. Dykes in the Wambo lease strike NS. The general seam splitting patterns follow three broad zones: EW, NE and NS trends (Figure 10-10). The EW trend is generally observed between Arrowfield and Glen Munro seams, whilst the NE trend that is parallel to NE thrusting dominates the interburdens between Glen Munro and Whybrow seams. The NS trend is generally observed in the eastern margin of the lease. The thick sandstone packages stack either side of these split lines. The lateral offset of sandstone packages and coalesced thick coal seams suggest that variability in the structure contours is more a function of differential compaction than tectonic deformation. In distributary channel systems the muds and peats formed in adjacent floodplains or in-filled water bodies compact more than sand-filled channels and create topographic lows in the process that attract overbank sedimentation and subsequent relocation of the channel during deposition. During subsequent burial and coalification, the finer grained sediments beneath the sandstone channels compact more readily and coal seams structure is often preferentially steeper near the margins of channel sandstone bodies. Coals beneath channel margins under these circumstances are often slickensided and faulted (Weisenfluh and Ferm, 1991). 10.6 Zoning of Roof Conditions and Implications for Mining The thickness and proximity of competent massive sandstones and weak coal seams in the roof will impact on the goafing behaviour during mining. In order to evaluate areas with potentially different behaviour, the stratigraphic sequence between the Arrowfield and Glen Munro seams was characterized and classified into one of the seven qualitative groups. Classification relied on the position of the Woodlands Hill Ply WHD3, relative to the roof of the Arrowfield seam, and on the thickness and proportion of sandstone units within seam interburdens. Examples of these zones are shown in Figure 10-12. Although each zone can probably be subdivided into tighter categories, this 181 qualitative method gives a quick indication of roof type and potential goafing behaviour. Due to the qualitative nature of the method, each borehole was re-assigned to the zones to see if there was any discrepancy between two assignments and the repeatability was found to be acceptable (method was developed after Esterle and Fielding, 1996). Finally, a "roof zone" map was constructed after assigning each drill hole record to a zone (Figure 10-13). 182 Zone 2 has an immediate roof of siltstone (1 to 5 m thick) and the first 40 m is characterized by interbedded siltstone/sandstone units in which the siltstone is the dominant lithology. Further up, thick sandstone units develop. The strength of this zone is again expected to be weak. It develops in the east of the Homestead mine and in north-west corner of the lease and is closely associated with faulting in these areas. Zone 3 has an immediate roof of siltstone (1 to 4 m thick) and is similar to the first zone. It is characterized by interbedded siltstone/sandstone units in the first 40 m in which the siltstone is the dominant lithology and sandstone layers are less than 3m thick. Further up in the sequence, coarser sandstone units characterize the zone. This zone is especially associated with the split lines of the Arrowfield (AFA) and Woodlands Hill seams (WHD3A). This zone is also expected to be weak and is associated with faulting in the eastern margin and southwestern corner of the lease. Zone 4 is dominated by interbedded siltstone/ sandstone units in the first 40 m in which sandstone is the dominant lithology and sandstone beds range from 3 m to 7 metres. This zone is transitional to the sandstone-dominated zones 5 and 6. Further up in the sequence, the coarser and thicker sandstone units characterize the zone. This zone is expected to be weak to moderate in strength. It is observed in the east of Homestead mine and in the northwestern corner of the lease and associated with the faulting. Zone 5 has an immediate roof of siltstone (1-3 m) and the first 40 m is characterized by sandstone units with beds ranging from 5 m to 10 metres. Further up in the sequence, the coarser and thicker sandstone units characterize the zone. This zone is expected to be moderate to relatively strong in strength. IVis observed in the northwestern corner of the lease and in some parts of the Homestead mine. Zone 6 has an immediate roof of siltstone (1-3 m) and the first 40 m is characterized by sandstone units greater than 10 m in thickness, but they are still intercalated with finer grained units. It is observed in much of the Homestead mine with a NE-SW alignment. This zone expected to be strong in strength. Zone 7 has an immediate roof of siltstone generally less than 1 m thick. The first 40 m is characterized by sandstone units ranging from 10 to 20 m in thickness and sometimes contains conglomerates within these beds. This zone is expected to be strong. It is observed in the central portion of the Homestead mine. 185 The transition from Zone 1 to Zone 7 is characterized by the development of the thick sandstone units above the Arrowfield seam. The increasing proportion and thickness of sandstone units above the merged seam suggests that these zones will be stronger. Peng and Chiang (1983) used a similar approach in the United States to identify typical goafing zones from strong to weak (i.e. sandstone to mudstone dominated) and their respective behaviours. According to their observations, weak immediate roof caves immediately behind the support advance and fills up the whole goaf space. Where stiff or strong, thick sandstone units occur within a distance less than four times the mining height, the weak unit fails but cannot fill up the goaf space. Under such conditions, the thick sandstone unit in the upper roof, lacking any support, will move vigorously and periodically. Where hard and strong sandstone units occur in the immediate and upper roof, it breaks only when it overhangs for a certain distance in the goaf. This type of stratigraphic sequence induces a strong periodic weighting. The caving of strong, massive sandstones can also be accompanied by wind blasts. The mass strength of the thick sandstone units is reduced if they are well jointed, or if they are well bedded or interbedded with finer grained rock types. In these well-bedded sandstones, the roof can sag gradually until it touches the floor in the goaf and form a semi-arch. In addition to this qualitative roof condition zone method, a quantitative approach was also employed to determine the immediate roof and floor conditions. For this purpose, thicknesses of the finer grained units i.e. siltstone and mudstone in the immediate floor and roof units for the Arrowfield seam were calculated in the model and amalgamated into a distribution map. This exercise showed that weak floor and roof conditions exist east of the Homestead mine and are associated with tha thrust faults (Figure 10-14 and Figure 10-15). 186 10.7 Relationship between Seam Gas, Structure And Sandstones Total virgin gas content in Bowfield and Arrowfield seams is directly related to the seam structure and thickness and to the seam splitting patterns of the immediate roof. The total gas content in both seams increases with increasing depth to the southwest with a NW-SE trend (Figure 10-16 and Figure 10-17). However, towards the deepest part of the lease the gas content decreases. The low concentration of gas content shown in this area comes from measurements made in only one borehole (WA6) and it is possible that the patterns shown may be erroneous. The highest concentration of total gas content (more than 12m3/t) in the Bowfield seam occurs mainly to the east of the NS running dyke and coincides with the Bowfield seam divergence and the thickest seam development in this area. This is the area where a synclinal structure develops from a broad local anticline that strikes and plunges SW (Figure 10-16 and Figure 10-21). The gas content is the lowest in the eastern flank of this anticline. 189 10.8 Sedimentary Environment The interval from the Bowfield seam to the Whybrow seam in the Wambo lease belongs to the Jerrys Plains Subgroup. The subgroup is interpreted as a series of alluvial fan and high energy fluvial deposits adjacent to the Hunter Thrust which dissipate into a series of distributary channel systems to the south. Sniffin and Beckett, (1995) interpreted these sedimentary facies as river dominated delta sequences beginning and ending with a marine incursion. The source areas are thought to be outside the Hunter Valley coalfield north of Scone and the sources are thought to have controlled the major delta building episodes. The relative position of individual deltas directly controlled the development of different coal seams in the sequence. A major delta lobe is thought to have prograded towards the south and southwest in a similar depositional pattern to that of underlying Foybrook Formation. The depositional axis of this lobe was parallel to the Hunter Thrust for some distance and bent to the west of the Muswellbrook Anticline (Figure 10-2). Another major delta lobe can be recognized in the western limb of the Lobi Anticline and with a source area probably to the northwest of Camberwell. More rapid subsidence on the northern or Muswellbrook lobe resulted in a marine incursion between the deposition of the Wyhynot and Wambo seams (Figure 10-2). The river dominated lower delta plain coals are thin and are commonly split by crevasse splay and channel deposits whilst coals formed in upper delta plain environments tend to be thicker and laterally more continuous than those in the lower delta plain. Channel sandstones and conglomerates in the latter environment grade upwards through point bar sediments to levee deposits and then to floodplain and coal mires. The en echelon arrangement of the thick stacked sandstone channels in the Wambo area are represented by the NE-SW and EW oriented channel belts that occur between Bowfield and Woodlands Hill (WHD3) seams. The sandstone units developed between the Woodlands Hill seam ply WHD3 and Whybrow suggest periodic shifts in the depositional centres. This interval is thought to be dominated by interdistributary bay and crevasse splays. In the Wambo area this is seen as a shift from linear to more lobate sandstone bodies. The depositional pattern observed locally within the Wambo lease follows the regional pattern of southward flowing distributary channel systems in the Jerrys Plains Subgroup. 196 10.9 Conclusions At the Wambo mine, six distinctive lithofacies were identified from the borehole photographs. These are coal, tuff, mudstone/siltstone, interbedded siltstone/sandstone, sandstone and conglomerate. Only sandstone units were correlated, coded and input to the coal seam model as these would have the greatest impact on the caving behaviour of the overburden. The thickness and areal extent define the geometry of a sandstone unit. In plan view sandstones are distributed as linear belts oriented NE-SW and EW, or as broader lobes. The section from the Bowfield to Whybrow seams consists of a series of interburden packages that become progressively thinner up section and offset in their thickness distributions. Where thick, interburdens contain a series of stacked sandstone channels and/or lobes. The position of thick stacked sandstone packages shifts around the hingelines that developed in zones with orientations trending NE-SW, EW and NS. Significant seam splitting occurs along these hingelines. The spatial relationship between the local anticline, the position of split lines and thick sandstones suggest that much of the structure in the Bowfield and Arrowfield seams originated from differential compaction rather than tectonic deformation. The most widespread sandstone development occurred at the following stratigraphic intervals: between Bowfield and Arrowfield seams, within the Arrowfield seam plies, between Arrowfield and Woodlands Hill seams, within the Woodlands Hill seam plies, between the Woodlands Hill and Glen Munro seams (WG series), between the Blakefield and Whynot seams (WB series), within the .Whynot seam plies (WT series), between the Whynot and Wambo seams (WN series), within the Wambo seam plies (WM, WA series), between the Wambo Rider and Redbank Creek seams (WR series), within the Whybrow seam plies (WW series), and finally, between the Whybrow seam and Monkey Place Creek tuff (SS series). The presence of these thick sandstones in the mine lease impacts on the goafing behaviour during mining. Seven qualitative goafing zones were identified between Arrowfield and Woodlands Hill seams which were based on the proportion and thickness of sandstone units and the position of the coal seam above the Arrowfield seam. These zones can be correlated to overlying roof strength, with increasing strength from Zone 1 to Zone 7. Strong roof zones occur in an area covering much of Homestead mine and may cause some weighting problems. Weak roof zones associated with faulting and split zones occur in the eastern portion and northwestern corner of the lease. 197 In addition to qualitative goafing zoning methods, a quantitative approach was also employed to determine the immediate roof and floor conditions. For this purpose, thicknesses of the finer grained units i.e. siltstone and mudstone in the immediate floor and roof units for the Arrowfield seam were calculated in the model and amalgamated into a distribution map. This showed that weak floor and roof conditions exist east of the Homestead mine and are associated with the thrust faults. Also, a split-line-zoning map overlaid on top of the goafing zones showed that the relatively weak zones were associated with the maximum numbers of intersections of the split zones. These areas were also coupled with the fault zones. The relationship between seam gas, structure and sandstones was also investigated in this study. It was found that the total virgin gas, CH4 and C02 contents in Bowfield and Arrowfield seams follow the seam structure, depth and thickness. The total gas content in both seams increases with increasing depth to the southwest with a NW-SE trend. The highest concentration of total gas content (more than 12m3/t) in the Bowfield seam to the east of NS running dyke coincides with the Bowfield seam divergence and the thickest seam development in this area. This is the area where a syncline shaped structure develops from a broad local anticline that strikes and plunges SW. The gas content is the lowest in the eastern flank of this anticline. The highest gas concentration in the Arrowfield seam is observed both to the east and west of NS striking dyke and in the axis of a broad anticline in the western margin. It also appears that the highest concentration of total gas content in the lease is observed where thick sandstone units develop over the thickest parts of the seam. Where total gas content is high, the CH4 content also increases to more than 80% both in Bowfield and Arrowfield seams. High concentration areas in the Bowfield seam are observed immediately to east of the NS striking dyke whilst the highest concentration in the Arrowfield seam is observed in an area running NE-SW direction in the Homestead mine. These areas for both seams correspond more or less to the synclinal structure. Conversely, the C02 content for Bowfield seam increases with depth on the flanks and the nose of anticline plunging to the SW and highest concentration is observed to the north-west of BFA split line and NS running dyke. The same trend is also observed for Arrowfield seam and the higher concentrations are seen to the west of the NS striking dyke. The depositional pattern in the Wambo lease follows the regional pattern of southward/ southwest flowing distributary channel systems in a delta plain environment. 198 10.10 References Beckett J, and McDonald I, 1984. The depositional geology of the Jerrys Plains Subgroup. 18th Symposium on Advances in the Study of the Sydney Basin. Dept of Geology, University of Newcastle, pp.44-46. Guo H, Mallett C, Xue S, Kahraman H, Boland J, Sliwa R, Zhou B, Soole P, Poulsen BA, Harbers C, and Maconochie AP, 2000. Predevelopment Studies for Mine Methane Management and Utilisation, CSIRO Exploration and Mining Report 699C, 2000 EsterleJS, and Fielding CR, 1996. Sedimentological analysis of goafing interval for longwall mining at Goonyella Mine, Bowen Basin. Symposium on Geology in Longwall Mining, 12-13 November, 1996. Eds. G. H. McNally and C. R. Ward, pp. 27-34 Peng SS, and Chiang HS, 1983. Longwall Mining. John Wiley & Sons, New York, Chichester, Brisbane, Toronto, Singapore. Sniffin MJ, and Beckett J, 1995. Sydney Basin-Hunter Valley coalfield in Ward, CR, Harrington HJ, Mallett CW, and Beeston, JW (eds) Geology of Australian Coal Basins. Geological Society of Australia Special Publication No. 1, pp177-195. Uren RE, 1985. The margin of a marine incursion in the Whittingham, Coal Measures, Upper Hunter Valley. 19th Symposium on Advances in the Study of the Sydney Basin. Dept of Geology, University of Newcastle, pp. 45-48. Weisenfluh GA, and Perm JC, 1991. Roof control in the Fireclay Coal Group, Southeastern Kentucky. Journal of Coal Quality, v. 10, no 3, pp 67-74. 199 11 3D GEOTECHNICAL CHARACTERISATION AND MODEL 11.1 Introduction This section presents an initial geotechnical classification of the rock strata located between the Whybrow and Bowfield seams at the Wambo mine. The basis of this classification is an interpretation and correlation of the geophysically logged boreholes and the application of the “strength from velocity ” formula developed for use at the Wambo mine. It is expected that this initial classification will be extended and refined as further drilling and geophysical logging is undertaken. Currently the borehole spacing in the Arrowfield seam is 500-1000 m and this needs to be reduced to increase the detail in the geotechnical model. The objective of the geotechnical classification is to assist in the predictive gas and stress modelling to be undertaken of the Wambo mine in 2001 and 2002. This chapter presents the initial classification and describes, in detail, how it was derived. 11.2 GeotechnicalClassification Geotechnical classification was undertaken to identify and map the rock units that could be expected to behave in a similar manner under the influence of mining of the Arrowfield coal seam. The classification method is based on the LogTrans interpretation of the geophysically logged boreholes and on the “strength from velocity ” relationship developed for the Wambo mine. In summary, the geotechnical classification involves grouping the LogTrans classes to identify unique rock units. While the LogTrans classes can each be related to a lithology, sandstone or mudstone for example, the geotechnical units will usually contain more than one litho type. Hence the geotechnical units are referred to by their relative strengths and the descriptive terms “strong", “moderately strong", “moderately weak ” and “weak ” are used to differentiate the units. The classification procedure has reduced the six LogTran classes into four geotechnical units plus coal. The classes used in the geotechnical classification are defined as: • Unit 1: A strong strata, predominately sandstone, coloured yellow in the figures; • Unit 2: A moderately strong strata, predominately sandstone with some inter- laminated siltstone and mudstone, coloured orange in the figures; • Unit 3: A moderately weak strata, predominately mudstone and siltstone, coloured green in the figures; 223 • Unit 4: A weak strata, predominately mudstone, coloured blue in the figures; and • Unit 5: Coal, coloured black in the figures. 11.2.1 Strength from Velocity Together with the LogTrans analysis, the “strength from velocity ” relationship, as developed for the Wambo mine from 185 UCS tests, was used to guide the classification. As outlined in Section 14 this formula may be written as: UCS= 1.140 e (°-0011 v) where v is the sonic velocity in m/s and UCS is in MPa. 11.2.2 LogT rans Analysis As outlined in Section 8, after consideration of a number of possible petro-physical groupings, the LogTrans analysis program was “trained" to produce the six “lithological ” classes described in Table 11-1. Table 11-1 LogTrans classes used as basis for geotechnical classification LogTrans Corresponding Median Median Median Derived Class Lithology Density Gamma Velocity UCS (MPa) (CODE (GRDE GAPI) (VL4F m/Sec) from g/cm 3) median velocity SS Sandstone 2.51 98.87 3809 75 SSST Interlaminated 2.59 122.64 3763 72 Sandstone/ Siltstone MSSS Interlaminated 2.50 131.12 3672 65 Mudstone/ Sandstone ST Siltstone 2.49 127.89 3378 47 MS Mudstone 2.28 122.61 2854 26 CO Coal 1.42 42.46 2395 Once the “training ” process on five control boreholes had been completed LogTrans was applied to all the WA series holes at the Wambo mine. The results of the LogTrans analyses were then incorporated into the Vulcan borehole database allowing for the display of all the holes in 3D. Correlation and classification into the geotechnical classes was then undertaken. 224 11.2.3 Classification Method The following description of the steps in the classification of the rock strata relate to the EW section at 1 392 000 N, a section with the closest borehole spacing in the mine lease. The same procedure is followed for all other sections. The boreholes on the section colour coded by the LogTrans interpretation, are shown in Figure 11-1. Vertical exaggeration of this, and all other cross sections, is ten times horizontal. A preliminary manual classification and correlation was then undertaken on an A1 plot of this, and all other, sections. The coal-only and interburden geological models (see Section 10) were used as a guide for the correlation of coal seams for the geotechnical classification. An EW section of the interburden model at 1 392 000 N is shown in Figure 11-2. The manual classification was then digitised to create a series of design strings in the Vulcan software as shown in Figure 11-3. These design strings were then smoothed by application of a bi-cubic spline. The resulting strings were joined to form polygons and coloured by the classification colour scheme which is shown in Figure 11-4. This step was undertaken on section lines to produce the plots shown in these figures, however as the strings have been digitised in true 3D, they may be joined between section lines to produce a volumetric model such as shown in Figure 11-16. A plot of a vertical log through the model at 298 250 E, 1 392 000 N (mid-way between holes WA48 and WA64 at the approximate middle of the section) is shown in Figure 11-5. An example of the classification for a section of hole WA20 is shown in Figure 11-6. This figure shows, on the left, the original LogTrans petro-physical classification into the six classes described in Table 11-1 and, on the right, the derived UCS trace coloured by the Geotechnical Classification. An example of an interpolated section based on the geotechnical interpretation of boreholes WA20, WW44 and WA27 is shown in Figure 11-7. This is a NS section at 296 500 E. 225 11.2.4 Cross-sections Four vertical cross sections from the Whybrow to the Bowfield Seams (section 11.2.5) and a further three cross sections from the Woodlands Hill to the Bowfield Seams (section 11.2.6) have been completed and presented in this report. The location of these sections is shown in Figure 11-8. Table 11-2 List of geotechnical sections Section From To Figure Number EW at 1 392 000N Whybrow Seam Bowfield Seam Figure 11-9 EW at 1 391 000N Whybrow Seam Bowfield Seam Figure 11-10 NS at 296 500E Whybrow Seam Bowfield Seam Figure 11-11 NS at 299 500E Whybrow Seam Bowfield Seam Figure 11-12 EW at 1 392 SOON Woodlands Hill Seam Bowfield Seam Figure 11-13 EW at 1 393 000N Woodlands Hill Seam Bowfield Seam Figure 11-14 EW at 1 394 000N Woodlands Hill Seam Bowfield Seam Figure 11-15 230 The 3D geotechnical model is to be extended to the south and refined with the results from further drilling during 2001. This will allow further evaluation of mining conditions at the Wambo mine. 11.3 Geotechnical Properties During the rock strength testing program of 1999-2001 a total of 185 DCS tests from eight boreholes were performed at the CSIRO rock testing laboratory. The mean values of the UCS and Young ’s Modulus results, grouped by lithology, are summarised in Table 11-3. Included in this table is the derived UCS from the median velocity of the LogTrans control holes. A total of 44 shear tests were performed on core samples with natural discontinuities. The average friction angle and (non-zero) cohesions are summarised in Table 11-4. Table 11-3 Summary of strength properties from rock testing holes (mean values). Lithology Derived UCS from UCS UCS Range E median velocity (MPa) (MPa) (GPa) (MPa) Sandstone 75 80 26-130 20.7 (74 tests) 1 Interlaminated 72/65 74 31-134 13.7 Siltstone or (72 tests) Mudstone / Sandstone Siltstone 47 51 22-130 9.6 2 3 (25 tests) Mudstone 26 46 19-69 7.9 (6 tests) Notes 1. includes conglomerates and sandstone with pebbles, which generally have low UCS, and sideritic sandstones with high UCS 2. removed two very high (60 GPa results) 3. 2000 tests give 12.3 MPa, 2001 tests give 5.6 MPa It was noted that there is significant variation in the modulus and UCS results for siltstone in the 2000 and 2001 tests. The reasons for, and implication of, this variation is currently under investigation. 242 Table 11-4 Summary of shear tests on natural discontinuities. Lithology Cohesion (MPa) Friction angle (°) Sandstone 0.1 29 (3 non-zero) Interlaminated Siltstone or Mudstone 0.17 29 / Sandstone (11 non-zero) Siltstone 0.25 27 (2 non-zero) Mudstone 0.11 21 (4 non-zero) It is known that strength properties are strongly scale dependent, and that the mass strength properties will usually be less than the properties measured at laboratory scale. Based on the results of testing and judgement a preliminary estimate has been made of the mechanical properties for the rock units of the classification scheme at mass scale and is shown in Table 11-5. These properties need to be verified with further geotechnical analysis, modelling and field data. Table 11-5 Preliminary mechanical properties for geotechnical units Unit Average Average Average Average ucs Density Young’s Cohesion Friction (MPa) (kg/m 3) modulus (MPa) angle (GPa) (°) Unit 1 (strong) 2500 18 12.4 45 40-80 Unit 2 2500 14 8.7 40 30-45 (moderately strong) Unit 3 2500 10 5.4 35 20-35 (moderately weak) Unit 4 (weak) 2300 8 4.3 30 10-20 Unit 5 (coal) 1400 4 1.2 35 4.3-5.0 243 11.4 Conclusions A geotechnical classification of the rock strata has been undertaken. This classification identified four rock units and coal. The rock units are defined as: • Unit 1: A strong strata, predominately sandstone • Unit 2: A moderately strong strata, predominately sandstone with some inter- laminated siltstone and mudstone • Unit 3: A moderately weak strata, predominately mudstone and siltstone • Unit 4: A weak strata, predominately mudstone A number of sections, and a 3D model, are shown. It is expected that the classification of the rockmass will be extended and refined as further drilling is undertaken. An analysis of the rock testing results has produced recommended mechanical properties for use in the predictive gas and stress numerical modelling to be undertaken in 2001 and 2002. 244 12 GAS RESERVOIR CHARACTERISATION OF ARROWFIELD AND BOWFIELD SEAMS Coal seam gas reservoir properties must be evaluated to determine the quantity of gas-in- place and its rate of production and migration. These properties are critical and necessary prior to planning mine ventilation systems and mine gas drainage, or starting gas production, or considering gas utilisation options. Gas can exist in a coal seam in two ways. It can be present as free gas within the natural porosity of the coal (joints and fractures), and it can be present as an adsorbed layer on the internal surfaces of the coal. The fine micropore structure of coal has a very high storage capacity for both methane and carbon dioxide. Because the bulk porosity of the coal cleat system is small (less than five percent) and the initial gas saturation in the coal cleats is typically low (less than ten percent), most of the gas-in-place in coals (greater than ninety percent) is adsorbed in the coal matrix. The movement of gas is considered to be a two-step process. Once there is mining/degasification activity in a seam, pressure in the cleat decreases, making the coal less capable of retaining gas in adsorbed form. Consequently gas is desorbed at the matrix- cleat interface. As a result, a gas concentration gradient is established between the cleats and the coal matrix. This migration is a diffusion process since the diameter of the micropores is small compared with the mean free path of the gas molecules, and is controlled by the diffusivity of the coal matrix. Once a cleat is reached, the flow becomes viscous, and is controlled by the permeability of the coal. To estimate coalbed gas-in-place, one must determine both the gas content of the coal as it exists at initial reservoir conditions (pressure and temperature) and the desorption isotherm, which describes how gas will be released as pressure is reduced. To determine gas flow/movement from coal, one must determine the properties of coal diffusion and permeability and the cleat network must be evaluated. This chapter describes these reservoir properties for the Arrowfield seam and Bowfield seams within Wambo mining lease and how they are measured or estimated. 12.1 Gas Sorption Isotherm This section describes the background and measuring method of gas sorption isotherm and presents the results of the gas sorption testing aimed at defining the adsorption isotherm parameters for the Arrowfield and Bowfield seams in boreholes WA55, WA58 and WA69. Sorption capacities were determined for both CH4 and C02. 245 The scope covers: • Background of gas sorption theory; • Measuring sorption isotherm; • Sorption isotherm of Arrowfield seam and; • Sorption isotherm of Bowfield seam. 12.1.1 Background Gas is stored in coal primarily by sorption on coal matrix surfaces, although a small fraction of gas may be stored as free gas in the fracture system. Gas adsorption on coal is a physical phenomenon. During the physical sorption, fluid molecules experience a net attraction to a solid surface. Because of the attraction, the density of the fluids near the pore walls is increased, which increases the bulk density of the fluid in the sorbed state. The increased density means that at low pressure, greater volumes of gas can be stored by sorption than by compression. A variety of models have been used to describe the sorption process. The most common model in use for coal is the Langmuir isotherm, which relates the capacity of coal to store gas to the external pressure of the gas. As the name implies, an isotherm is evaluated at a constant temperature (reservoir temperature). A form of the Langmuir isotherm that can be used for single component gas is given by the equation below: p+p, Where: V Gas storage capacity, m3/t P Absolute gas pressure, kPa VL Langmuir volume, m3/t. Pi- Langmuir pressure, kPa Langmuir volume VL represents the maximum amount of gas, of a particular type that can be adsorbed on the surface of the coal. Langmuir pressure PL represents the pressure at which half the gas of a particular type that can be adsorbed on the surface of the coal will be adsorbed. Many factors affect the sorption isotherm. These factors include ash content, moisture content, temperature and gas composition. The ash content is a non-coal component and acts as a diluent and reduces the gas storage of coal. Moisture does not act as a simple diluent, it also competes for the sorption sites with other molecular species and reduces the 246 storage capacity of non-water molecules. Thus, isotherm is measured at the insitu moisture content. For a given coal sample, the sorption isotherm is strongly affected by changes in temperature. The amount of gas stored will decrease as temperature increases. Gas composition has a significant effect on sorption capacity. Coal adsorbs more carbon dioxide than methane under the same condition of temperature and pressure because coal has a greater affinity to carbon dioxide. In the case that seam gas is a mixture of methane and carbon dioxide, a separate sorption isotherm is measured for methane and carbon dioxide and extended Langmuir theory is used to compute the isotherm of the mixed gas. 12.1.2 Measuring Sorption Isotherm Measuring the sorption isotherm involves the following steps: • Selecting and preparing samples; • Performing isotherm tests and; • Performing proximate analysis. The method employed to measure isotherm tests for both Arrowfield and Bowfield seams are described below. A gravimetric method was used to measure the high pressure CH4/CO2 sorption isotherm. The total mass of gas adsorbed is defined by the difference between the mass of the sample with and without gas. Initially, the coal is crushed to 75% less than 125 pm and 100% less than 250 pm, and then approximately 70 g of coal is placed into the high-pressure adsorption bomb. The equipment consists of a gas supply system, which is connected to a manifold through a pressure regulator valve. The system is thermostatically controlled in an air bath at the required temperature. An “as received" state of coal (i.e., with the moisture content) is used for determination of the amount of adsorbed gas. The samples are evacuated at the required temperature over 3 hours with the isotherm determination starting at atmospheric pressure. For isotherm testing the volume of adsorbed gas on a known weight of coal is measured at gauge pressures of about 150, 300, 700, 1300, 2500, 3500 and 4500 kPa. From 2 to 12 hours is required at each pressure to reach sorption equilibrium. Free space (i.e. the space between the coal particles) is calculated from the weight and specific gravity of the coal and the bomb volume. The Langmuir isotherm curve is made to fit the methane and carbon dioxide experimental data through the following: 247 • A linear regression is performed for P/V versus P chart and; • The Langmuir volume VL and Langmuir pressure PL were calculated from the slope and intercept. Proximate analyses were performed on all samples to determine moisture level, ash content and volatile matters. 12.1.3 Sorption Isotherm of Arrowfield Seam Sorption isotherm measurement was conducted for three core samples taken from Arrowfield seam. These core samples were from three boreholes, namely WA55, WA58 and WA69. Each sample consisted of several sections of coal taken from several plies of the core of Arrowfield seam. The elevation and length of these sections are shown in Table 12-1. Table 12-1 Coal samples taken for adsorption isotherm tests from Arrowfield seam of WA55, WA58 and WA69 Borehole/Sample Seam/Ply Elevation, m Thickness Number Top Base m WA55AF AFA 296.510 296.600 0.09 AFA 297.560 297.660 0.10 AFB 298.140 298.180 0.04 AFC 298.775 298.895 0.12 AFC 299.675 299.745 0.07 WA58AF AFA 225.625 225.633 0.08 AFB 226.840 226.940 0.10 AFC . 227.820 227.910 0.09 WA69AF AF 488.34 488.46 0.12 AF 488.74 488.84 0.10 AF 489.86 489.91 0.05 AF 490.49 490.56 0.08 The results of adsorption isotherm of these three samples are summarised in Table 12-2 and shown in, Figure 12-1, Figure 12-2 and Figure 12-3 . Results of proximate analyses of these samples are shown in Table 12-3. Test details are covered in the Appendix. 248 Table 12-4 Summaryof adsorption isotherm parameters (daf) of Arrowfield seam. Borehole/Sample Gas Temperature Isotherm Parameters Number Type °C Langmuir Langmuir Volume, VL Pressure, PL (m=/t) (kPa) WA55AF CH4 30 23.07 2313.96 WA55AF C02 30 62.78 1721.63 WA58AF CH4 30 23.09 2162.29 WA58AF C02 30 62.01 1675.95 WA69AF CH4 40 24.45 2264.34 WA69AF C02 40 59.50 1972.07 12.1.4 Sorption Isotherm of Bowfield Seam Sorption isotherm measurement was conducted for three core samples taken from Bowfield seam. These core samples are taken from three boreholes, namely WA55, WA58 and WA69. Each sample comprises of several section of coal taken from several plies of the core of Bowfield seam. The elevation and length of these sections are shown in Table 12-5. Table 12-5 Coal samples taken for adsorption isotherm tests from Bowfield seam of WA55, WA58 and WA69. Borehole/Sample Seam/Ply Elevation, m Thickness Number Top Base m WA55BF BFA 315.675 315.725 0.05 BFA 316.355 316.435 0.08 WKA 317.605 317.705 0.10 WKA 317.705 317.805 0.10 WA58BF BFA 236.710 236.800 0.09 BFA 237.805 237.865 0.06 WKA 238.605 238.665 0.06 WKC 239.685 239.735 0.05 WA69BF BF 531.47 531.53 0.06 BF 532.51 532.58 0.07 BF 533.41 533.49 0.08 BF 533.73 533.80 0.07 251 m3/t, while Langmuir pressures are 2158.24, 2239.77, and 2303.60 kPa respectively for these three samples. Results show that at 1 MPa, adsorption capacity is around 6 m3/t for CH4 and 19 m3/t for C02. At 2 MPa, adsorption capacity is around 9 m3/t for CH4 and 27 m3/t for C02. Results clearly show that C02 adsorption is two to three times higher than CH4 adsorption. It should be noted that the adsorption isotherms shown here are in the “as received ” or “in situ” state, implying that no correction is made for ash and moisture contents. Ash and moisture contents have no effect on Langmuir pressure, however they do affect the value Langmuir volumes. The corrected Langmuir volumes (daf) are listed in Table 12-8. Table 12-8 Summary of Adsorption Isotherm Parameters (daf)of Bowfield seam Borehole/Sample Gas Temperature Isotherm Parameters Number Type °C Langmuir Langmuir Volume, VL Pressure, PL (m3/t) (kPa) WA55BF CH4 30 22.24 2158.24 WA55BF C02 30 58.20 1605.99 WA58BF CH4 30 24.58 2239.77 WA58BF C02 30 65.86 1757.09 WA69BF CH4 40 23.15 2303.60 WA69BF C02 40 61.36 2145.00 12.2 Gas Content This section describes the background and measuring method of gas content of coal and presents the results of the gas content for Arrowfield and Bowfield seams. The scope covers: • Background of gas content theory; • Measuring gas content; • Gas content of Arrowfield seam and; • Gas content of Bowfield seam. 254 12.2.1 Background 12.2.1.1 General Coal is a porous medium with comparatively very low hydraulic conductivity. Although coal is in the family of reservoir rocks, it differs remarkably from conventional reservoirs in that the volume of gas, which it can store, is far beyond its pore volume capacity. In fact the gas stored in coal is mainly adsorbed onto the pores, large internal surface. For a given coal, the quantity of gas adsorbed is a function of the number of free gas molecules (gas pressure), and total volume adsorbed is limited by the available free pore surface. The relationship between the volume of adsorbed gas and pressure at a given temperatures is called the adsorption isotherm and is described in previous section. If the in situ seam gas pressure is suddenly changed to a new lower pressure, gas will be released from the coal until a new equilibrium state between the new pressure and adsorbed gas is reached. The time required to reach the new equilibrium state depends on the flow properties of coal. If the gas pressure is reduced to zero absolute, then all the stored gas will be released. The definition of gas content is based on the physics of gas storage in coal and will be given in next section. 12.2.1.2 Gas content based on physics of gas storage in coal Total gas content of coal is defined as the total volume of gas stored in a unit mass of coal. This gas, theoretically, can be released if the ambient gas pressure is reduced to absolute zero; assuming that there is sufficient time or the coal permeability is sufficiently high for all the gas to desorb. Desorbable gas content is defined as the volume of gas released per unit mass of coal, when the ambient gas partial pressure is kept at 1 atmosphere. 12.2.1.3 Measuring Gas Content Methods for the estimation of the gas content of coal can be grouped into two categories, viz. direct and indirect. Direct methods are based upon extracting a coal sample, enclosing it in a sealed container and measuring the volume of the gas evolved. Indirect methods are based upon either the gas adsorption characteristics of coal under a given pressure and temperature condition, or upon other empirical data, obtained from existing mines, that relate the gas content of coal to such other parameters such as coal rank, depth of cover or gas emission rate. The direct method (Australian Standard AS3980-1999) was used in estimating the gas content of the Arrowfield and Bowfield seams. For detailed information on the theory and procedures for determining the gas content of coal, see Australian Standard - Guide to the determination of gas content of coal - Direct desorption method. 255 Estimating the gas content of coal using the direct method requires estimates of three components: lost gas, desorbed gas, and residual gas. The lost gas is the volume of gas that is lost before sealing a sample in s desorption canister. Desorbed gas is the volume of gas that is released from the desorption canister as a function of time and measurement conditions. Residual gas is the volume of gas that remains sorbed on the coal at the conclusion of the desorption test To compare gas content estimates from coal seams or samples of differing composition, all gas content estimates were reported on dry, ash-free basis (DAF) and corrected to STP (0°C and 1 atmosphere pressure). 12.2.2 Gas Content of Arrowfield Seam Thirty four gas content tests were conducted for core samples taken from Arrowfield seam. The gas content estimates are shown in Table 12-9. Statistical summaries of the gas content of Arrowfield seam are as follows: Minimum value: 2.92 m3/t Maximum value: 13.06 m3/t First quartile: 7.87 m3/t Median: 9.30 m3/t Third quartile: 10.17 m3/t Mean: 9.04 m3/t Standard deviation: 1.89 m3/t The statistical summary indicates that gas content ranges from 2.92 to 13.06 m3/t and most of the gas content values are between 7.5 to 10.5 m3/t. It also shows the standard deviation of these gas content values is 1.89 m3/t and these gas values are fairly evenly distributed. 256 Table 12-9 Gas content in Arrowfield seam. Borehole Depth Gas content number m m3/t, daf WA02 157.900 2.92 WA03 230.315 7.02 WA04 307.980 6.47 WA06 474.770 6.29 WA07 405.980 8.96 WA08 317.530 8.81 WA10 540.360 10.60 WA11 495.850 9.98 WA12 263.600 7.37 WA17 384.190 7.14 WA19 315.620 8.81 WA21 569.400 10.22 WA23 560.165 13.06 WA25 401.960 8.52 WA31 177.100 8.15 WA32 329.660 8.55 WA35 244.730 6.28 WA37 261.325 9.94 WA39 246.990 11.01 WA40R 306.495 8.69 WA41 288.310 9.01 WA42 267.795 9.58 WA43 213.265 9.89 WA44 272.680 7.85 WA45 261.4575 10.64 WA46 325.4725 9.96 WA47 339.9625 10.51 WA48 342.5875 10.07 WA49 294.740 10.20 WA50 289.335 9.84 WA51 392.103 11.23 WA55 298.043 9.82 WA58 226.650 8.68 WA69 489.233 11.27 • Mean: 9.06 m3/t • Standard deviation: 2.16 m3/t The statistical summary indicates that gas content ranges from 3.91 to 13.0 m3/t and most of the gas content values are between 7.5 to 11.0 m3/t. It also shows the standard deviation of these gas content values is 2.16 m3/t and these gas values are fairly evenly distributed. Table 12-10 Gas content in Bowfield seam. Borehole Depth A. 7 Gas content number m m3/t, daf WA02 175.650 3.91 WA04 330.610 5.39 WA06 516.680 4.13 WA07 441.390 10.16 WA08 407.610 10.19 WA10 567.700 7.75 WA11 529.670 8.65 WA12 281.080 6.04 WA17 399.860 8.29 WA19 331.820 7.48 WA21 589.510 8.91 WA23 595.730 9.37 WA25 429.770 7.57 WA31 208.500 8.71 WA32 376.670 7.91 WA35 259.070 7.15 WA37 302.820 10.37 WA39 258.500 9.67 WA40R 317.770 8.84 WA41 337.240 11.85 WA42 275.880 10.97 WA43 235.980 9.06 WA44 293.765 9.16 WA45 282.170 10.68 WA46 371.178 11.60 WA47 389.083 12.32 WA48 363.565 8.74 WA49 308.080 8.29 WA50 307.670 10.70 WA51 441.25 13.00 WA55 316.418 10.68 WA58 238.135 11.35 WA69 532.620 9.99 259 12.3 Gas Composition This section presents the results of the gas composition from Arrowfield and Bowfield seams. 12.3.1 Gas Composition of Arrowfield Seam Gas samples were taken during desorption tests of Arrowfield seam and these samples were analysed for their composition. The results are reported as air-free basis and shown in Table 12-11. Table 12-11 Gas composition in Arrowfield seam Borehole Depth CH4 C02 N2 number m % % % WA02 157.900 61.5 13.70 24.70 WA03 230.315 76.50 17.90 5.60 WA04 307.980 81.40 12.80 5.80 WA06 474.770 81.70 6.90 11.40 WA07 405.980 87.44 12.45 0.11 WA08 317.530 53.56 17.59 28.82 WA10 540.360 59.38 27.96 12.51 WA11 495.850 45.55 46.22 8.05 WA12 263.600 81.96 5.17 12.73 WA17 384.190 59.22 30.14 10.60 WA19 315.620 60.75 32.13 7.06 WA21 569.400 68.21 28.92 2.85 WA25 401.960 73.89 18.26 7.79 WA40R 306.495 58.64 23.37 17.98 WA41 288.310 73.71 22.53 3.70 WA42 267.795 64.06 31.94 3.99 WA43 213.265 71.81 25.99 2.19 WA44 272.680 80.77 14.86 4.27 WA45 261.4575 78.65 15.37 5.94 WA46 325.4725 78.59 17.96 2.92 WA47 339.9625 84.87 14.16 0.94 WA48 342.5875 62.33 33.15 4.37 WA49 294.740 57.41 29.37 13.18 WA50 289.335 65.60 27.80 6.60 WA51 392.103 80.61 19.14 0.16 WA55 298.043 58.84 40.45 0.69 WA58 226.650 70.27 29.32 0.39 WA69 489.233 52.65 45.35 1.60 261 Statistical summaries of the gas composition of Arrowfield seam are given as follows: For CH4 Minimum value: 46 % Maximum value: 87 % First quartile: 59 % Median: 69 % Third quartile: 79 % Mean: 69 % Standard deviation: 11 % Minimum value: 5 % Maximum value: 46 % First quartile: 15 % Median: 23 % Third quartile: 30 % Mean: 24 % Standard deviation: 11 % The statistical summary indicates that CH4 ranges from 46 to 87 % and the average CH4 is 69%. The results also indicate that C02 ranges from 5 to 46% and the average C02 is 24%. Figure 12-9 show the relationship between the gas composition of Arrowfield seam and its depth. The figure shows that: • CH4 percentage of Arrowfield seam decreases as the cover depth increases; • C02 percentage of Arrowfield seam increases with increase of cover depth and; • CH4/C02 ratio varies and changes from around 80/20 to about 60/40 as the cover depth increases from 200 to 600 metres. 262 Table 12-12 Gas composition in Bowfield seam. * 8 Borehole Depth A. 1 CH4 N2 Number m % % WA02 175.650 80.90 9.10 10.00 WA04 330.610 74.10 13.10 12.50 WA06 516.680 39.70 47.90 12.40 WA07 441.390 89.41 10.50 0.09 WA08 407.610 81.33 18.49 0.03 WA10 567.700 49.77 44.45 5.55 WA11 529.670 45.06 54.12 0.24 WA12 281.080 79.03 3.92 16.92 WA17 399.860 56.93 28.19 14.64 WA19 331.820 65.48 26.52 7.60 WA21 589.510 60.51 34.64 4.76 WA25 429.770 72.27 15.81 11.81 WA41 337.240 64.36 22.47 13.17 WA42 275.880 69.62 26.36 4.01 WA44 293.765 48.36 27.06 24.11 WA45 282.170 69.47 13.93 1.03 WA46 371.178 84.74 13.42 1.59 WA47 389.083 70.98 16.04 12.93 WA48 363.565 64.57 32.24 2.88 WA49 308.080 61.00 31.82 6.96 WA50 307.670 67.00 25.90 7.00 WA51 441.25 68.37 31.02 0.33 WA55 316.418 53.77 45.03 1.18 WA58 238.135 60.72 30.45 8.81 WA69 532.620 33.57 58.54 7.55 Statistical summaries of the gas composition of Arrowfield seam are given as follows: For CH4 Minimum value: 34 % Maximum value: 89 % First quartile: 57 % Median: 65 % Third quartile: 72 % Mean: 64 % 264 12.4 Coal Seam Thickness This section presents the results of the coal thickness of Arrowfield and Bowfield seams. 12.4.1 Thickness of Arrowfield Seam Thicknesses of the Arrowfield seam in Wambo lease were obtained through coring and logging of exploration boreholes. The results are shown in Table 12-13. Table 12-13 Thickness of Arrowfield seam. Borehole Thickness Borehole Thickness Borehole Thickness number m number m number m WA01 3.30 WA22 3.99 WA43 4.15 WA02 3.67 WA23 3.50 WA44 4.40 WA03 4.07 WA24 3.31 WA45 3.73 WA04 3.64 WA25 3.45 WA46 3.76 WA05 3.57 WA26 3.44 WA47 3.62 WA06 3.72 WA27 3.59 WA48 3.44 WA07 4.35 WA28 3.42 WA49 3.07 WA08 4.05 WA29 3.60 WA50 3.49 WA09R 3.35 WA30 3.65 WA51 3.70 WA10 3.45 WA31 4.12 WA52 3.53 WA11 3.90 WA32 4.10 WA53 3.55 WA12 3.75 WA33 3.31 WA54 4.11 WA13 3.20 WA34 3.49 WA56 4.43 WA14 3.60 WA35 2.97 WA57 3.33 WA15 3.50 WA36 4.05 WA58 3.07 WA16 3.45 WA37 3.60 WA61 3.73 WA17 3.25 WA38 3.44 WA62 3.24 WA18 3.05 WA39 3.56 WA63 2.75 WA19 3.15 WA40 4.52 WA63 4.24 WA20 3.30 WA41 3.65 WA65 3.56 WA21 3.55 WA42 3.80 WA69R 3.41 Statistical summaries of the thickness of Arrowfield seam are given as follows: Minimum value: 2.75 m Maximum value: 4.52 m First quartile: 3.42 m Median: 3.56 m Third quartile: 3.76 m Mean: 3.61 m Standard deviation: 0.37 m 266 The statistical summary indicates that the thickness of Arrowfield seam ranges from 2.75 to 4.52 metres and the average thickness is around 3.61 metres. The results also indicate that in most of the boreholes the seam thickness is mostly from 3.4 to 3.8 metres. 12.4.2 Thickness of Bowfield Seam Thickness of Bowfield seam in Wambo lease was obtained through coring and logging of exploration boreholes. The results are shown in Table 12-14. Table 12-14 Thickness of Bowfield seam. Borehole Thickness Borehole Thickness Borehole Thickness number m number m number m WA01 3.35 WA22 3.77 WA43 3.20 WA02 4.02 WA23 3.45 WA44 3.58 WA03 3.64 WA24 3.36 WA45 3.56 WA04 3.67 WA25 3.49 WA46 3.93 WA05 3.78 WA26 3.60 WA47 3.89 WA06 3.62 WA27 3.38 WA48 3.36 WA07 4.25 WA28 3.60 WA49 3.15 WA08 3.70 WA29 3.38 WA50 3.32 WA09R 3.80 WA30 3.80 WA51 4.31 WA10 3.80 WA31 3.28 WA52 3.59 WA11 3.30 WA32 3.50 WA53 3.52 WA12 3.40 WA33 3.32 WA54 3.41 WA13 3.35 WA34 3.74 WA56 3.31 WA14 3.50 WA35 3.14 WA57 3.22 WA15 3.45 WA36 3.20 WA58 2.87 WA16 3.05 WA37 3.55 WA61 3.30 WA17 3.20 WA38 * 3.72 WA62 3.05 WA18 2.93 WA39 3.23 WA63 3.28 WA19 3.15 WA40 3.55 WA63 3.74 WA20 3.39 WA41 3.95 WA65 3.95 WA21 3.30 WA42 3.00 WA69R 4.25 Statistical summaries of the thickness of Bowfield seam are given as follows: Minimum value: 2.87 m Maximum value: 4.31 m First quartile: 3.30 m Median: 3.45 m Third quartile: 3.71 m Mean: 3.50 m Standard deviation: 0.32 m 267 The statistical summary indicates that the thickness of Bowfield seam ranges from 2.87 to 4.31 metres and the average thickness is 3.50 metres. The results also indicate that in most of the boreholes the seam thickness is mostly from 3.3 to 3.7 metres. 12.5 Gas Diffusivity through Coal Matrix This section describes the background and estimating method of gas diffusivity through coal matrix and presents the results of the gas diffusivity for Arrowfield and Bowfield seams in boreholes WA55, WA58 and WA69. The scope covers: • Background of gas diffusion theory • Estimating gas diffusivity • Diffusivity of Arrowfield and Bowfield seams 12.5.1 Background of Diffusion Theory Mass transfer of gas through the coal primary porosity is assumed to be dominated by diffusion because of the small size of the micropores. It is driven by concentration gradients. Pick's Law is used to relate mass transfer to concentration gradients by assuming that the mass flow rate across a surface is proportional to the concentration gradient across the surface, the area of the surface, and diffusion coefficient of the coal through which diffusion occurs. Mathematically, m — —DVC where: m = mass flow, D = diffusion coefficient VC = concentration gradient. Theoretically, the diffusion phenomena is discussed in terms of three components: bulk diffusion, Knudsen diffusion, and surface diffusion. Bulk diffusion is dominated by intermolecular interactions and includes diffusion of one molecular species through a mixture of different molecular species. Knudsen diffusion is dominated by molecule and pore wall interactions. During surface diffusion, mass transfer occurs by movement through the orbed state fluid without mass transfer into the free gas state. In practice, diffusion includes all three types and no attempt is made to distinguish between the three processes. 268 It is generally assumed that gas is transported through the coal primary porosity by diffusion towards the secondary porosity. Upon reaching the secondary porosity, the molecules desorb from the primary porosity surface and enter the free gas state within the secondary porosity. The concentration difference that controls diffusion is the difference between the average gas concentration within a matrix element and the gas concentration at the primary- secondary porosity interface. The sorbed gas molecules at the interface are assumed to be in equilibrium with the free gas molecules within the secondary porosity. The gas concentration at the interface is equal to that computed from the sorption isotherm gas storage capacity at the pressure of the secondary porosity. Thus the sorption isotherm relationship functions as the boundary condition at the primary-secondary porosity interface. 12.5.2 Measuring/Estimating Gas Diffusivity It is difficult to measure the diffusion coefficient, instead a more common approach is to determine a characteristic parameter called the sorption time (x). The sorption time is defined using the equation below: 1 where T sorption time a coal matrix shape factor D dffusion coefficient Sorption time is a characteristic parameter in the standard slow desorption test of gas content in coal. When desorption occurs in* coal at constant pressure, desorption rate is described by following equation: Af) - crDt — e where AQ cumulative desorbed gas content Qo initial gas content Substituting the sorption time definition for t in above equation, we have: Meaning that the sorption time x is equal to the time required to desorb 63.2% of the initial gas volume. 269 It should be noted that for the coal seam gas reservoirs that have been commercially developed to date, diffusion has a minor and usually negligible effect on estimates of gas productivity. Gas flow rates are generally limited by permeability rather than by diffusivity. These coal seam gas reservoirs are characterised by a well developed fracture system that results in relatively short diffusion distances and large coal matrix surface area to volume ratios. However, for the purpose of coal mining operations, where coal seam reservoir is appraised based upon the economics of coal production, diffusion may play a role to some extent in gas flow rate. 12.5.3 Sorption Time of Arrowfield and Bowfield Seams From the data available on gas desorption test results, the values of sorption time have only derived for three boreholes namely WA55, WA58 and WA69R. These values are shown in Table 12-15. Table 12-15 Sorption time of Arrowfield and Bowfield seams. Borehole Seam Sorption time,days WA55 Arrowfield 34 WA55 Bowfield 21 WA58 Arrowfield 40 WA58 Bowfield 32 WA69R Arrowfield 8 WA69R Bowfield 18 12.6 Reservoir And Desorption Pressure Reservoir pressure is the pore pressure that governs the initial fluid flow through natural fracture systems in coal seams. Desorption pressure is the pressure at which gas in coal seam starts to desorb and flow. These two pressures are very important parameters for both water and gas flow. 12.6.1 Reservoir Pressure Well tests were conducted to measure the reservoir pressure on the Arrowfield and Bowfield seams in boreholes WA55, WA58 and WA69R at the Wambo mine. Detailed descriptions of testing tool, operational and interpretational procedures of these well tests are covered in the appendices. Reservoir pressures measured from these tests are listed in Table 6.1. 270 Table 12-16 Reservoir pressure of Arrowfield and Bowfield seams. Borehole Seam Depth, m Water level, m Reservoir pressure, kPa WA55 Arrowfield 298 -11 2751 WA55 Bowfield 316 -11 2917 WA58 Arrowfield 227 -24 1926 WA58 Bowfield 238 -24 2088 WA69R Arrowfield 489 k 4556 These results demonstrate that a linear relationship exists between reservoir pressure (Pres ) and reservoir depth (H). A simple linear regression shows the relationship as follows: P„ = 9.9H-262 12.6.2 Desorption Pressure An attempt was made in multi-phase well tests to estimate desorption pressures on the Arrowfield and Bowfield seams in boreholes WA55, WA58 at the Wambo mine. It should be noted, however, that because desorption process may not be characterised by a definite pressure response in some cases, the estimation of desorption pressure from multiphase tests can be very difficult and may lead to inaccurate results in some cases. Detailed descriptions of testing tool, operational and interpretational procedures of these well tests are covered in the appendices. Desorption pressures measured from these tests are listed in Table 12-17. 271 Table 12-17 Desorption pressure of Arrowfield and Bowfield seams. Borehole Seam Desorption pressure, kPa WA55 Arrowfield Not initiated WA55 Bowfield 1075 WA58 Arrowfield 920-1000 WA58 Bowfield 505-625 These results will be compared with the desorption pressures derived from the adsorption isotherm using gas content measurement. This is discussed in Section 12.9. 12.7 Seam Permeability 12.7.1 Permeability of Arrowfield and Bowfield Seams Permeability is the formation property which relates pressure drop and flow rate through the formation. Well tests (Production buildup, drawdown buildup, and step-rate) were conducted to measure the permeability of the Arrowfield and Bowfield seams in boreholes WA55, WA58 and WA69R at the Wambo mine. Detailed descriptions of testing tools, operational and interpretational procedures of these well tests are covered in the appendices. Seam permeability measured from these tests is listed in Table 12-18. Table 12-18 Permeability of Arrowfield and Bowfield seams. Borehole Seam Permeability, mD WA55 Arrowfield 0.56-0.61 WA55 Bowfield 0.050-0.064 WA58 Arrowfield 8.35-8.38 WA58 Bowfield 14.6-14.7 WA69R Arrowfield 0.005-0.014 WA69R Bowfield N/A Results show that there is a quite a degree of variability in measured permeability for the Arrowfield and Bowfield seam in the three holes tested. Values range from 0.005 to 8.38 mD and from 0.050 to 14.7 mD for the Arrowfield and Bowfield seams respectively. 272 12.7.2 Factors Affecting Seam Permeability The variability of permeability is due to many factors including effective stress and cleat infilling, as well as the effect of geological structures. 12.7.2.1 Effective Stress Table 12-19 shows the measured effective stress in the Arrowfield and Bowfield seams as measured in the three boreholes WA55, WA58 and WA69R. Figure 12-11 shows a marked decrease in permeability with increasing effective stress (the total measured minimum coal stress less reservoir pressure). Furthermore, if the result from borehole WA69R is excluded, the effective stress gradients are almost the same, suggesting a significant reduction in permeability occurs for only a slight increase in effective stress (Figure 12-12). There is also a difference in the measured minimum horizontal coal stresses in WA55 and WA58 which at around 60% to 65% of overburden are quite moderate compared with. WA69R where the stress is notably higher at around 87% of overburden pressure. Table 12-19 Effective stress of Arrowfield and Bowfield seams. Borehole Seam Effective stress, kPa WA55 Arrowfield 1649 WA55 Bowfield 1983 WA58 Arrowfield 1474 WA58 Bowfield 1712 WA69R Arrowfield 6044 WA69R Bowfield N/A 273 12.7.2.2 Cleat Infillinqs Coal seam permeability of the Arrowfield seam in borehole WA69R is quite low (0.005 to 0.014). Apart from the effect of the effective stress of 6,044 kPa, the permeability may also be significantly affected by cleat infillings. The coal cores taken from the Arrowfield seam in this hole have cleat that is extensively filled with calcite. No quantitative analysies has been done on the influence of the infill but it is considered that the extent of calcite infill would have greatly reduced the permeability of the seam. 12.7.2.3 Geological Structure Structural complexity usually affects permeability. No such geological complexity was apparent in the three holes where the permeability tests were conducted. 12.8 Relative Permeability Relative permeability is the ratio of the effective permeability to a base permeability. It is a measure of relative flow easiness of one phase such as gas in the presence of another phase such as water. Relative permeability is normally measured through history matching of production data of well testing. Due to practical difficulties in multiphase tests, relative permeability was defined for the Arrowfield and Bowfield seams in the borehole WA58 only. The relative permeability was estimated by curve matching the multiphase well test results using a reservoir simulator. The reservoir simulator uses the adsorption isotherm, gas content and composition results, and the results of the multiphase well testing for this study data from borehole WA58 was used. Simulation procedures and parameters used for relative permeability estimation are covered in detail in the appendices. The result of the relative permeability for both Arrowfield and Bowfield seams in WA58 is defined in Table 12-20 and shown in Figure 12-13. 275 12.9 Gas Saturation Actual gas content needs to equal the theoretical storage capacity of coal. If a coal seam contains the maximum gas content that is theoretically possible, as defined by the sorption isotherm determined in the laboratory, the coal seam is defined as saturated and gas flow begins as soon as at start of de-watering and pressure drawdown commence, as shown in Figure 12-14. In this example a pressure drop of AP will release the amount of gas between points 1 and 2 as governed by the sorption curve. Sorption C. k ~2q Figure 12-14 Saturated coal seam Sorption i^-2f Figure 12-15 Undersaturated coal seam An undersaturated coal seam contains less gas than the coal seam can adsorb. This means that de-watering and pressure drawdown are required even to elicit gas flow as shown in Figure 12-15. In this example a pressure drop of AP will not result in any gas being released. 277 Table 12-21 shows the results of gas saturation of Arrowfield and Bowfield seams in the boreholes of WA55, WA58 and WA69R. The results indicate that all the boreholes tested are undersaturated to a varying degree. The degree of undersaturation ranges from 23% to 45%. Table 12-21 Gas saturation of Arrowfield and Bowfield seams. Boreholes Seams Gas Desorption pressure Reservoir Degree of content (abs), kPa saturation pressure (in situ) m3/t, in situ Isotherm curve Well tests kPa % kPa kPa WA55 Arrowfield 9.70 1411 - 2751 55 WA55 Bowfield 8.66 1177 1175 2917 66 WA58 Arrowfield 8.84 1271 1020-1100 1926 63 WA58 Bowfield 10.70 1831 605-725 2088 77 WA69R Arrowfield 11.87 1507 - 4556 - WA69R Bowfield 10.08 1122 - - - 12.10 Summary Reservoir characterisation of the Arrowfield and Bowfield seams within Wambo mining lease (study area) has been conducted and they are summarised in this section. The seams are characterised by: • Moderate to high gas content, gas content increasing with depth (up to 12m3/t); • Low to very low permeability sharply decreasing with effective stress (depth) and greatly influenced by cleat infill (as low as 0.005 mD); • Carbon dioxide constituting a significant portion of seam gas, its portion increasing with depth (up to 50%) and; • Arrowfield and Bowfield seams are closely spaced (about 10 to 30 m). These characteristics may pose some significant challenges in mine development and operation, in the deeper part of the Wambo mine (below 300 m depth of cover). Issues to be considered include: • Seam gas has to be substantially drained prior to mine development and operation in order to control gas emission and potential gas/coal outbursts because of high gas content. 278 • Conventional gas pre-drainage may not be effective because of low permeability and high carbon dioxide contents. Innovative pre-drainage techniques need to be developed. • Effective goaf drainage might have to be practised to combat gas emission from the overlying/underlying seam. This requires a clear understanding of gas flow mechanisms, a reliable prediction of gas emissions during mining, and further development of current post-drainage techniques. Major reservoir properties of the Arrowfield and Bowfield seams within Wambo mining lease (study area) are summarised below. 12.10.1 Arrowfield Seam Adsorption Isotherms • Three coal samples were taken from the Arrowfield seam in the boreholes WA55, WA58 and WA69R for adsorption isotherm measurement for both CH4 and C02. • For CH4 adsorption isotherms (daf) of these three samples, Langmuir volumes are 23.07, 23.09 and 24.45 m3/t, Langmuir pressures are 2314, 2162 and 2264 kPa. • For C02 adsorption isotherms (daf) of these three samples, Langmuir volumes are 62.78, 62.01 and 59.50 m3/t, Langmuir pressures are 1722, 1676 and 1972 kPa. Gas Content • Gas content of the Arrowfield seam ranges from 2.9 to 13.1 m3/t and most of the gas content values are between 7.5 to 10.5 m3/t. The average gas content is 9.0 m3/t. • Gas content is more than 6 m3/t at a cover depth of 200 metres. Gas content continues to increase with depth. Gas Composition • Gas in the Arrowfield seam is a mixture of CH4 and C02. • CH4 percentage decreases as the cover depth increases. • C02 percentage increases with the increase of cover depth. • CH4/C02 ratio varies from around 80/20 to about 60/40 as the cover depth increases from 200 to 600 metres. Seam Thickness • Thickness of the Arrowfield seam ranges from 2.75 to 4.52 m with an average of 3.61 metres. 279 Sorption Time (Gas Diffusivitv) • The values of sorption time was derived from desorption tests on three coal samples taken from the Arrowfield seam in boreholes WA55, WA58 and WA69R. These values are 34, 40 and 8 days respectively. Reservoir Pressure • Reservoir pressures in the Arrowfield seam were measured in boreholes WA55, WA58 and WA69R. The reservoir pressures were 2751, 1926 and 4556 kPa respectively. • A linear relationship exists between the reservoir pressure and reservoir depth. Desorption Pressure • Desorption pressures of the Arrowfield seam are estimated from well tests and they are compared with those derived from the adsorption isotherm. The comparison indicates that in some cases there are some noticeable differences. It is revealed that the estimated desorption pressures from the well tests may not be accurate in these cases because desorption process may not be characterised by a definite pressure response. • Desorption pressures from the coal samples taken from the boreholes WA55, WA58 and WA69R were 1411, 1271 and 1507 kPa respectively. Seam Permeability • Well tests were conducted in boreholes WA55, WA58 and WA69R to measure the permeability of the Arrowfield seam. . • Measured permeabilities in the boreholes WA55, WA58 and WA69R were 0.56-0.61, 8.35-8.38, and 0.005-0.014 mD respectively. Results show quite a degree of variability in measured permeability for the Arrowfield seam. • The variability of the measured permeability is due to effective stress and cleat infilling. • A marked decrease in permeability is observed with increasing effective stress. • The coal samples of the Arrowfield seam were visually checked and it was observed that coal cleats are very much filled by calcite in the sample taken from the borehole WA69R. This cleat infilling reduces coal permeability significantly. A combination of effective stress and cleat infilling may explain the very low permeability of 0.005- 0.014 mD that was measured in the Arrowfield seam of the borehole WA69R. 280 Gas-Water Relative Permeability • Relative permeability of the Arrowfield seam was estimated by curve matching the multiphase well test results using reservoir simulator. • Irreversible water saturation is around 0.7. The relative permeability curve is shown in Figure 12-13. Gas Saturation • Gas saturation of the Arrowfield seam was analysed using gas contents, adsorption isotherms and the reservoir pressures. Results indicate that the Arrowfield seam is undersaturated. The degree of undersaturation ranged from 37 to 45%. 12.10.2 Bowfield Seam Adsorption Isotherms • Three coal samples were taken from the Bowfield seam in the boreholes WA55, WA58 and WA69R for adsorption isotherm measurement for both CH4 and C02. • For CH4 adsorption isotherms (daf) of these three samples, Langmuir volumes were 22.24, 24.58 and 23.15 m3/t, Langmuir pressures were 2158, 2240 and 2304 kPa. • For C02 adsorption isotherms (daf) of these three samples, Langmuir volumes are 58.20, 65.86 and 61.36 m3/t, Langmuir pressures are 1606, 1757 and 2145 kPa. Gas Content • Gas content of the Bowfield seam ranges from 3.9 to 13.0 m3/t and most of the gas content values are between 7.5 to 11.0 m3/t. The average gas content is 9.1 m3/t. • Gas content ranges from 4.1 to 13.0 m3/t once the cover depth is over 200 metres and it does not increases with the cover depth. Gas Composition • Gas in the Bowfield seam is a mixture of CH4 and C02. • CH4 percentage decreases as the cover depth increases. • C02 percentage increases with the increase of cover depth. • CH4/C02 ratio varies from around 80/20 to about 50/50 as the cover depth increases from 200 to 600 metres. 281 Seam Thickness • Thickness of the Bowfield seam ranges from 2.87 to 4.31 m and the average thickness is 3.50 metres. Sorption Time (Gas Diffusivitv) • The values of sorption time was derived from desorption tests on three coal samples taken from the Bowfield seam in boreholes WA55, WA58 and WA69R. These values are 21, 32 and 18 days respectively. Reservoir Pressure • Reservoir pressures in the Bowfield seam were measured in boreholes WA55 and WA69R. The reservoir pressures were 2917 and 2088 kPa respectively. • A linear relationship exists between the reservoir pressure and reservoir depth. Desorption Pressure • Desorption pressures of the Bowfield seam are estimated from well tests and they are compared with those derived from the adsorption isotherm. The comparison indicates that in some cases there are some noticeable differences. It is revealed that the estimated desorption pressures from the well tests may not be accurate in these cases because desorption process may not be characterised by a definite pressure response. • Desorption pressures from the coal samples taken from the boreholes WA55, WA58 and WA69R were 1177, 1831 and 1122 kPa respectively. Seam Permeability • Well tests were conducted in the boreholes of WA55and WA58 to measure the permeability of the Bowfield seam. • Measured permeabilities in the boreholes WA55 and WA58 were 0.050-0.064 and 14.6-14.7 mD respectively. Results show quite a degree of variability in measured permeability for the Bowfield seam. • A marked decrease in permeability is observed with increasing effective stress. • If the measured permeabilities from the boreholes WA55 and WA58 are compared for both the Arrowfield and Bowfield seams, two very similar effective stress gradients are apparent for each seam, suggesting a significant reduction in permeability occurs for only a slight increase in effective stress. 282 Gas-Water Relative Permeability • Relative permeability of the Bowfield seam was estimated by curve matching the multiphase well test results using reservoir simulator. • Irreversible water saturation is around 0.7. The relative permeability curve is shown in Figure 12-13. • Relative permeability is found to be the same for both the Arrowfield and Bowfield seams, indicating a similar behaviour in this regard. Gas Saturation • Gas saturation of the Bowfield seam was analysed using gas contents, adsorption isotherms and the reservoir pressures. Results indicate that the Bowfield seam is undersaturated. The degree of undersaturation ranges from 23 to 34%. 12.11 Reference Australian Standard 1999. Guide to the determination of gas content of coal - Direct desorption method 283 • Model orthogonality for computational purposes and; • Cover depth of Arrowfield and Bowfield seams. The model that was constructed had the following dimensions: X- direction (west-east): 8000 m Y- direction (north-south): 10000 m Z - direction (depth): 550 m 13.2 Model Mathematics Coal is treated as a dual-porosity medium. Gas flow through the dual-porosity cleat and matrix reservoir system is assumed to be a two-step process: diffusional flow through coal matrix (Pick's Law) and Darcy flow in cleats, and it is also assumed that equilibrium is reached at matrix/cleat interface. The process is schematically shown in Figure 13-2 and the mathematical description of the process is described below. Gas diffusion governed by concentration gradients Sorption equilibrium controls transfer of gas at matrix/cleat interface Matrix block (micropores) Gas-water flow governed by pressure gradients in cleat Figure 13-2 Schematic view of duai-porosity gas flow in coal. 285 13.2.1 Diffusional Flow Process in Coal Matrix (Pick’s Law) DK=^. F dr dt Where D Diffusion coefficient F Coal matrix shape factor Cg Gas concentration In terms of sorption time (x), the pseudo-steady state diffusion equation (from Pick’s Law) can be expressed as follows: K dt 13.2.2 Porous Flow Process in Cleats (Darcy ’s Law) ju dL Where V = Volumetric flow rate K = Permeability = Viscosity dP/dL = Pressure gradient 13.2.3 Sorption Mechanism (Langmuir Equation) C vLP 8 PL+P Where Cg Adsorbed gas content VL Langmuir volume Pl Langmuir pressure P Pressure 286 13.3 Model Properties Theoretically, all reservoir description data and fluid PVT properties should be measured/estimated and assigned to the model, however in a practical term, it is very difficult to obtain all of these data and properties. In this case, the model is assigned with main properties. These properties include: • Initial gas content • Gas composition • Adsorption isotherm • Coal seam thickness • Reservoir pressure • Desorption pressure • Cleat permeability • Relative permeability 13.3.1 Gas Content Gas content ranges from 3 to 13 m3/t in both the Arrowfield and Bowfield seams. Contour maps of gas content for the seams are shown in Figure 13-3 and Figure 13-4 respectively. The contour maps were used as the input data of gas content assigned in the model. 287 This function was used to calculate reservoir pressure of the Arrowfield and Bowfield seams and the results were used as the input data of reservoir pressure assigned in the model. 13.3.6 Desorption Pressure Desorption pressure is derived from the adsorption isotherm using actual gas content. In the case that seam gas is a mixture of methane and carbon dioxide, the desorption pressure is not only a function of gas content, but of gas composition as well. The desorption pressure can be calculated by the extended Langmuir isotherm equation as follows: 2 p n„ = 1 V, This equation was used to calculate desorption pressure of the Arrowfield and Bowfield seams and the results were used as the input data of desorption pressure assigned in the model. 13.3.7 Seam Permeability Cleat permeability of the Arrowfield and Bowfield shows quite a degree of variability. It is considered that effective stress and cleat infillings are the two major factors affecting the permeability of the Arrowfield and Bowfield seams. A relationship between seam permeability (k) and effective stress (a) was established as follows: k = a + eba This equation was used to calculate permeability of the Arrowfield and Bowfield seams and the results was used as the input data of seam permeability assigned in the model. 301 The rock samples were selected over the whole depth of the boreholes, whereas the coal samples were from the Arrowfield Seam and the Bowfield Seam only. All samples were HQ size (61 mm diameter). The rock UCS tests were conducted following the methods suggested by ISRM (1981). The elastic modulus was measured as the maximum slope in the axial stress versus axial strain in the elastic regime. No Poisson ’s ratio is given from the current tests since its value is highly variable depending upon the loading level. All coal samples were conducted in triaxial compression using CSIRO’s triaxial loading cell. This laboratory set-up has been used to test coal from several mines for highwall mining design. All tests were conducted in single stage triaxial compression. A range of confining pressures were used. The elastic modulus was calculated from the axial deformation using LVDTs mounted on the jacketed sample. The slake durability tests were performed in accordance with the recommended ISRM standard. The second cycle slake-durability index is reported. As recommended, tap water at 20° C was used as a slaking fluid. 14.2 Summary of Laboratory Test Results All together 69 rock samples were tested in uniaxial compression and 34 coal samples were tested in triaxial compression. Fifteen slake durability tests were conducted on samples of the floor rocks of four seams: Whybow, Woodlands Hill, Arrowfield, and Bowfield. The tests results are summarised in Table 14-1-Table 14-3. Detailed discussion on coal and rock strength is given in sections below. 304 Table 14-1 Summary of the laboratory test results on rock samples. Sample Sample Young’s ucs Borehole Sample ID, depth (m) Rock type Diameter Length Density Moisture Modulus Sample location (MPa) (mm) (mm) (Qfcc) (%) (GPa) GT1 70.44-70.31 Conglomerate 61.07 122.26 2.35 3.67 25.91 4.27 24.5m above Redbank Creek GT2 82.15-82.28 Conglomerate 61.15 122.26 2.40 1.80 48.98 3.42 13.1m above Redbank Creek GT4 160.16-160.28 Siltstone 60.87 115.04 2.40 2.59 28.72 4.06 1 9m above Whynot GTS 193.19-193.31 Siltstone 61.15 121.03 2.33 2.26 43.13 4.27 7.5m above Woodlands Hill A GT6 212.55-212.68 Medium grained sandstone 61.00 124.08 2.54 1.39 113.92 4.53 4.7m above Woodlands Hill B Interbedded siltstone and 60.75 93.96 2.40 2.41 34.41 4.06 0.2m below Woodlands Hill B GT7 220.065-220.165 sandstone Interbedded siltstone and 61.07 122.45 2.35 2.38 47.77 5.1m below Woodlands Hill B GTS 224.94-225.065 sandstone 3.81 GT9 228.34-228.47 Conglomerate 61.09 123.64 2.39 2.26 38.04 2.91 8.49m below Woodlands Hill B Interbedded sandstone and GT10 235.45-235.58 60.94 121.14 2.51 2.66 60.24 3.66 23m above Woodlands Hill C WA55 siltstone GT11 244.59-244.72 Conglomerate 61.06 123.20 2.45 2.4 42.25 3.71 13.8m above Woodlands Hill C Medium grained sandstone 61.10 123.15 2.38 42.60 8.9m above Woodlands Hill C GT12 249.49-249.62 with conglomeratic parts 4.08 3.11 Fine to medium grained 113.44 7.8m above Woodlands Hill C GT12A 250.70-250.80 sandstone 61.06 2.60 2.33 64.10 3.77 GT14 260.12-260.25 Tuff 61.07 121.78 2.30 5.92 20.87 3.51 1 6m above Woodlands Hill D Interbedded sandstone and 22.1m above Arrowfield GT15 273.79-273.92 siltstone 61.00 121.54 2.47 2.22 71.63 3.46 Interbedded sandstone and 19m above Arrowfield GT16 276.90-277.03 siltstone 61.03 122.99 2.56 1.71 96.68 3.79 Interbedded sandstone and 6.1m above Arrowfield GT16A 289.80-289.93 siltstone 60.99 121.39 2.66 1.45 93.47 4.29 Interbedded siltstone and 123.03 0.7m above Arrowfield GT18 295.15-295.28 sandstone with sideritic parts 60.99 2.49 1.59 74.62 3.87 Fine to medium grained GT24 300.44-300.56 sandstone 61.05 118.93 2.46 2.17 71.30 4.27 0.4m below Arrowfield Fine to medium grained 0.9 below Arrowfield GT19 300.93-301.06 sandstone with sideritic parts 61.05 121.32 2.61 0.98 132.75 4.66 GT21 313.74-313.86 Siltstone 60.65 116.70 2.32 2.92 28.40 4.01 0.9m above Bowfield GT 22 321.98-322.11 Siderite 61.07 121.90 3.47 0.25 94.15 4.01 3.9m below Bowfield GT2 24.52-24.65 Tuff 60.32 114.39 2.13 9.22 11.13 0.657 1 6m above Redbank Creek 305 Sample Sample Young's ucs Borehole Sample ID, depth (m) Rock type Diameter Length Density Moisture Modulus Sample location (MPa) (mm) (mm) (g/cc) (%) (GPa) Interbedded sills tone and 60.77 120.54 2.36 2.44 49.04 4.22 0.9m above Wambo Rider C GT3 50.02-50.15 sandstone Fine to medium grained 61.01 124.04 2.30 1.01 53.13 15.39 7.2m below Wambo B GT4 61.82-61.95 sandstone GTS 97.56-97.69 Siltstone 60.82 123.54 2.47 2.35 39.06 5.8 3.8m above Whynot B GT6 112.98-113.10 Siltstone 60.74 121.54 2.40 2.9 41.15 7.24 0.7m above Glen Munro GT7 133.50-133.63 Fine grained sandstone 60.89 123.29 2.48 2.34 63.08 7.34 2m above Woodlands Hill A4 GTS 147.61-147.74 Conglomerate 61.02 122.65 2.51 0.95 114.85 83.54 11.8m below Woodlands Hill A4 Medium grained sandstone GT9 163.88-164.01 61.04 122.18 2.64 0.9 194.22 48.61 2.0m above Woodlands Hill B with sideritic bands WA58 GT10 179.06-179.19 Medium grained sandstone 61.02 122.70 2.41 1.01 91.11 17.47 3.2m above Woodlands Hill C2 GT10A 179.06-179.19 Tuff 60.06 123.83 2.17 6.06 12.00 0.983 3.2m above Woodlands Hill C2 GT11184.73-184.86 Siltstone 60.75 122.50 2.33 2.97 26.73 4.60 3.2m above Woodlands Hill D3A GT12 188.87-188.99 Siltstone 60.86 113.09 2.39 2.35 30.49 4.35 1,5m below Woodlands Hill D3C Interbedded sandstone and 126.54 1.96 53.71 4.92 6.5m below Woodlands Hill D3C GT13 193.84-193.97 siltstone 60.74 2.46 Interbedded sandstone and 17.8m above Arrowfield GT14 207.29-207.41 siltstone 60.81 122.52 2.45 2.24 53.94 5.04 GT14A 215.20-215.33 Medium grained sandstone 60.98 122.53 2.34 1.05 68.38 16.14 9.9m above Arrowfield GT15 225.41-225.54 Medium grained sandstone 60.84 123.24 2.41 1.62 85.99 13.35 0.3m above Arrowfield Interbedded sandstone and 0.05m above Arrowfield GT15 225.07-225.195 siltstone 60.80 122.53 2.46 1.57 87.27 13.94 Interbedded siltstone and GT16 227.85-227.98 60.84 2.47 0.52 86.42 10.70 0.4m below Arrowfield B1 sandstone 122.18 GT20 228.34-228.48 Siltstone 60.74 121.68 2.37 2.11 44.77 5.97 0.15 below Arrowfield GT17229.58-229.71 Siltstone 60.79 113.94 2.38 2.35 38.29 6.56 1 4m below Arrowfield C GT19 243.07-243.19 Siderite 60.91 122.48 3.30 1.51 120.00 101.89 3.1m below Bowfield Interbedded siltstone and 63.4m above Monkey Place GT1 49.085-49.215 sandstone 60.91 128.07 2.45 0.93 61.47 6.97 Creek Tuff GT2 61.88-61.99 Fine grained sandstone with 61.07 134.43 2.58 2.62 136.33 28.75 50.6m above Monkey Place sideritic bands Creek Tuff Fine grained sandstone with GT3 124.44-125.50 Monkey Place Creek Tuff sideritic bands 61.05 129.47 2.46 4.05 61.51 9.11 Medium grained sandstone 40.3m above Whybrow GT4 203.050-203.135 with sideritic parts 61.05 129.32 2.65 1.27 154.98 42.49 306 Sample Sample Young’s ucs Sample ID, depth (m) Rock type Diameter Length Density Moisture Modulus Sample location Borehole (MPa) (mm) (mm) (g/cc) (%) (GPa) GT6 237.91-238.15 Fine grained sandstone 61.02 133.92 2.31 2.74 53.13 12.63 5.5m above Whybrow GT7 255.005-255.135 Tuff 60.77 128.83 2.36 8.61 41.93 5.08 6.4m above Redbank Creek Medium grained sandstone 60.89 133.77 2.70 0.81 199.74 51.14 1 9m above Whynot A GT10 316.115-316.225 with iron staining Medium grained sandstone 60.91 129.67 2.68 3.35 198.92 58.86 2.2m above Whynot A GT10a 315.85-315.98 with iron staining Interbedded sandstone and WA69R 120.09 2.36 84.84 13.38 1.8m above Whynot B GT11 328.115-328.245 siltstone 60.79 2.51 Interbedded sandstone and 131.09 2.61 2.32 98.64 27.26 6.1m above Glen Munro GT27 371.885-372.015 siltstone 60.96 GT12 385.75-385.87 Medium grained sandstone 61.06 130.29 2.53 1.75 96.23 30.53 5.8m above Woodlands Hill A GT13 411.115-411.245 Medium grained sandstone 61.04 127.83 2.43 3.63 59.22 14.75 17.5m above Woodlands Hill B3 Interbedded sandstone and 61.01 108.22 2.53 2.31 76.52 17.55 3.8m above Woodlands Hill B3 GT14 424.935-425.07 siltstone GT15 434.675-434.805 Tuff 61.01 134.45 2.49 4.46 61.81 15.65 1 3m above Woodlands Hill D Interbedded siltstone angl 133.75 2.56 2.03 70.05 14.71 17.8m above Arrowfield GT18 469.995-470.125 sandstone 60.99 GT19 483.075-483.262 Fine grained sandstone 60.99 131.51 2.52 2.76 75.52 18.35 4.7m above Arrowfield Interbedded siltstone and 1.1m above Arrowfield GT20 486.641-486.771 sandstone 60.86 112.21 2.79 2.07 92.50 20.68 GT22 492.29-492.42 Medium grained sandstone 60.96 131.52 2.55 2.01 102.30 25.38 1.1m below Arrowfield GT23 495.740-495.870 Coarse grained sandstone 61.09 134.01 2.52 2.36 83.26 23.95 4.6m below Arrowfield GT24 496.28-496.41 Sandstone 60.99 111.00 2.65 1.94 93.69 18.25 5.1m below Arrowfield Interbedded sandstone and 3.2m below Arrowfield GT17 494.39-494.52 siltstone 60.79 126.52 2.55 2.15 95.84 17.02 GT29 500.47-500.60 Sandstone 60.69 131.24 2.52 1.99 93.13 21.79 9.3m below Arrowfield GT30 523.07-523.22 Medium grained sandstone 61.06 128.05 2.67 0.83 169.56 40.72 7.5m above Bowfield GT32 530.41-530.55 Fine grained sandstone 61.05 131.79 2.52 1.40 112.29 19.81 0.2m above Bowfield GT26 535.75-535.88 Tuff 60.99 121.93 2.52 4.43 41.48 6.26 0.4m below Bowfield Interbedded sandstone and 2.4m below Bowfield GT33 537.89-538.01 siltstone 61.03 112.13 2.61 2.46 81.36 23.46 Interbedded siltstone and 61.04 2.6m below Bowfield GT31 537.92-538.05 sandstone 127.74 2.56 1.47 133.97 31.44 307 Table 14-2 Summary of the laboratory test results on coal samples. Sample Sample Peak Av Confining Young’s Borehole Seam Sample ID, depth Diameter Length Stress Pressure Modulus Sample Brightness (mm) (mm) (MPa) (MPa) (GPa) ED8 296.19-296.32 61.00 127.10 36.92 1.54 4.27 C3 ED9 297.02-297.14 61.03 117.20 26.19 1.83 3.42 C3 ED10 298.21-298.33 61.02 118.46 36.20 2.32 4.06 C2 AF ED11 298.70-298.81 61.02 121.26 50.98 6.14 4.27 C4 WA55 ED11 298.91-299.04 61.09 106.00 69.11 10.53 4.53 C3 ED24 299.775-299.895 61.14 121.63 65.28 14.56 4.06 C2 ED13 315.82-315.95 60.99 129.80 50.39 1.79 3.81 C3 BF ED14 316.185-316.305 61.13 120.31 28.50 2.30 2.91 C3 ED14 316.415-316.535 61.05 123.39 43.10 6.13 3.66 C4 ED17 225.585-225.225 61.05 122.21 27.90 1.64 3.71 C4 ED17 226.11-226.245 61.09 125.95 24.33 1.52 3.11 C3 ED17 226.185-226.055 ' 61.03 115.90 31.38 1.80 3.77 C3 ED18 226.66-226.83 C3 AF 61.01 126.07 73.13 16.02 3.51 ED18 226.69-226.47 61.05 122.39 39.64 2.20 3.46 C2 ED18 227.13-226.95 61.01 123.05 58.09 5.97 3.79 C3 ED19 227.64-227.47 61.02 122.41 73.68 10.40 4.29 C4 WA58 ED19 228.05-227.87 61.04 122.24 74.53 15.10 3.87 C3 ED20 236.71-236.51 61.05 122.85 39.28 1.53 4.66 C2/C4 ED20 237.15-237.01 61.03 117.11 46.96 1.82 4.01 C4/C5 ED25 239.885-239.745 61.06 123.34 82.03 9.24 4.01 C3 BF ED31 237.645-237.515 61.02 117.70 37.62 2.31 4.27 C2/C3 ED31 237.835-237.645 60.99 123.27 57.22 6.28 4.06 C3 ED32 238.925-238.815 61.04 104.23 79.53 9.92 4.40 C5 AF-3X1 488.06-488.21 61.03 126.46 24.96 1.48 3.98 C4/C5 AF-3X2 488.55-488.71 61.05 125.32 29.79 1.78 4.01 C3/C5 AF-3X3 489.19-489.32 61.07 103.1 39.72 2.20 4.48 C3/C5 AF AF-3X4 489.32-489.45 61.01 121.61 48.65 5.95 3.83 C4 308 Sample Sample Peak Av Confining Young’s Borehole Seam Sample ID, depth Diameter Length Stress Pressure Modulus Sample Brightness (mm) (mm) (MPa) (MPa) (GPa) WA69R AF-3X5 489.55-489.67 61.05 113.69 68.79 10.63 4.36 C4 BF- 3X1 531.53-531.70 61.01 124.57 40.66 1.50 4.68 C4/C5 BF- 3X2 531.86-532.04 61.02 124.68 22.84 1.77 3.84 C3 BF- 3X3 532.06-532.22 61.03 124:47 29.42 2.26 4.26 C3 BF BF- 3X4 533.21-533.32 61.02 100.65 67.48 5.97 4.84 C4 BF- 3X5 533.91-534.02 61.06 100.47 51.82 10.32 4.41 C2/C3 BF- 3X7 534.09-534.19 60.99 93.55 87.82 15.85 5.03 C2 309 Table 14-3 Slake durability test results. Hole Sample Slake Durability Index (%) Lithology Sample Location GT34 77.1 Siltstone (tends to Mudstone) 0.02m below Arrowfield Carbonaceous mudstone (tends to 0.05m below Arrowfield WA51 GT35 89.0 Mudstone) GT36 74.0 Siltstone 0.21m below Arrowfield GT51 42.3 Mudstone 0.01m below Bowfield GT52 78.9 Mudstone 0.06m below Bowfield GT53 92.4 Siltstone 0.19m below Bowfield GT20 97.7 Siltstone 4.8m below Arrowfield GT23 98.0 Mudstone 0.83m above Bowfield WA55 GT24 95.5 Fine to medium grained sandstone 0.28m below Arrowfield GT25 82.2 Mudstone 0.44m below Bowfield GT17 95.8 Siltstone 1 40m below Arrowfield GT18 94.6 Siltstone 1 34m below Bowfield WA58 GT20A 64.8 Siltstone 0.01m below Arrowfield GT21A 67.9 Siltstone 0.01m below Bowfield GT7 95.4 Siltstone 0.5m above Why brow GT15 97.3 Siltstone 0.54m below Woodlands Hill C2 WA69R GT21a 84.7 Siltstone 0.5m below Arrowfield GT21b 87.6 Siltstone 0.4m below Arrowfield GT25a 95.3 Mudstone 0.16m below Bowfield GT25b 96.6 Mudstone 0.25m below Bowfield GT26 89.3 Tuff 0.96m below Bowfield 14.3 Rock Strength The tested rock DCS is highly dependent upon the rock types as can be seen in Table 14-1. For the purpose of roadway roof stability and longwall cavability assessment, the test results are discussed based on their stratigraphic locations rather than the rock type. Figure 14-2 plotted the tested DCS against the samples depth. The relative location of the tested samples to the Arrowfield and Bowfield seam can be observed. The strength classification of Table 14-4 is used in the following discussion. Table 14-4 Rock strength classification after Geological Society Working Party (1970) and ISRM (1981). ROCK STRENGTH CLASSIFICATION Code Description Field Identification Estimated UCS (MPa) S1 Very soft soil Easily penetrated several cm by fist. <0.25 S2 Soft soil Easily penetrated several cm by thumb. 0.025 - 0.05 S3 Firm soil Can be penetrated several cm by thumb with 0.05-0.10 moderate effort. S4 Stiff soil Readily indented by thumb but penetrated only 0.10-0.25 with great effort. S5 Very stiff soil Readily indented by thumb nail. 0.25 - 0.50 S6 Hard soil Indented with difficulty by thumbnail. >0.50 RO Extremely weak rock Crumbles in hand. <0.50 R1 Very weak rock May be broken in hand with difficulty. 0.5 -1.25 R2 Weak rock Cut with a knife. Core SOD x 150L breaks by 1.25-5.00 hand. R3 Moderately weak rock Peeled with knife. Pick indents 5 mm. Core 5.0-12.5 SOD x 150L difficult to break by hand. R4 Moderately strong rock Scored with knife. Breaks with 1 hammer blow. 12.5-50 R5 Strong rock Requires >1 hammer blow to break. 50-100 R6 Very strong rock Rings when struck. Requires several hammer 100-200 blows to break. R7 Extremely strong rock Sharp ring when struck. Hammer blow can >200 only chip. 311 Main roof of Arrowfield seam The overburden rocks above the Arrowfield seam were tested to be generally competent. The rock DCS is mostly in the range 25 MPa-100 MPa. In boreholes WA55 and WA58, the test results show a general trend of higher UCS close to the Arrowfield seam, whereas in borehole WA69R this trend is not very obvious. Rocks within 5 m above the Arrowfield seam were tested to have a UCS mostly in the range of 50-100 MPa. Immediate roof of Arrowfield seam Five tests were conducted on samples collected within 5 m above Arrowfield seam. The tested UCS ranges from 75-92 MPa with an average of 83 MPa, indicating that the immediate roof rock is strong to very strong. Immediate floor of the Arrowfield seam Ten tests were conducted on samples collected within 5 m below the Arrowfield seam The UCS ranges from 38-132 MPa with an average of 75 MPa. The average UCS indicates a strong floor condition. However, it is expected that variable floor conditions may exist from moderately strong to very strong. Immediate roof of Bowfield seam Only three tests were conducted on the immediate roof samples of the Bowfield seam. The results suggest a wide range of UCS form 28 MPa to 16 9 MPa with an average of 103 MPa. The limited number of tests, however, makes it difficult to draw any conclusions on the general roof condition of the Bowfield seam. Immediate floor of Bowfield seam The test results from 5 samples indicate that the immediate floor of the Bowfield Seam has a UCS range from 41 MPa to 134 MPa with an average of 94 MPa. This average strength represents that of a competent and strong rock. 312 14.4 Coal Strength 14.4.1 Analysis of Laboratory Tests Results The test results given in Table 14-2 are plotted separately for the AF seam and the BF seam in Figure 14-3 and Figure 14-4. The test data are interpreted using the Hoek-Brown strength formula (Brady BHG and Brown ET, 1985) given below: where cr, and a3 are respectively the loading stress and the confining stress used in the tests. A Hoek-Brown strength curve is obtained for each seam by varying the oc and m values to best-fit the test results. The resultant crc and m values for the two seams are: AF Seam: crc =23.0MPa; m=9.7 BF Seam: crc =26.9MPa; m=10.7 The best-fit Hoek-Brown curves are also plotted in Figure 14-3 and Figure 14-4. 314 14.4.2 Coal Seam Brightness The brightness profiles of the AF and BF seams from the three boreholes are plotted in Figure 14-5 and Figure 14-6. On average, the AF seam has an overall brightness of C3. The BF seam is marginally duller than the AF seam and has an overall brightness of C3/C4. The AF seam has a similar brightness in boreholes WA55 and WA58, but is duller in borehole WA69 R. The BF seam is brighter in borehole WA6 R than in boreholes WA55 and WA58. The seam brightness is an indicator of the intensity of microfracturing in coal. Bright coal has more fractures than dull coal, and hence often has less compressive strength. For this reason, it is expected that the BF seam is stronger than the AF seam. The test results appear to have confirmed this. The brightness profiles of the AF and BF seams are also compared with the typical brightness profile of the DU seam at Moura Mine given in Figure 14-7. The AF seam has an overall brightness similar to the Moura DU seam, and the BF seam tends to be slightly duller than the Moura DU seam. A comprehensive study has been conducted on the strength of the Moura DU seam by CSIRO (Medhurst, 1996) and its mass coal strength is well understood. For the Moura DU seam, an average UCS of 25.8 MPa was obtained from laboratory tests on coal samples. 316 WA55 - AF WA58 -AF WA69r - AF Brightness Brightness Brightness C6 C5 C4 C3 C2 C1 C6 C5 C4 C3 C2 C1 C6 C5 C4 C3 C2 C1 295.90 225.13 487.76 296.44 488.27 225.67 296.97 488.80 226.22 E" 297.50 ? r 489.35 Q. S 298.04 Q 226.75 Q © © o o o 489.88 X X 298.57 227.27 490.39 299.09 227.80 299.63 490.93 Figure 14-5 Brightness profiles of the AF seam. WA55 - BF WA58-BF WA69r - BF Brightness Brightness Brightness C6 C5 C4 C3 C2 C1 C6 C5 C4 C3 C2 C1 C6 C5 C4 C3 C2 C1 314.68 236.29 530.58 531.13 315.22 236.85 531.67 -g 532.19 f g 532.72 0 1 533.27 533.79 239.48 534.34 317.93 Figure 14-6 Brightness profiles of the BF seam. modified Hoek-Brown criterion. Using his method, Medhurst found that the mass UCS of the Moura DU seam was 4.8MPa. This coal strength is considered to be representative for the Moura coal after the successful design and mining in more than 10 pits in Moura Mine. Based on the Moura test data, Medhurst suggested that the Hoek-Brown parameters mb and s are scale dependent. They follow the following equations: s = 91.58(LZ2)" 1045 mb =25.85m(L/2)' 07803 where L is the length of a cylinder sample in millimetres. A sample with a length of 1600 mm and a diameter of 800 mm was considered by Medhurst to be the mass sized sample for the Moura DU coal. Although the above equations were developed based on the Moura DU Seam data, it is believed that they can be applied to the AF and BF seams at the Wambo mine which have similar brightness profile in general as the Moura DU seam. Hence the following mass coal strength parameters are derived for the two seams: AF Seam: crc =23.0MPa; mb=1.4, s=0.075 (Mass UCS=4.3MPa) BF Seam: crc =26.9MPa; mb=1.5, s=0.075 (Mass UCS=5.0MPa) The above Hoek-Brown strength parameters produce a mass unconfined compressive strength (UCS) of 4.3MPa for AF Seam and of S.OMPa for BF Seam. Note that the mass UCS differs from the conventional mass coal strength of a coal cube. It represents the strength of a cylindrical coal mass with a height to diameter ratio of 2:1. The mass UCS was reported to be often 25% less than the cube coal strength (Townsend, 1977). The equivalent cube strength for the two seams are therefore estimated to be: AF seam: Cube mass coal strength = 5.7MPa BF seam: Cube mass coal strength = 6.7MPa Recent publications suggested that the mass strength of a cube of coal in situ often lies in a narrow range. Mark and Barton (1996) suggested the range to be 5.4 - 7.4 MPa, whereas Gale and Mills (1995) suggested a range of 4 - 8 MPa. The mass cube strengths of the AF and BF seams at the Wambo mine fall within these ranges. 320 14.5 Slake Durability of the Seam Floors The test results given in Table 14-3 is grouped according to sample stratigraphic locations, see Table 14-5. Table 14-5 Slake-durability index grouped by stratagraphic location | Hole Sample Slake Durability index (%) Lithology Coal seam WA69R GT7 95.4 TF WHYBROW AVERAGE 95.4 WHYBROW Minimum Maximum WA69R GT15 97.3 TF WOODLANDS HILL AVERAGE 97.3 WOODLANDS HILL Minimum Maximum WA51 GT34 77.1 ST (TENDS TO MS) ARROWFIELD WA51 CARBONACEOUS MSARROWFIELD GT35 89.0 (TENDS TO MS) WA51 GT36 74.0 ST ARROWFIELD WA55 GT20 ARROWFIELD (BELOW 97.7 ST AFD NOT AFC SEAM) WA55 GT24 95.5 SSFM ARROWFIELD WA58 GT17 95.8 ST ARROWFIELD WA58 GT20A 64.8 ST ARROWFIELD WA58 GT21A 67.9 ST ARROWFIELD WA69R GT21A 84.7 ST ARROWFIELD WA69R GT21B 87.6 ST ARROWFIELD AVERAGE 83.4 ARROWFIELD Minimum 64.8 WA58/GT20A/ST Maximum 95.8 WA58/GT 17/ST WA51 GT51 42.3 MS BOWFIELD WA51 GT52 78.9 MS BOWFIELD WA51 GT53 92.4 ST BOWFIELD WA55 GT23 98.0 MS BOWFIELD WA55 GT25 82.2 MS BOWFIELD WA58 GT18 94.6 ST BOWFIELD WA69R GT25A 95.3 MS BOWFIELD WA69R GT25B 96.6 MS BOWFIELD WA69R GT26 89.3 TF BOWFIELD AVERAGE 85.5 BOWFIELD Minimum 42.3 WA51/GT51/MS Maximum 98.0 WA55/GT23/MS 321 One slake-durability test was performed on a sample from the floor of the Whybrow (WWC) and Woodlands Hill (WHC2) seams. In both cases the rock lithology is a tuff. Both tests give a High durability classification. However given the number of samples and the highly variable nature of tuffs in the full stratigraphic core photographic record (from competent to extremely weathered / friable) it is not possible to draw any conclusion on floor conditions of these two seams. The durability of the Arrowfield Seam floor was tested with ten samples comprising of mudstone(1), siltstone(S) and sandstone(l). The tested slake durability for the Arrowfield seem floor ranges from 64.8-95.8%, with an average of 83.4%. The average value represents Medium to Medium High durability. A visual inspection of the core photographs indicates poor floor conditions where the AFD seam is in close proximity to the main Arrowfield Seam, the AFC seam. Such a situation occurs in hole WA58 and two tests from this hole give indices of 64.8% and 67.9% which is considerably below the average. In other locations where there is no other coal seam close to the Arrowfield Seam floor, the test results indicate Medium High durability on average. Durability of the Bowfield Seam floor was tested by nine samples comprising mudstone (6), siltstone (2) and tuff (1). It ranges from 42.3% to 98.0% with an average of 86.5%. High variability in durability was evident in the samples with both the lowest test result 42.3% and the highest, 98.0% recorded (both mudstones). High variability was also evident in tests from the one hole with the three samples from WA51 giving values of 42.3%, 78.9% and 92.4%. These three samples were 0.14m apart. On average the test results indicate Medium High durability. However variable floor conditions is expected to exist as have been seen from the test results. 14.6 Development of rock strength formula for the Wambo mine 14.6.1 Empirical formula of sonic velocity - rock strength Sonic velocity, as recorded during routine geophysical logging, is often used to provide an estimate of the rock strength. The resulting, near continuous, distribution of rock strength in the logging interval is invaluable for the geotechnical classification and interpretation of the rock strata. An empirical relation between the sonic velocity and the rock strength (often refers to the unconfined compressive strength DCS) is required. This relation is often site dependent. To provide a site specific relation for future use, strength test results on rock samples from nine boreholes at the Wambo mine, including the recent three boreholes WA55, WA58 and WA69R, was used to correlate to sonic velocity obtained during 322 geophysical logging.Table 14-6 outlines the number of the tested samples and sonic sampling used in regression analysis. The hole locations are shown in Figure 14-1. Table 14-6 Number of strength tests per hole Hole Number of samples Number of samples Number of samples tested with strength value with VL4F velocity value WA44 15 15 15 WA45 18 18 18 WA48 20 10 10 WA49 19 17 17 WA50 32 30 29 WA51 33 32 28 WA55 19 19 17 WA58 21 21 21 WA69R 27 27 27 The LAS formatted geophysical logs provided by EarthData for the Wambo mine have several sonic logs that could be used in the regression analysis. These logs usually include 20, 40 and 60cm channels, VL2F, VL4F and VL6F respectively plus other compensated velocity logs. In practice it was found that the 40cm log VL4F provided the most, non-null, readings at the selected sample depths. The equation between UCS and velocity is generally assumed to be an exponential function of the form: UCS = A e" bt where A and b are site specific constants and t is the sonic velocity “slowness ” expressed in uSec/ft or uSec/m. Geophysical logging companies have increasingly adopted velocity in m/s rather than “slowness ” (the inverse of velocity) for the velocity logs. Given this it is useful to express the function as: UCS = C e dv where C and d are site specific constants and v is the sonic velocity expressed in m/s. As the relationship between velocity and “slowness ” is non-linear there will be a slight difference if using velocity converted to “slowness ” in the original equation. For the same reason, regression analysis has been carried out on the velocity values direct from the LAS file rather than converting them to “slowness ” and then analysing them. 323 14.7 Summary A program of laboratory testing of coal and rock samples was undertaken by CSIRO. The program comprised of 69 unaxial compression tests of rock samples, 34 triaxial tests of coal samples and 15 slake durability tests of floor samples. The samples were collected from three boreholes (Boreholes WAS5, WA58 and WA69 R). The coal samples were selected from the Arrowfield and the Bowfield seams. The test results suggested that the immediate roof and floor of both the Arrowfield and the Bowfield seams are competent with an average UCS 70-90 MPa. Strong variation in the test results however exist, suggesting that variable roof and floor conditions could be encountered. The overburden rock above the Arrowfield seam is generally strong, with a UCS mostly greater that 50 MPa. The coal of the Arrowfield and the Bowfield seams was tested to be reasonably competent. The equivalent UCS of the laboratory size samples is estimated to be 23 MPa for the Arrowfield seam and 27 MPa for the Bowfield seam. The Arrowfield seam is slightly brighter and hence weaker than the Bowfield seam. The estimated mass UCS of the two seams on a mass scale is 4.3 MPa for Arrowfield and 5.0 MPa for Bowfield. The floor slake durability index for both the Arrowfield seam and the Bowfield seam are about 85%. The value represents Medium to Medium High durability. Generally competent floor conditions are expected. Based on the current and previous test results, a rock strength - sonic velocity relationship has been developed for application at the Wambo mine. This formula can be used with some confidence given that it was developed from 183 tests of the collieries rock samples. The formula models low to medium rock strength well but overestimates rock strength for velocities above 4500 m/s. The strength from velocity relationship is being used for the geotechnical classification and assessment of the Wambo mine. 326 14.8 References McNally GH, 1987. Estimation of Coal Measures Rock Strength Using Sonic and Neutron Logs. Geoexploration, 24 (1987) 381-395 Ward B. Rock Strength from Sonic Velocity Logs. Geotechnical Consulting Services Pty Ltd, Report No 105 Brown ET, (ed) 1981. Rock Characterisation Testing and Monitoring. ISRM Suggested Methods, Pergamon Press 1981, pp92-94 Brady BHG and Brown ET, 1985. Rock Mechanics for Underground Mining. George, Allen & Unwin. Gamble JC, 1971. Durability-plasticity classification of shales and other argillaceous rocks. PhD Thesis, University of Illinois (1971) Mark C, and Barton TM, 1996. A New Look at the Uniaxial Compressive Strength of Coal. Proc. 2nd North American Rock Mech. Symp., ed. M. Aubertin, F. Hassani and H. Mitri. 405- 412. Rotterdam: A.A. Balkema. Medhurst TP, 1996. Estimation of the In situ Strength and Deformability of Coal for Engineering Design. Ph.D. Thesis. Department of Mining, Mineral and Material Engineering. University of Queensland. Gale WJ, and Mills KW, 1995. Coal Pillar Design Guidelines - P531. Australia Mineral Industries Research Association Limited, Report AM 10157. Townsend JM, Jennings WC, Haycocks C Neall GM, and Johnson LP, 1977. A Relationship Between the Ultimate Compressive Strength of Cubes and Cylinders for Coal Specimens. Proc. 18 th U.S. Symp. On Rock Mech., ed. F. Wang and G.B. Clark, 4A6-1-4A6-6. Golden: Colorado School of Mines Press. 327 15 IN SITU STRESS MEASUREMENTS In situ stress measurements were carried out to determine the direction and magnitude of principal horizontal stresses in the key strata about the Arrowfield and Bowfield seams using the hydraulic fracture technique. The magnitude of the minimum effective stress in the Arrowfield and Bowfield seams was measured using the step rate test method. The result of the measurements is summarised in this section. The full details are provided in Appendix A. 15.1 Rock Stress Measurements 15.1.1 Data Selection The pressure records for all tests successfully completed in the four holes tested (WA55, WA58, WA69, WA69R), and the corresponding impressions, where available are contained in Appendix A. Generally, the pressure records were similar and consistent with the simplest understanding of hydraulic fracturing (Enever, 1993). The available impression records and pressure record diagnostics (Enever, 1993) indicated predominantly vertical or near vertical fracture development allowing for ready interpretation of the horizontal secondary principal stress field. The methods used to interpret the salient data required for analysis from the records are illustrated in Figure 15-1 and Figure 15-2. The salient data is summarised in Table 15-1 15.1.2 Data analysis The following procedure was employed for analysis of the horizontal stress field: • use of fracture orientation to directly estimate the orientation of the major horizontal secondary principal stress (cth), • use of shut-in pressure to directly estimate the magnitude of the minor horizontal secondary principal stress (a h), • use of the following expression(Enever, 1993) to estimate the magnitude of crH, Pr is the crack re-opening pressure (Figure 15-1) and Po the ambient pore pressure derived from the standing water level in the hole. The results of analysis are summarised in Table 15-2. Also included in Table 15-2 are estimates of fracture strength made from the pressure records (Fracture Strength = Fracture Initiation Pressure (Pi) - Fracture Re-opening Pressure (Pr)). 328 A program of laboratory testing was undertaken, as illustrated in Figure 15-3, to measure fracture strength on samples prepared from core corresponding to representative field test horizons. All tests involved internal pressurisation to failure with water at a constant pressurisation rate of 3.5 MPa/min to simulate field conditions. A range of test hole diameters was employed to investigate the impact of scale effect on fracture strength. The results of the laboratory program are summarised in Figure 15-4. The individual test by test results are contained in Appendix A. Superimposed on Figure 15-4 is the range of field measured fracture strength as given in Table 15-2, plotted at the hole size used in the field. In a general sense, Figure 15-4 suggests reasonable agreement between the laboratory and field data in the context of the commonly observed impact of size effect on fracture strength (Enever and Wu, 2000). On this basis the results of the above analysis can be viewed, in the first instance, as being internally consistent. On an individual test by test basis, however, some specific tests did not show the same level of agreement between laboratory and field information as for the combined data, leading to the assignment of reliability ratings to the results of analysis as indicated in Table 15-2. 15.1.3 Discussion The orientational data from Table 15-2 is summarised on a spatial basis with respect to the local geological setting in Figure 15-5. The relationship of the major horizontal stress orientation with depth of cover is shown in Figure 15-6. Figure 15-5 and Figure 15-6 suggest: • an approximate alignment of the major horizontal secondary principal stress with the north-west trend of the nearby Redmanvale Fault in holes WA69 and WA69R; • an approximately east-west orientation from the single impression obtained in WA55; • a degree of scatter in the measured orientations in WA58, with the deeper data (below the Glen Munro seam) generally being clustered around an orientation just south of east and; • at greater than 100 m depth of cover, the orientation of the stress direction is more consistent, between 90° and 160°. Scrutiny of the hole locations with respect to existing mine workings (Figure 15-7) suggests the distinct possibility of the shallower measurements in WA69 having been influenced by proximity to workings, but probably hot so the deeper measurements in WA69R. The proximity of WA69 to the Triassic escarpment (Figure 15-8) is also likely to have influenced the measurements in WA69 but probably not in WA69R. The deeper measurements in WA58 are unlikely to have been influenced by the nearby Wambo and Whybrow seam workings 329 (Figure 15-7) and/or proximity to the escarpment. WA55 was located remotely from any likely extraneous influences. Apart from the above mentioned relationship between the measured stress field orientation in WA69/69R and the trend of the Redmanvale Fault, the remainder of the orientational data does not show any obvious general relationship to the local geological structure. Figure 15-9 summarises the orientational data with respect to the corresponding magnitude data for holes WA58 and WA55 combined (assuming WA69/69R to have possibly been influenced by local circumstances). What clearly emerges from Figure 15-9 is evidence of relatively higher stress magnitude around an orientation of 80° to 110°, independent of depth of measurement. Relatively lower stress magnitudes are also evident, scattered across the orientational spectrum. The predominant measured orientation can be placed in the regional context by reference to Figure 15-10 which summarises measurements made throughout the Hunter Valley (Enever et al, 1993) (Figure 15-3). The current Wambo data (free of local influences) appears to be consistent with a pervasive, approximately E-W, stress field orientation (from ENE to ESE) measured regionally throughout the Hunter Valley. The magnitude data from Table 15-2 is summarised in Figure 15-11, Figure 15-12 and Figure 15-13 for WA55, WA58 and WA69/69R respectively. In each case the most dubious results from Table 15-2 have been omitted, and the less reliable data based on fracture strength matching identified (open symbols). The locations of the measurements are shown with respect to the stratigraphic sequence in each case. No obvious pattern appears in the data when related to the relative stratigraphic location. There is, however, a broadly similar trend to the respective profiles with depth from the three holes, with the minor horizontal secondary principal stress generally varying.between the corresponding magnitude of the vertical (overburden) stress based on depth of cover, and approximately one and a half to twice this value (somewhat lower in WA55). As Figure 15-9 shows, the relatively higher stress magnitudes appear to correspond to an approximately E-W stress field orientation for that data postulated to not be influenced by local circumstances. Figure 15-14 is similar to Figure 15-9 but for the data from WA69/69R. Figure 15-14 suggests a somewhat different average stress field orientation to the remainder of the data, and a generally lower magnitude, possibly due to the impact of local effects. It should be noted, however, that in WA69R there were some tests where fracture initiation could not be achieved up to the pressure limit of the equipment, possibly reflecting some points of very high stress in the sequence. In Figure 15-15 and Figure 15-16, the magnitude data for all holes is combined for the minor and major horizontal secondary principal stress respectively. Viewed in this way, the 330 magnitude data follows a similar pattern to the rest of the Hunter Valley (Figure 15-17 and Figure 15-18), albeit with a somewhat lower limit to the upper bound trends of stress magnitude with depth for both stress components. 331 Table 15-1 Salient Data Hole/ Depth Po Pi Pr Shut-in Description of Fracture (m) (MPa) (MPa) (MPa) (MPa) Test WA55 #1 224.3 - 2.24 10.0 - 7.24 6.55-6.20-6.37 No impression** 224.72 12.4* #2 232.0 - 2.36 11.0 - 6.55 6.89-6.89-6.89 Near vertical fracture, rotating 232.42 13.1* at bottom, striking @ 74° mag #3 251.0 - 2.51 11.4 - 5.86 6.20-6.20-6.20 No impression ** 251.42 13.4 #4 275.0 - 2.75 11.7 - 10.0 9.65 - 9.30 - 9.30 No impression** 275.42 13.4* #6 286.5 - 2.87 16.2 - 10.70 10.0-10.0-10.00 No impression ** 286.92 19.3* WA58 #2 63.5- 0.64 6.9-9.0* 2.41 2.72 - 2.41 - 2.72 Steeply inclined fracture 63.92 striking @ 66° mag #4 84.5- 0.85 4.8-7.6* 3.10 2.41 - 2.04 - 2.04 Inclined fracture striking @ 84.92 354°mag #5 88.3- 0.89 6.9- 2.07 2.20 - 2.07 - 2.07 Near vertical crack rotating at 88.72 10.0* top and bottom, striking @ 10° mag #6 146.0- 1.46 12.1 - 5.86 6.03-5.17-5.17 One sided vertical fracture @ 146.42 13.8* 94° mag #7 151.0- 1.51 14.3- 5.51 4.82-7-4.31 One sided inclined fracture 151.42 16.4* striking @114° mag #8 156.5- 1.55 13.1 - ? 3.62-7-4.13 No impression 156.92 15.5* #9 159.5- 1.60 12.2- 5.51 4.13-4.13-4.13 Segmented sub vertical 159.92 14.5* fracture @ 330° mag #10 197.0- 1.97 14.1 - 8.96 6.55 - 6.55 - 7.6 Extensive vertical fracture @ 197.42 19.7* 98° mag #11 201.0 - 2.00 15.8 - 6.20 8.16-6.03-6.03 Extensive vertical fracture @ 201.42 17.0* 88° mag #12 212.0 - 2.12 18.3 - 10.34 10.00-7.92-8.26 Extensive vertical fracture @ 212.42 20.7* 80° mag #13 216.0 - 1.92 31.0 - 5.51 7-5.51-7 Rotating fracture 216.42 35.1* #14 219.5 - 2.07 18.6 - ? 7-10.7-6.9-7.9 No impression 219.92 22.1* #15 221.0 - 2.07 24.1 - ? 7-13.4-17.2-11.4 No impression 221.42 28.3* -10.3 #16 234.0 - 2.07 11.7 - 5.17 4.82 - 4.82 - 4.82 Horizontal fracture 234.42 15.2* #17 210.0 - 1.89 16.5 - 11. 7.92-10.3-10.3 Vertical fracture under top 210.42 20.0* 71 packer @ 96° mag 332 1 Me/ Depth Po Pi Pr Shut-in Description of Fracture . vSt (m) (MPa) (MPa) (MPa) (MPa) WA69 #1 140.0- 1.03 3.6-6.9* 2.41 1.53-2.41 -2.41 Breakout zone @ 52° mag 140.42 #2 143.0- 1.10 3.7-6.2* 2.76 2.76-2.41 -2.41 Extensive vertical fracture 143.42 0 332° mag + breakout zone @ 64° mag #3 118.0- 0.86 6.2- 2.07 2.07-1.73-1.73 Extensive vertical fracture 118.42 10.5* @ 304° mag #4 116.0- 0.86 6.4-9.5* 2.38 2.76 - 2.41 -2.41 Complex, near vertical 116.42 fractue striking @ 93° mag #5 240.0 - 2.14 17.9- 11.71 ? - 9.30 - 9.30 Short near vertical 240.42 21.4* fracture striking @ 102° mag #6 276.5 - 2.62 11.7- 6.55 8.61 - 7.58 - 7.58 Extensive vertical fracture 276.92 14.5* @ 120° mag #7 366.5 - 3.48 20.3- 15.64 7-15.16-15.16 No impression** 366.92 23.1* #7(a) 362.0 - 3.03 11.7- 10.34 9.65-10.0-10.3 No impression ** 362.42 15.5* Ti O 369.0 - 3.41 No fracture initiation to 42 MPa 369.42 #8(a) 384.0 - 3.24 10.2- 8.61 8.26 - 8.26 - 8.26 No impression ** 384.42 13.4* #9 418.5- 3.58 No fracture initiation to 42 MPa 418.92 #10 444.0 - 3.72 22.7- 19.98 18.30-18.6- No impression ** 444.42 27.2* 17.91 #11 477.0 - 4.0 No fracture initiation to 42 MPa 477.42 #12 481.5- 4.0 22.4 - ?* 11.71 7-11.7-11.7 Short segment of rotating 481.92 fracture of uncertain orientation #13 495.7 - 4.17 20.7- 13.09 13.43-13.43- Short segment of rotating 496.12 23.8* 13.43 fracture of uncertain orientation #14 505.5 - 4.41 24.8- 17.23 15.50-15.50- No impression** 505.92 28.9* 15.50 #15 520.5 - 4.58 25.8- 19.98 17.91 -18.3-18.3 No impression** 520.92 30.0* Packer pressure at fracture initiation * Diagnostics of pressure record suggest vertical fracture development _ 'ues of shut-in used to define aH 333 Table 15-2 Results of analysis Fracture Hole/ Depth °H Orientation Strength Test (m) (MPa) (MPa) of (MPa) WA55 #1 224.3 - 274.72 6.4 9.6 2.8 #2 232.0 - 232.42 6.9 11.8 86° 4.1 #3 251.0-251.42 6.2 10.2 - 5.5 #4 275.0 - 275.42 9.3 15.2** - 1.7 #6 286.5 - 286.92 10.0 16.4*** - 5.5 WA58 #2 63.5 - 63.92 2.7 5.1 78° 4.5 #4 84.5 - 84.92 2.4 3.3** 6° 1.7 #5 88.3 - 88.72 2.2 3.6*** 22° 4.8 #6 146.0-146.42 5.2 8.3 106° 6.2 #7 151.0-151.42 4.6 6.8** 126° 8.8 #8**** 156.5- 156.92 - - - - #9 159.5-159.92 4.1 5.2** 162° 6.7 #10 197.0-197.42 6.6 8.8 110° 5.1 #11 201.0-201.42 6.0 9.8** 100° 9.6 #12 212.0-212.42 8.1 11.9 92° 8.0 #13**** 216.0-216.42 - - - - #14**** 219.5-219.92 - - - - #15**** 221.0-221.42 - - - - #16**** 234.0 - 234.42 - - - - #17 210.0-210.42 10.3 17.3 108° 5.5 #1 140.0-140.42 2.4 154°* #2 143.0-143.42 2.4 3.3 164° 2.4 #3 118.0-118.42 1.7 . 2.1 136° 4.1 #4 116.0-116.42 2.4 3.9 105° 4.0 WA69/R #5 240.0 - 240.42 9.3 14.1 114° 8.9 #6 276.5 - 676.92 7.6 13.6 132° 5.1 #7 366.5-366.92 15.2 26.5 - 4.7 #7(a) 362.0 - 362.42 10.0 16.7** - 1.4 #8 369.0 - 369.42 - - - - #8(a) 384.0 - 384.42 8.3 13.1*** - 1.6 #9 418.5-418.92 - - - - #10 444.0 - 444.42 18.3 31.2** - 2.7 #11 477.0-477.42 - - - - #12 481.5-481.92 11.7 23.4 - 10.7 #13 495.7-496.12 13.4 22.9 - 7.6 #14 505.5 - 505.92 15.5 24.9 - 7.6 #15 520.5 - 520.92 18.3 30.3 - 5.8 Based on orientation of breakout ** Tentative result based on fracture strength match *** Dubious result based on fracture strength match **** Analysis not attempted due to uncertain data or horizontal fracture development 334 15.2 Coal Stress Measurements 15.2.1 Data and Analysis The Step Rate Tests conducted to measure the magnitude of the absolute minimum stress in the Arrowfield and Bowfield seams in WA55, WA58 and WA69 R are summarised in Appendix A. In each case the test was conducted on a whole of seam basis. The results are given in Table 15-3. 15.2.2 Discussion The results from Table 15-3 are presented graphically in Figure 15-19. Figure 15-19 suggests a similar trend with depth as has been found throughout the eastern coal basins (Enever et ai 2000), with minimum stress increasing systematically with depth above about 300 metres depth, and being approximately 50% of the corresponding overburden pressure. Below approximately 300 metres depth the minimum coal stress tends toward the corresponding overburden pressure as indicated by the single result in WA69R in Figure 15-19. Table 15-3 Results of Step Rate Tests Hole/Seam Depth Min. Stress (MPa) WA55 - Arrowfield 294.5-301.94 4.4 WA55 - Bowfield 311.06-318.5 4.9 WA58 - Arrowfield 221.56-229.0 3.4 WA58 - Bowfield 233.7-241.14 3.8 WA69R - Arrowfield 486.5 - 493.32 10.6 335 15.3 Summary The results of these measurements indicate that stress conditions when mining the Arrowfield and Bowfield seams at depth will be significantly higher than those encountered in most of currently operating mines in Australia. As a result, a relative high degree of strata deformation and fracturing during mining can be expected, and significant ground control will be required. The key results of the stress measurements are summarised as follows: • The orientation of the major horizontal principal stresses measured from WA55 (324m), WA58 (246m), WA69 (227m) and WA69R (541m) ranges from the east-west direction to north-west, south-east direction. These measurements suggest that the stress field orientation at the Wambo mine is consistent with those measured regionally throughout the Hunter Valley. • The magnitude of the major horizontal principal stresses measured generally varied between the corresponding magnitude of the vertical (overburden) stress based on the depth of cover, and approximately twice this value. At a depth of about 500 m, the major principal horizontal stress is high, ranging from 23 MPa to 30 MPa which is more than twice the approximate vertical stress (12.5 MPa). • The magnitude of the minimum effective stress (effective stress) in the Arrowfield and Bowfield seams increases systematically with depth. From surface level to about 300 m depth, the value of the stress is approximately 50% of the corresponding overburden pressure. Below 300 m depth, the value of the minimum effective stress in the coal approaches the corresponding overburden pressure as indicated by the measurement (10.5 MPa) at 500 m. This trend with depth has been found throughout the eastern coal basins. 15.4 References Enever JR, 1993. Comprehensive Rock Eng, 3, Rock Testing and Site Characterisation, Ch 24, Pergamon Press. Enever JR, Glen RA, and Beckett J, 1998. The Stress Field and Structural Environment of the Hunter Valley. Aust. Geomech. Soc. Newcastle Branch. Enever JR, Jeffrey R, and Casey D, 2000. The Relationship between Stress in Coal and Rock, Fourth North American Rock Mechanics Symposium. Enever, JR, and Wu B, 2000. Scale Effects in Hollow Cylinder Tests, 2nd Asian Geomechanics Conference, to be held in Beijing. 336 15.5 Figures Figure 15-1 Sample pressure record. 337 STRATA-TEK PTX LTD. Fracture Orientation Record 'H'Series Bcre Hole. FUt. n Viewed Down Hole. Figure 15-2 Sample impression record 338 Axial lead Test fluid injection. Spherical seats Figure 15-3 Laboratory test arrangement Orientation of Major Horizontal Stress All Boreholes Orientation (degrees M) 0 30 60 90 120 150 180 No valid results from below 276.71 metres Figure 15-6 Orientation of the major horizontal stress 342 1 5 o 10 ■ CD 0.09' <0 COB 8 £ v-07- C/3 ,N 0,06 O 0U5' x o 0.04' nT 0.03 t3 Figure 15-9 Stress field orientation v magnitude for WAS5 and WA58 344 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 0 90 180 Orientation of Major Horiz. Stress (deg) Figure 15-14 Stress field orientation v magnitude for WA69 / 69R nominal \ overburden SG>=E.5 Mag. of Minor Horiz. Stress (MPa) Figure 15-15 Summary of minor horizontal stress magnitude for all data combined -400 ~ •500 - • nominal \ overburden SG.=aL5 -600 Mag. of Major Horiz. Stress (MPa) Figure 15-16 Summary of major horizontal stress magnitude for all data combined ■ 1 M - -300 - overburden SilaiLS Mag. ol Minor Horiz. Stress (MPa) Figure 15-17 Summary of regional data - minor horizontal stress Figure 15-18 Summary of regional data - major horizontal stress nominal overburden SG=2.5 -200- * Coal Depth 69fR (m> * Coal Depth 55 (m) •300- WA 55 A Coal Depth 58 (m) -400 - 50% of overburden Min. Stress (MPa) Figure 15-19 Summary of step rate test results C. J-" 21 351 16 GAS AND ROCK BEHAVIOUR SIMULATION SOFTWARE 16.1 Background Underground coal mining by full extraction methods, such as longwall mining involves extracting coal in large panels. The panel extents are typically 200-300 m wide, up to 3 km long and up to 3 m in height. In this method, roof rock behind the advancing face is allowed to collapse in the mine void (goaf). This will induce rock fracturing in the strata overlying the coal seam resulting in roof caving. This, in turn, may result in a strongly heterogeneous and anisotropic hydraulic field and may, as well, induce large pore fluid pressures due to the transient nature of the rock deformations. Both of these factors will have an impact on rock deformation and fluid-gas flow behaviour. In order to be able to simulate such a process, first it is important that fluid and gas flow and rock mass mechanical behaviour is fully coupled and secondly the change in fluid conductivity due to mining induced rock deformation is properly incorporated. Biot (1941) first introduced a fully coupled mechanical-fluid flow poro-elasticity theory. He recognised the fact that the change in pore fluid pressure resulting from pore volume change will also impact on the deformation of solid phase. Biot ’s theory has since been used by many researchers in one or another form for simulating the coupled mechanical flow problems. 16.2 The trend in model development A coal seam and surrounding coal strata are porous in nature and often contain distinctive fracture systems. A common feature of coal strata is the presence of both a persistent solid matrix and a void space. So gas and water flow in these rocks takes place through a complex network of pores and channels comprising the void space. This flow is bounded by the solid-water interface in the microscopic level. In principle, the flow of gas and water in a porous medium may be treated at a microscopic level. However, this approach is usually impractical due to our inability to describe the complex configuration of this solid-water interface within the porous medium. In order to overcome these difficulties, another level of description is needed. This is the macroscopic level, at which quantities can be measured and boundary-value problems can be solved. To obtain a description of the flow at this level, one adopts the continuum approach. Hence in this approach, the fluid flow problem is solved for porous media utilising average flow properties. At the present, there are two distinctive class of flow models: 352 1) Single porosity models based on Biot ’s theory of poro-elasticity - these models utilise average flow properties of the formation incorporating both pores and fissures. 2) Double porosity models - in these models, rock mass is treated as a porous medium with well developed fissure/fracture networks. 16.2.1 Single Porosity Model Henry Darcy (1856) derived the first simplified equation for fluid flow through a porous medium. His formulation, now known as Darcy ’s law, is used in almost all fluid flow calculations. Karl Terzaghi first introduced the fluid-flow-stress coupling problem in 1924. His one-dimensional consolidation theory has been widely used in practice to calculate ground settlement. Since then, Biot (1941, 1955 and 1956) has made a substantial contribution in advancing the science of fluid-flow-stress coupling process by introducing a more general three-dimensional case, based on a linear stress-strain constitutive relationship and Darcy’ s flow law. At the present, Biot ’s theory is widely used in coupled flow problems. In this approach, the flow of fluid and the deformation of the porous solid are continuously interrelated through the concept of effective stress. Accordingly, a simultaneous solution is sought for two dependent variables: pressure in the fluid and deformation of the solid matrix. Since the early 1970 ’s, the use of numerical simulation has gained wide acceptance throughout most of the major industries, and in particular the petroleum and civil engineering branches. A book titled “Flow through porous media ” and edited by Roger J.M. De West (1969) covers a broad aspects of fluid flow theory such as fundamental principles of groundwater flow, porosity and hydraulic conductivity of porous medium, saturated and unsaturated flow etc. A book titled “Modelling groundwater flow and pollution ” authored by Jacob Bear and Arnold Verruijt (1987) covers the principle of simulating fluid flow through porous media including the mechanical coupling process. Roland W. Lewis and Bernard A. Schrefler (1987) describes in detail the aspects of numerical modelling of multi-phase fluid flow and rock deformation problems. 16.2.2 Double Porosity Model Barrenblatt et al (1960) for the first time employed the concept of double porosity to present a model for simulating flow through rigid, fissured porous media. Since then this approach has been used in oil reservoir simulation problems. In these models, a fractured porous medium is simulated as an entity having two porosities; one representing a fracture network and another a continuum porous rock. Thus in this approach, the flow behaviour is mainly described by the interaction of the basic components, namely the porous medium and the 353 surrounding fracture system. Mechanistically, the fractures provide rapid hydraulic connection but little fluid mass storage, whereas the porous medium represents high storage but low hydraulic connection. In this approach, the basic mechanism of fluid flow in a fractured porous medium can be described as follows. The imposed external loads, for example, mining induced stresses, may create a pressure gradient between the fluid within the matrix pores and the fluid in the adjacent fractures. Once such a pressure gradient is created, fluid may flow from the pores towards the fractures and vice versa depending upon the stress induced fluid pressure gradient. The double porosity approach has been extensively developed to represent both single phase and multiphase flow in petroleum reservoirs. The theory presented by Aifantis (1977, 1980) and Khaled et al (1984) provides a suitable framework in which the flow-deformation behaviour of dual-porosity media may be fully coupled to the deformation field as a multiphase continuum. Derek Elsworth of Pennsylvania State University, USA and Roland W Lewis of University of Wales, Swansea, UK has published a number of research papers in this field. 16.2.3 Hydraulic conductivity Hydraulic conductivity of a porous medium is either derived by field measurements or through theoretical/empirical formulations. There are different formulae proposed in the literature for estimating the hydraulic conductivity of porous medium depending upon whether the porous medium is intact or contains a network of fractures. In any simulation exercise, it is not only important to estimate the hydraulic conductivity correctly, but also very important to compute its possible variation induced by mining. It is widely recognised that mining induced deformation may considerably alter the hydraulic conductivity a porous rock. However, most of the researchers seem to correlate the variation in hydraulic conductivity as a sole function of the variation in rock porosity. The group in Pennsylvania State University (Derek Elsworth et al) has taken up a different approach such that the change in hydraulic conductivity of a porous rock is computed as a function of the mine induced strain. Such a derivation posses a definitive appeal as far as mine induced stress-flow simulation is concerned. Depending upon whether the porous rock is modelled as an intact porous rock with uniformly distributed solid matrix and void space or as a fractured porous rock with a uniformly distributed fracture network, they have proposed two distinctive formulations for estimating the change in hydraulic conductivity as a function of mine induced deformation. 354 16.3 Practical Applications There exists a very large number of published papers on coupled mechanical and fluid flow problems. Most of these papers deal only with the theoretical formulation of the problem. The papers, that cite mine induced stress-flow problems, are limited to a few. Among them, the majority of the papers are found to be published by one group based at the Pennsylvania State University. As almost all the papers, published by the group from the Pennsylvania State University, report the data from one mining site, it is not feasible to draw general conclusions due to the lack of a broad database. The earliest reported work on this subject appears to be the one by Neate et al (1979). They described a field study carried out to investigate changes in in situ hydraulic conductivity and ground water conditions due to longwall mining under the North Sea in UK. They observed that the change in hydraulic conductivity due to mining varied spatially i.e. both vertically and horizontally. They reported a difference in absolute change in hydraulic conductivity measured ahead of the central part of the face and that ahead of the face-ends. They reported that the degree of change in hydraulic conductivity was very much dependent on the type of strata and its position relative to the face centre line. Ouyang and Elsworth (1993) and Bai and Elsworth (1994) formulated a coupled one-phase fluid flow mechanical model that takes into consideration the change in rock mass hydraulic conductivity due to mining induced stresses. They performed simple elastic stress calculations coupled with transient pore pressure distribution in order to predict surface subsidence and change in hydraulic conductivity of the strata over a longwall mining panel located in West Virginia. Ouyang and Elsworth (1993) in their study represented the porous rock as a continuum with a well-distributed fracture network. This assumption allowed them to represent the porous rock by an equivalent continuum with anisotropic hydraulic conductivity controlled by fracture spacing and mean aperture. The essence of such an idealization of a porous medium is that the change in mining induced strain may be readily incorporated in calculating the corresponding hydraulic conductivity changes. Bai and Elsworth (1994) also utilized the above approach proposed by Ouyang and Elsworth (1993). In addition, Bai and Elsworth (1994) further explored an option of introducing mine- induced change in hydraulic conductivity in intact porous media. For this, they replaced the porous medium by a medium comprised of regularly packed spherical grains of uniform size such that a change in the hydraulic conductivity may be expressed through the variation in grain size or pore space due to the effect of changing intergranular stresses. Liu and Elsworth (1997) adopted the approach proposed by Ouyang and Elsworth (1993) and used a 355 three dimensional model to study the water flow patterns in the same mine located in West Virginia. Valliappan and Wohua (1996) formulated a coupled model for describing the methane gas flow behaviour in a deforming coal seam. The study takes into account the effect of diffusion of absorbed methane gas from the solid matrix to the voids space by considering a two- phase gas flow model. On the basis of their numerical calculations they put forward explanations for gas outbursts. Choi et al (1997) described a study carried out to investigate the impact of longwall mining of the Bull! Seam on gas and water flow patterns at Appin Colliery in the NSW Southern Coalfield. In their study, they used reservoir models derived from the petroleum industry and coupled mechanical-fluid flow models derived from mining geomechanics applications. With these tools they identified that the major causes of excess gas emissions were: a) destressing of the lower seams, b) drop in reservoir pressure in the lower seam due to destressing and c) increased hydraulic conductivity of existing faults due to mining. 16.4 Software for Simulation of Gas and Rock Behaviours To achieve the aim of developing an efficient integrated simulation method capable of predicting the transient process of ground deformation and subsequent gas emission during mining, firstly a review of existing software and modelling methods was carried out. Existing commercially available computer codes can be grouped into following three categories: • Empirical/semi-empirical methods • Flow simulators (Uncoupled with mechanical deformation) • Coupled Mechanical - Flow Simulators 16.4.1 Empirical/Semi-Empirical Approach In these codes, gas emission into roadway excavations and on longwall faces and pillar panels is estimated using empirical or semi-empirical methods based upon historical measurements in mines. Flow of gas into roadway excavation may be radial or linear depending upon the shape of the excavation. Field observations have shown that gas emission from a heading at a constant rate of advance can be approximated by an exponential function ( dV/dt = aVt )or a power function (V = At"). The constant a is dependent on local conditions and can be measured over a period of time for gas emission from a heading advancing at a fixed rate. 356 Gas emission from longwall faces and pillar panels comes from the seam being mined as well as from the surrounding medium, which may contain very large amounts of gas particularly in surrounding seams. Several empirical methods have been developed to estimate gas emission into mine workings when longwall systems are used. For emission of gas from the worked seam most methods are independent of advance rate and are based on the assumption that 100% of gas from the seam being mined is emitted. Emission from adjacent seams is estimated in the range of 0 to 100% depending upon the definition of emission zone and method used. The accuracy of these methods largely depends upon localised caving characteristics in relation to geological and geotechnical conditions. These empirical/semi-empirical methods have been widely used all over the world for mine planning, gas control and ventilation design and will continue to play a significant role. However, there are some drawbacks using empirical/semi-empirical approach for gas emission estimation. These include: reliance on measurements made during mine development and operation, gas emission estimation is static, not real-time and dynamic and; consideration of caving characteristics is generally limited due to the lack of detailed modelling techniques of the extent and nature of relaxation and fractures of the surrounding strata. Further research and development is required to predict gas emission prior to mine development and operation, particularly, in a new mining environment where no previous experience or limited data are available. 16.4.2 Flow simulators (Uncoupled with mechanical deformation) There are a number of coal seam gas simulators available. These simulators are distinguished by their underlying physics, boundary conditions, grids and other features. To illustrate current capability of these flow simulators, two commonly used flow simulators, COALGAS and COMETS are briefly described in this section. Both of them are comprehensive and commercially available. COALGAS is a three-dimensional, two-phase, dual-porosity reservoir simulator. It is designed to model flow of gas and water in coal seam gas reservoir and dual-porosity reservoir. COALGAS numerically models the processes that control the behaviour of coal seam gas reservoir: Darcy flow, Fickian diffusion and gas sorption (Langmuir isotherms). COALGAS model accounts for pressure dependent porosity and permeability, and saturation dependent capillary pressure and relative permeability. It should be noted that COALGAS could not handle binary gas sorption. COMETS is another three-dimensional, two-phase, fully implicit, finite-difference fractured reservoir simulator. COMETS treats the release and transport mechanisms of desorption, diffusion and Darcy flow through a dual-porosity/single permeability network in coal seam 357 (matrix and fractures). For gas-bearing shale, it models the process of the release of transport mechanisms through triple porosity/dual permeability network. COMETS defines the non-linear relationship between free and adsorbed multi-component gas mixtures (methane-carbon dioxide and methane nitrogen) as a function of methane concentration using extended Langmuir isotherms. COMETS can model fault blocks, dipping reservoirs, layed systems, and stress-sensitive permeability and porosity. The model also accounts for saturation dependent capillary pressure and relative permeability. In terms of its capabilities of simulating gas flow in various mining operations such as roadway development, gas drainage and longwall operation, COMET3 is more comprehensive and realistic because of its underlying multi-component seam gas approach. 16.4.3 Coupled Mechanical - Flow Simulators The number of research papers published in this topic is extensive. However, when dealing with a coupled mechanical and multiphase flow problem, the number of commercially available codes is small. CSIRO has a license for the Itasca codes - UDEC and FLAG. Both of these codes are only two-dimensional. UDEC handles the coupled mechanical and single phase flow problems for incompressible fluid. This code is well suited for simulating the flow through fissures and joints. FLAG handles coupled mechanical and two-phase flow problems. This code is based on the Biot ’s theory of poro-elasticity and handles two-phase flow in a single porosity medium. However, the codes do not update the change in fluid flow properties associated with mechanical deformation, hence such a formulation needs to be added separately. ABACUS - a general purpose three dimensional finite element package developed by Hibbit, Karlsson and Sorensen, inc. incorporates an option for conducting coupled mechanical and two phase fluid flow calculations. However, its two-phase calculation is rather simplified by assuming that the non-wetting phase (i.e. gas) does not contribute to pore pressure such that the pore pressure change is only generated by wetting fluid (i.e. water). ELFEN - a three dimensional package marketed by Rockfield Software Ltd. also handles coupled mechanical and multiphase flow problems. It has recently appeared in the market and hence, at this stage, it is hard to judge its applicability in solving real mining problems. A group at the University of Swansea, UK have developed a three dimensional finite element model for simulating coupled mechanical and multiphase flow problems. At this stage we have initiated a dialogue with them and are trying to assess the features of their code, such as, capacity to handle material non-linearity (plasticity) and stage excavation problems problem solution schemes and user-friendliness. 358 CSIRO has been developing a three-dimensional finite element computer code called “PartCoss ” for stress-strain analysis. This code has been developed specially for simulating the behaviour of bedded rock strata, which is normally encountered in the coal-mining environment. The code is developed on the basis of Cosserat continuum theory, which describes the behaviour of bedded (discontinuum) rock strata in a continuum fashion. In order to be able to model coupled mechanical and flow problems within a coal mining environment, it is necessary that both mechanical and flow process be simulated appropriately. The Cosserat continuum formulation has a major advantage over conventional continuum models in that it can efficiently handle rock breakage and slip as well as separation along the bedding planes. Any opening/closure along the bedding plane may introduce a strong anisotropy in fluid flow properties of a porous medium. This, in turn, will impact on fluid/gas flow behaviour of the porous medium. Thus, it is imperative that such a process be modelled correctly in analysing the mine induced rock deformation and subsequent changes in gas/fluid flow patterns. 16.5 Proposed Integrated Simulation Strategy To be able to predict strata behaviour and associated gas emission during mining, and to assess performance of gas control measures, a reliable integrated simulation method must have the capability to accurately determine: • mining induced rock deformation, fractures and subsequent changes in reservoir pressure, permeability, and gas/fluid flow patterns, and • gas emission as a result of mining, and effectiveness of gas drainage measures. The review of existing software has indicated that none of them can fully simulate coupled mining induced mechanical and multi-phase problems. To achieve the aim of an efficient and integrated simulation, it will be necessary to: • further develop the current CSIRO software PartCoss into a three-dimensional fully coupled mechanical and two-phase flow model that can provide a reliable prediction of mining induced changes in strata conditions, reservoir pressure and properties; and • develop an interface so that the reservoir property change can be imported to the existing commercial softwares such as COALGAS and COMETS to simulate the production and control of gas and/or gases during mining as shown in Figure 16-1. 359 16.6 Summary There are a large number of published papers on coupled mechanical and fluid flow problems. Most of these papers deal only with the theoretical formulation of the problem. The papers, that cite mine induced stress-flow problems, are limited. At the present, there are following two distinctive class of flow models: a) single porosity models based on Biot ’s theory of poro-elasticity - these models utilise average flow properties of the formation incorporating both pores and fissures and b) double porosity models - in these models, rock mass is treated as a porous medium with well developed fissure/fracture networks. There are a number of coal seam gas simulators available such as COALGAS and COMETS. These codes are distinguished by their underlying formulation specifically aimed at simulating the gas flow behaviour in coal seams. However, these codes treat the rock matrix as an elastic entity and only account for the effect of pore pressure in updating the rock matrix deformation. Hence rock deformation coupling is not fully realised as these codes do not consider rock fracturing and its subsequent effects in fluid flow behaviour. The number of commercially available code suitable for solving a proper coupled mechanical and multiphase flow problem is rather very slim. UDEC, a two dimensional code, handles the coupled mechanical and single-phase flow problems for incompressible fluid. This code is well suited for simulating the flow through fissures and joints. FLAG, a two dimensional code, handles coupled mechanical and two-phase flow problems in a single porosity medium. There are some three dimensional commercial codes such as ABACUS, ELFEN, however, they also lack in proper modelling capability a*s outlined above. It is recommended to: • further develop the current CSIRO software PartCoss into a three-dimensional fully coupled mechanical and two-phase flow model that can provide a reliable prediction of mining induced changes in strata conditions, reservoir pressure and properties and; • develop an interface so that the reservoir property change can be imported to the existing commercial softwares such as COALGAS and COMETS to simulate the production and control of gas and/or gases during mining as shown in Figure 16-1. 361 16.7 Bibliography Mantis, E.C. (1977). Introducing a multi-porous medium, Developments in Mechanics, Vol. 8, pp. 209-211. Mantis, E.C. (1980). On the problem of diffusion in solids, Acta Mechanica, Vol. 37, pp. 265-297. Bai., M. and D. Elsworth (1993). Transient poroelastic response of equivalent porous media over a mining panel, Engng. Geology, Vol. 35, pp. 49-64. Bai., M. and D. Elsworth (1994). Modelling of subsidence and stress dependent hydraulic conductivity for intact and fractured porous media, Rock Mech. Rock Engng., Vol. 27 (4), pp. 209-234. Bai., M., F. Meng, D. Elsworth, Y. Abousleiman and J.-C. Rogiers (1999). Numerical modelling of coupled flow and deformation in fractured rock specimens, Int. J. Numer. Anal. Meth. Geomech., Vol. 23, pp. 141-160. Bear J. and A. Verruijt (1987). Modelling groundwater flow and pollution, D. Reidel Publishing Co., Boston, pages 414. Choi, S.K., M.B. Wold and J. Wood (1997). Modelling of interburden gas flows at Appin Colliery, Symposium on Safety in Mines: The Role of Geology, 24-25 November, Doyle et al. (eds), pp. 105-117. De West, R.J.M. (1969). Flow through porous media, Academic Press, New York, pages 530. Detournay, E. and A. H.-D. Cheng, Fundamentals of poroelasticity, Comp. Rock Engng., Fairhurst (ed), Vol. 2, pp. 113-172. Elsworth, D. (1992). Flow-deformation response of dual-porosity media, ASCE J. Geotech. Engng., Vol. 118 (1), pp. 107-124. Elsworth, D. (1993). Computational Methods in Fluid Flow, Comp. Rock Engng., Fairhurst (ed), Vol. 2, pp. 173-189. Elsworth, D., J. Liu and Z. Ouyang (1994). Some approaches to determine the potential influence of longwall mining on ground water resources, Proc. Int. Land Reclamation and Mine Drainage Conference, Pittsburgh, April, Vol. 4, pp. 172-179. Ezzedine, S. (1996). A discrete fracture double porosity model, Rock Mechanics, Hassani & Mitri (eds), Balkema, Rotterdam, pp. 1399-1406. 362 Ghafouri, H.R. and R.W. Lewis (1996). A finite element double porosity model for heterogeneous deformable porous media, Int. J. Numer. Anal. Meth. Geomech., Vol 20, pp. 6831-844. Khaled, M.Y., D.E. Beskos, and E.C. Aifantis (1984). On the theory of consolidation with double porosity - III, a finite element formulation, Int. J. Numer. Anal. Meth. Geomech., Vol 8, pp. 101-123. Lewis, R.W. and B. A. Schrefler (1987). The finite element method in the deformation and consolidation of porous media, John Willey & Sons, New York, pages 344. Lewis, R.W. and H.R. Ghafouri (1997). A novel finite element double porosity model for multiphase flow through deformable fractured porous media, Int. J. Numer. Anal. Meth. Geomech., Vol 21, pp. 789-816. Lewis, R.W. and Y. Sukirman (1993). Finite element modelling of the three-phase flow in deforming saturated oil reservoirs, Int. J. Numer. Anal. Meth. Geomech., Vol 17, pp. 577-598. Liu, J. and D. Elsworth (1997). Three-dimensional effects of hydraulic conductivity enhancement and desaturation around mined panels, Int. J. Rock. Mech. Min. Sci, Vol. 34 (8), pp. 1139-1152. Matetic, R.J., J. Liu and D. Elsworth (1995). Modelling the effects of longwall mining on the ground water system, Rock Mechanics, Daemen & Schultz (eds), Balkema, Rotterdam, pp. 639-644. Matetic, R.J., M. A. Trevits, and T. Swinehart (1991). A case study of longwall mining and near-surface hydrological response, Proc. American Min. Congr. Coal Convention, Pittsburgh, PA, June 2-5, pp. 445-472. Neate C.J. and B.N. Whittaker (1979). Influence of proximity of longwall mining on strata permeability and ground water, 20th US Symp. Rock Mech., Austin, Texas, June 4-6, pp. 217-223. Ouyang, Z. and D. Elsworth (1993). Evaluation of groundwater flow into mined panels, Int. J. of Rock Mech. Min. Sci. Geomech. Abstr, Vol. 30 (2), pp. 71-79. Stoner, J.D. (1983). Probable hydrologic effects of subsurface mining, Ground Water Monitoring Rev., Winter, pp. 128-138. Sukirman, Y. and R.W. Lewis (1993). A finite element solution of a fully coupled implicit formulation for reservoir simulation, Int. J. Numer. Anal. Meth. Geomech., Vol 17, pp. 677- 698. Valliappan, S. and Z. Wohua (1996). Numerical modelling of methane gas migration in dry coal seams, Int. J. Numer. Anal. Meth. Geomech., Vol 20, pp. 571-593. 363 17 DISCUSSION The implications of the project findings to date on future mining in the region are briefly discussed here, in particular, in relation to the research work required for the remainder of the project. It is recommended that detailed assessment of potential mining and gas conditions be carried out during this project. The results of the research work have highlighted that future deep coal mining in the Hunter Valley Coalfield will face significant economical, technical and environmental challenges. The challenges are a direct result of the deep, gassy, multi-seam environment, characterised by low permeability, high in situ stress, and complex geology. The potential impact of these difficult conditions is that mining costs with current mining technology could sharply increase for a large amount of future deep coal reserves, in one of the largest coal producing areas in Australia. In extreme cases, these difficult conditions may render the recovery of some of these reserves uneconomic. There are also significant opportunities for integrated coal mining and gas utilisation operations in the region that could profit from both coal and mine gas production, and reduce the environmental impacts. To capture the opportunities, development and application of advanced technologies will be essential in gas drainage and utilisation, predictive simulation of ground condition and gas emission, and mining and gas drainage/control optimisation. 17.1 Stress level at depths The magnitude of the major horizontal principal stresses measured in rocks at the Wambo mine generally varied between the corresponding magnitude of the vertical (overburden) stress based on the depth of cover, and approximately twice this value. At a depth of about 500 m, the major principal horizontal stress is high, ranging from 23 MPa to 30 MPa, which is more than twice the approximate vertical stress (12.5 MPa). The pattern of stress magnitude with depth observed at the Wambo mine is similar to the rest of the Hunter Valley Coalfield (Enever et al, 1998). The magnitude of the minimum effective stress (effective stress) in the Arrowfield and Bowfield seams increases with depth. From surface level to about 300 m depth, the value of the stress is approximately 50% of the corresponding overburden pressure. Below 300 m, the value of the minimum effective stress in the coal approaches the corresponding overburden pressure as indicated by the measurement of 10.5 MPa at 500 m. Again, this trend with depth has also been found throughout the eastern coal basins in Australia (Enever et al, 2000). 364 The results of these measurements indicate that stress level in the coal seams and rock strata at depths will be significantly higher than those encountered in most of currently operating mines in Australia. Some of key effects of the high stress conditions on mining are discussed as follows: At these measured in situ stress levels, the mining induced stress could easily reach or exceed the mass strengths of the coal and rocks, at and near the surfaces of the excavations, such as the roof, floor and the rib of the pillars. A relatively high degree of strata deformation and fracturing during mining can be expected, and significant and sophisticated ground control will be required. This condition will lead to greater levels of strata support and reinforcement, reduce roadway development rates and cause an increase in the amount of coal sterilised within pillars. A marked decrease in permeability is observed with increasing effective stress (associated with depth of cover) for both the Arrowfield and Bowfield seams. The sensitivity of permeability to the effective stress has also been observed from well testing in various areas of Australia ’s major coalfields (Enever et al, 2000). This relationship not only predicts a significant decrease of permeability in coal with depth of cover, but also highlights the critical role of the stress (in situ and mining induced) in coal seams for gas recovery and emission assessment in Australia. 17.2 Permeabilities and Gas Contents of Arrowfield and Bowfield seams at Wambo mine Reservoir characterisation of the Arrowfield and Bowfield seams within the study area at the Wambo mine has been conducted based on all available data at the time of this study. The seams are characterised by: • Moderate to high gas content in both the Arrowfield and Bowfield seams. Gas content increasing with depth (up to 12m3/t); • Moderate permeability measured at around 230 m depth. Low to very low permeability at and below 300 m depth of cover, permeability sharply decreasing with increasing effective stress (depth) in coal (as low as 0.005 mD). Permeability also influenced by cleat infill; • Carbon dioxide constituting a significant portion of seam gas, its portion increasing with depth (up to 50%) and; • The Arrowfield and Bowfield seams being closely spaced (about 10 to 30 m) in the area of the study. 365 These characteristics will pose some major challenges for mine development and operation, in the deeper part of the Wambo mine (below 300 m depth of cover), in the context of mine gas control and gas drainage. The high gas contents of the seams at depths highlight the need to substantially drain seam gas prior to mine development and operation, in order to control gas emission and potential gas/coal outbursts. The low in situ permeability measured at depth indicates that current pre-drainage methods with surface holes will probably not be viable particularly at cover depths greater than 300- 500 m. Without effective surface drainage techniques, costly full scale in mine post-drainage system and associated infrastructure will be required. As there are a number of closely spaced coal seams at depth, the overlying and underlying coal seams are likely to release a significant amount of gas into the workings during mining. This will increase the drainage needs and costs of cross measure and in-seam drainage methods. The costs of goaf drainage with surface holes will increase significantly as the future mining depth will be at least twice the current mining depths. Effective post-drainage will require a clear understanding of gas flow mechanisms, a reliable prediction of gas emission during mining, and further development of current pre-drainage and post-drainage techniques. 17.3 Geological and geotechnical complexity Geological, structural and geotechnical characterisation and modelling were undertaken this year. There are a set of significant structures such as the NS Dyke, the NE-trending thrust faults east of the Dyke, and the NE trending thrust faults, west of the NS Dyke. These structures may have important effects on gas emission and ground conditions during mining. The roof and floor geological and geotechnical conditions have been found to vary significantly across the area of the study. The effects of these structures and changes in strata conditions on mining may take the form of directly posing mining difficulties, for example, the expected strong (hard) section of the NS dyke at the Arrowfield and Bowfield seams and fractured strata within or near the faulted areas. The structures and spatial variation of strata conditions can also affect the mining induced stress field, strata caving, fracturing and deformation characteristics, and gas and water flow mechanisms and patterns. The experiences from some deep longwall mines in Australia have highlighted the importance of understanding the geological structures and geotechnical environment, and mining induced strata fractures and reservoir pressure changes to gas control (Ogden and Wood, 1996 and Choi et al, 1997). Geological structures may enhance or retard water and 366 gas flows depending on the nature and characteristics of the structures. Production delays can be caused by excessive gas emissions occurring along faults, joints or other weak strata. Effects of the local structures and strata conditions at the Wambo mine on future mining and gas emission need to be examined in detail. 17.4 Recovery and utilisation technologies In an environment of deep, gassy, multi-seams with low permeability such as that of the Arrowfield and Bowfield seams, current pre-drainage methods with surface holes will probably not be viable. It is important to develop and/or apply new and effective pre-drainage methods to substantially drain seam gas prior to mine development and operation. Research and development work have been undertaken in the recent years in Australia, to develop advanced technologies such as longhole in-seam drilling from surface and tight radius drilling with water jet. A new approach with radial mechanical drilling from a vertical hole has also been considered by CSIRO Exploration and Mining. Applicability of these new technologies needs to be examined in the above conditions Reliable and low cost post-drainage will be required to combat the gas emission from the overlying/underlying seam during mining. Effectiveness and cost of in-seam borehole and surface goaf well drainage methods need to be examined in the above conditions. A review of potential utilisation options of mine methane by CSIRO Exploration and Mining indicated that electricity and mine water management are two favoured options. Several technologies exist to potentially convert the mine gas into electricity: directly in gas reciprocating engines, as power station air, directly in gas turbines, or as part of a co-fired coal and gas fired turbine. Successful development and/or application of advanced utilisation technology will reduce the mining cost and environmental impacts. 17.5 Key technical issues and tasks Longwall mining in conditions such as those of the Arrowfield and Bowfield seams will present a new set of challenges. It is imperative that ground conditions and gas emission characteristics in this environment are accurately predicted, and cost-effective drainage technology is developed and applied to ensure coal mining remains economical. These issues will also be critically important for other key coal mining areas in Australia, as future mining progressively goes deeper. Given the importance and potential impact of the issues that this project is addressing on the future of coal production in Australia, it is recommended that, for the remainder of the project, the following tasks need to be carried out: 367 • Evaluation and development of new pre-drainage methods; • Development of predictive mining and gas simulation methods; • Post-drainage performance study of existing analogue sites; • Completion of assessment and evaluation of key mining and gas issues at the Wambo mine site as an example and; • Development and application of advanced utilisation technology to reduce the mining cost and environmental impacts. The recommended project work for the next year is illustrated in Figure 17-1. One of the key objectives of the next year work is to develop an integrated simulation method to predict strata behaviour and associated gas emission during mining, and to assess performance of gas control measures. Specifically, the integrated simulation method needs to accurately determine: • Mining induced rock deformation, fractures and subsequent changes in reservoir pressure, permeability, and gas/fluid flow patterns and; • Gas emission as a result of mining, and effectiveness of gas drainage measures. Existing commercial software such as COALGAS and COMETS are comprehensive in simulating gas desorption, diffusion and flow. However, no commercial software that has been reviewed fully couples gas emission processes with mining induced mechanical changes. To achieve the aim of an efficient and integrated simulation and take advantage of the existing commercial software, it will be necessary to: • Further develop the current CSIRO sbftware PartCoss into a three-dimensional fully coupled mechanical and two-phase flow model that can provide a reliable prediction of mining induced changes in strata conditions, reservoir pressure and properties and • Develop an interface so that the reservoir property change can be exported to the existing commercial software such as COALGAS and COMETS to simulate the production and control of gas and/or gases during mining. While the above simulation methods are being developed, it will be essential to carry out geotechnical and gas investigations at analogue sites with similar conditions in Australia. The investigations should obtain information in relation to strata caving, fracturing and deformation characteristics, and gas emission and mechanisms during mining. The simulation methods should then be validated and calibrated with the actual data from the analogue sites before being applied to the Arrowfield and Bowfield seams at the Wambo mine. 368 Additional gas and geotechnical data have been acquired while this report was being written. These data need to be analysed and the results need to incorporated into the gas and geological and geotechnical models. Evaluation of the following new pre-drainage technologies and concepts needs to be carried out for their applicability: • CSIRO’s mechanical radial drilling from vertical holes; • tight radius drilling with water jet and; • longhole in-seam surface drilling. Specific evaluation of potential utilisation methods needs to be undertaken in relation to use of drained gas and ventilation air with/without supplementary fuel. 17.6 References Choi, SK, MB Wold and J Wood, 1997. Modelling of interburden gas flows at Appin Colliery, Symposium on Safety in Mines: The Role of Geology, 24-25 November, Doyle et al. (eds), p 105-117. Enever JR, Glen RA, and Beckett J, 1998. The Stress Field and Structural Environment of the Hunter Valley. Aust. Geomech. Soc. Newcastle Branch. Enever JR, Jeffrey R, and Casey D, 2000. The Relationship between Stress in Coal and Rock, Fourth North American Rock Mechanics Symposium. Ogden AN and Wood J, 1996. Geological factors influencing longwall gas emissions at Appin Colliery. Geology in Longwall Mining, McNally GH and Ward CR, (eds), p 47-54. 369 371 APPENDIX A In Situ Stress Data Analysis J. Enever CSIRO Petroleum A ROCK STRESS MEASUREMENTS A. 1 Data Selection The pressure records for all tests successfully completed in the four holes tested (WA55, WA58, WA69, WA69R) are contained in Appendix 1, the corresponding impressions, where available, in Appendix 2. Generally, the pressure records were similar and consistent with the simplest understanding of hydraulic fracturing (Enever, 1993). The available impression records and pressure record diagnostics (Enever, 1993) indicated predominantly vertical or near vertical fracture development allowing for ready interpretation of the horizontal secondary principal stress field. The methods used to interpret the salient data required for analysis from the records are illustrated in Figures 1 and 2. The salient data is summarised in Table 1. A.2 Data analysis The following procedure was employed for analysis of the horizontal stress field: • use of fracture orientation to directly estimate the orientation of the major horizontal secondary principal stress (On), • use of shut-in pressure to directly estimate the magnitude of the minor horizontal secondary principal stress (Oh), use of the following expression(Enever, 1993) to estimate the magnitude of Oh, Pr is the crack re-opening pressure (Figure 1) and Po the ambient pore pressure derived from the standing water level in the hole. The results of analysis are summarised in Table 2. Also included in Table 2 are estimates of fracture strength made from the pressure records (Fracture Strength = Fracture Initiation Pressure (Pi) - Fracture Re-opening Pressure (Pr)). A program of laboratory testing was undertaken, as illustrated in Figure 3, to measure fracture strength on samples prepared from core corresponding to representative field test horizons. All tests involved internal pressurisation to failure with water at a constant pressurisation rate of 3.5 MPa/min to simulate field conditions. A range of test hole diameters A-1 was employed to investigate the impact of scale effect on fracture strength. The results of the laboratory program are summarised in Figure 4. The individual test by test results are contained in Appendix 3. Superimposed on Figure 4 is the range of field measured fracture strength as given in Table 2, plotted at the hole size used in the field. In a general sense, Figure 4 suggests reasonable agreement between the laboratory and field data in the context of the commonly observed impact of size effect on fracture strength (Enever and Wu, 2000). On this basis the results of the above analysis can be viewed, in the first instance, as being internally consistent. On an individual test by test basis, however, some specific tests did not show the same level of agreement between laboratory and field information as for the combined data, leading to the assignment of reliability ratings to the results of analysis as indicated in Table 2. A.3 Discussion The orientational data from Table 2 is summarised on a spatial basis with respect to the local geological setting in Figure 5. Figure 5 suggests: • an approximate alignment of the major horizontal secondary principal stress with the north-west trend of the nearby Redmanvale Fault in holes WA69 and WA69R, • an approximately east-west orientation from the single impression obtained in WA55, • a degree of scatter in the measured orientations in WA58, with the deeper data (below the Glen Munro seam) generally being clustered around an orientation just south of east. Scrutiny of the hole locations with respect to existing mine workings (Figure 6) suggests the distinct possibility of the shallower measurements in WA69 having been influenced by proximity to workings, but probably not so the deeper measurements in WA69R. The proximity of WA69 to the Triassic escarpment (Figure 7) is also likely to have influenced the measurements in WA69 but probably not in WA69R. The deeper measurements in WA58 are unlikely to have been influenced by the nearby Wambo and Whybrow seam workings (Figure 6) and/or proximity to the escarpment. WA55 was located remotely from any likely extraneous influences. Apart from the above mentioned relationship between the measured stress field orientation in WA69/69R and the trend of the Redmanvale Fault, the remainder of the orientational data does not show any obvious general relationship to the local geological structure. Figure 8 summarises the orientational data with respect to the corresponding magnitude data for holes WA58 and WA55 combined (assuming WA69/69R to have possibly been influenced by local circumstances). What clearly emerges from Figure 8 is evidence of relatively higher stress magnitude around an orientation of 80° to 110°, independent of depth of measurement. Relatively lower stress magnitudes are also evident, scattered across the orientational spectrum. The predominant measured orientation can be placed in the regional A-2 context by reference to Figure 9 which summarises measurements made throughout the Hunter Valley (Enever et al., 1993) (Figure 3). The current Wambo data (free of local influences) appears to be consistent with a pervasive, approximately E-W, stress field orientation (from ENE to ESE) measured regionally throughout the Hunter Valley. The magnitude data from Table 2 is summarised in Figures 10, 11 and 12 for WA55, WA58 and WA69/69R respectively. In each case the most dubious results from Table 2 have been omitted, and the less reliable data based on fracture strength matching identified (open symbols). The locations of the measurements are shown with respect to the stratigraphic sequence in each case. No obvious pattern appears in the data when related to the relative stratigraphic location. There is, however, a broadly similar trend to the respective profiles with depth from the three holes, with the minor horizontal secondary principal stress generally varying between the corresponding magnitude of the vertical (overburden) stress based on depth of cover, and approximately one and a half to twice this value (somewhat lower in WA55). As Figure 8 shows, the relatively higher stress magnitudes appear to correspond to an approximately E-W stress field orientation for that data postulated to not be influenced by local circumstances. Figure 13 is similar to Figure 8 but for the data from WA69/69R. Figure 13 suggests a somewhat different average stress field orientation to the remainder of the data, and a generally lower magnitude, possibly due to the impact of local effects. It should be noted, however, that in WA69R there were some tests where fracture initiation could not be achieved up to the pressure limit of the equipment, possibly reflecting some points of very high stress in the sequence. In Figures 14 and 15, the magnitude data for all holes is combined for the minor and major horizontal secondary principal stress respectively. Viewed in this way, the magnitude data follows a similar pattern to the rest of the Hunter Valley (Figures 16 and 17), albeit with a somewhat lower limit to the upper bound trends of stress magnitude with depth for both stress components. A.4 COAL STRESS MEASUREMETNS A.5 Data and Analysis The Step Rate Tests conducted to measure the magnitude of the absolute minimum stress in the Arrowfield and Bowfield seams in WA55, WA58 and WA69R are summarised in Appendix 4.In each case the test was conducted on a whole of seam basis. The results are given in Table 3. s A-3 A.6 Discussion The results from Table 3 are presented graphically in Figure 18. Figure 18 suggests a similar trend with depth as has been found throughout the eastern coal basins (Enever et al. 2000), with minimum stress increasing systematically with depth above about 300 metres depth, and being approximately 50% of the corresponding overburden pressure. Below approximately 300 metres depth the minimum coal stress tends toward the corresponding overburden pressure as indicated by the single result in WA69R in Figure 18. A.7 REFERENCES Enever, J.R. (1993). Comprehensive Rock Eng, 3, Rock Testing and Site Characterisation, Ch 24, Pergamon Pres. Enever, J.R., Glen, R.A. and Beckett, J. (1998). The Stress Field and Structural Environment of the Hunter Valley. Aust. Geomech. Soc. Newcastle Branch. Enever, J.R. Jeffrey, R. and Casey, D. (2000). The Relationship between Stress in Coal and Rock, Fourth North American Rock Mechanics Symposium. Enever, J.R. and Wu, B. (2000). Scale Effects in Hollow Cylinder Tests, 2nd Asian Geomechanics Conference, to be held in Beijing. A-4 TABLE 1: SALIENT DATA Hole/ Depth Po Pi Pr Shut-in Description of Fracture (m) (MPa) (MPa) (MPa) Test (MPa) WA55 #1 224.3 - 2.24 10.0- 7.24 6.55-6.20-6.37 No impression** 224.72 12.4* #2 232.0 - 2.36 11.0- 6.55 6.89-6.89-6.89 Near vertical fracture, rotating 232.42 13.1* at bottom, striking @ 74° maq #3 251.0- 2.51 11.4- 5.86 6.20-6.20-6.20 No impression ** 251.42 13.4 #4 275.0 - 2.75 11.7- 10.0 9.65-9.30-9.30 No impression** 275.42 13.4* #6 286.5 - 2.87 16.2- 10.70 10.0-10.0-10.00 No impression ** 286.92 19.3* WA58 #2 63.5- 0.64 6.9 - 9.0* 2.41 2.72 - 2.41 - 2.72 Steeply inclined fracture 63.92 striking @ 66° mag #4 84.5- 0.85 4.8 - 7.6* 3.10 2.41 - 2.04 - 2.04 Inclined fracture striking @ 84.92 354° mag #5 88.3- 0.89 6.9- 2.07 2.20 - 2.07 - 2.07 Near vertical crack rotating at 88.72 10.0* top and bottom, striking @ 10° mag #6 146.0- 1.46 12.1 - 5.86 6.03-5.17-5.17 One sided vertical fracture @ 146.42 13.8* 94° mag #7 151.0- 1.51 14.3- 5.51 4.82-7-4.31 One sided inclined fracture 151.42 16.4* striking @ 114° mag #8 156.5- 1.55 13.1 - ? 3.62-7-4.13 No impression 156.92 15.5* #9 159.5- 1.60 12.2- 5.51 4.13-4.13-4.13 Segmented sub vertical 159.92 14.5* fracture @ 330° mag #10 197.0- 1.97 14.1 - 8.96 6.55 - 6.55 - 7.6 Extensive vertical fracture @ 197.42 19.7* 98° maq #11 201.0- 2.00 15.8- 6.20 8.16-6.03-6.03 Extensive vertical fracture @ 201.42 17.0* 88° maq #12 212.0- 2.12 18.3- 10.34 10.00-7.92-8.26 Extensive vertical fracture @ 212.42 20.7* 80° maq #13 216.0- 1.92 31.0- 5.51 7-5.51 -7 Rotating fracture 216.42 35.1* #14 219.5- 2.07 18.6- ? 7-10.7-6.9-7.9 No impression 219.92 22.1* #15 221.0- 2.07 24.1 - ? 7-13.4-17.2-11.4 No impression 221.42 28.3* -10.3 #16 234.0 - 2.07 11.7- 5.17 4.82 - 4.82 - 4.82 Horizontal fracture 234.42 15.2* #17 210.0- 1.89 16.5- 11.71 7.92-10.3-10.3 Vertical fracture under top 210.42 20.0* packer @ 96° maq A-5 able 1 (cont ’d) ■)le/ Depth Po Pi Pr Shut-in Description of Fracture (m) (MPa) (MPa) (MPa) (MPa) Test WA69 #1 140.0- 1.03 3.6-6.9* 2.41 1.53-2.41 -2.41 Breakout zone @ 52° mag 140.42 #2 143.0- 1.10 3.7 - 6.2* 2.76 2.76-2.41 -2.41 Extensive vertical fracture 143.42 @ 332° mag + breakout zone @ 64° mag #3 118.0- 0.86 6.2- 2.07 2.07-1.73-1.73 Extensive vertical fracture 118.42 10.5* @ 304° mag #4 116.0- 0.86 6.4 - 9.5* 2.38 2.76 - 2.41 - 2.41 Complex, near vertical 116.42 fractue striking @ 93° mag 1M469A #5 240.0 - 2.14 17.9- 11.71 ? - 9.30 - 9.30 Short near vertical 240.42 21.4* fracture striking @ 102° mag #6 276.5 - 2.62 11.7- 6.55 8.61 -7.58-7.58 Extensive vertical fracture 276.92 14.5* @ 120° mag #7 366.5 - 3.48 20.3- 15.64 7-15.16-15.16 No impression** 366.92 23.1* *7(a) 362.0 - 3.03 11.7- 10.34 9.65-10.0-10.3 No impression ** 362.42 15.5* #8 369.0 - 3.41 No fracture initiation to 42 MPa 369.42 #8(a) 384.0 - 3.24 10.2- 8.61 8.26 - 8.26 - 8.26 No impression ** 384.42 13.4* #9 418.5- 3.58 No fracture initiation to 42 MPa 418.92 #10 444.0- 3.72 22.7- 19.98 18.30-18.6- No impression ** 444.42 27.2* 17.91 #11 477.0 - 4.0 No fracture initiation to 42 MPa 477.42 #12 481.5- 4.0 22.4 - ?* 11.71 7-11.7-11.7 Short segment of rotating 481.92 fracture of uncertain orientation #13 495.7 - 4.17 20.7- 13.09 13.43-13.43- Short segment of rotating 496.12 23.8* 13.43 fracture of uncertain orientation #14 505.5 - 4.41 24.8- 17.23 15.50-15.50- No impression** 505.92 28.9* 15.50 #15 520.5 - 4.58 25.8- 19.98 17.91 -18.3-18.3 No impression** 520.92 30.0* Packer pressure at fracture initiation * Diagnostics of pressure record suggest vertical fracture development __Values of shut-in used to define Oh A-6 TABLE 2: RESULTS OF ANALYSIS Fracture Hole/ Depth □ a □ h Orientation Strength Test (m) (MPa) (MPa) of (MPa) □ h WA55 #1 224.3 - 274.72 6.4 9.6 2.8 #2 232.0 - 232.42 6.9 11.8 86° 4.1 #3 251.0-251.42 6.2 10.2 - 5.5 #4 275.0 - 275.42 9.3 15.2** - 1.7 #6 286.5 - 286.92 10.0 16.4*** - 5.5 WA58 #2 63.5-63.92 2.7 5.1 78° 4.5 #4 84.5 - 84.92 2.4 3.3** 6° 1.7 #5 88.3 - 88.72 2.2 3.6*** 22° 4.8 #6 146.0-146.42 5.2 8.3 106° 6.2 #7 151.0-151.42 4.6 6.8** 126° 8.8 #8**** 156.5-156.92 - - - - #9 159.5-159.92 4.1 5.2** 162° 6.7 #10 197.0-197.42 6.6 8.8 110° 5.1 #11 201.0-201.42 6.0 9.8** 100° 9.6 #12 212.0-212.42 8.1 11.9 92° 8.0 #13**** 216.0-216.42 - - - - #14**** 219.5-219.92 - - - - #15**** 221.0-221.42 - - - - #16**** 234.0 - 234.42 - - - - #17 210.0-210.42 10.3 17.3 108° 5.5 A-7 Table 2 (Cont ’d) Fracture Hole/ Depth □ /i □ h Orientation Strength Test (m) (MPa) (MPa) of (MPa) □ w WA69 #1 140.0-140.42 2.4 . 154°* #2 143.0-143.42 2.4 3.3 164° 2.4 #3 118.0-118.42 1.7 2.1 136° 4.1 #4 116.0-116.42 2.4 3.9 105° 4.0 WA69/ R #5 240.0 - 240.42 9.3 14.1 114° 8.9 #6 276.5 - 676.92 7.6 13.6 132° 5.1 #7 366.5 - 366.92 15.2 26.5 - 4.7 #7(a) 362.0 - 362.42 10.0 16.7** - 1.4 #8 369.0 - 369.42 - - - - #8(a) 384.0 - 384.42 8.3 13.1*** - 1.6 #9 418.5-418.92 - - - - #10 444.0 - 444.42 18.3 31.2** - 2.7 #11 477.0 - 477.42 - - - - #12 481.5-481.92 11.7 23.4 - 10.7 #13 495.7-496.12 13.4 22.9 - 7.6 #14 505.5 - 505.92 15.5 24.9 - 7.6 #15 520.5 - 520.92 18.3 30.3 - 5.8 Based on orientation of brea (out ** Tentative result based on fracture strength match *** Dubious result based on fracture strength match **** Analysis not attempted due to uncertain data or horizontal fracture development TABLE 3: RESULTS OF STEP RATE TESTS Hole/Seam Min. Stress Depth (MPa) WA55 - Arrowfield 294.5-301.94 4.4 WA55 - Bowfield 311.06-318.5 4.9 WA58 - Arrowfield 221.56-229.0 3.4 WA58 - Bowfield 233.7-241.14 3.8 WA69R - Arrowfield 486.5-493.32 10.6 A-8 A.8 Figures Figure A-1 Sample pressure record. A-9 STRATA-TEK PTX LTD. Fracture Orientation Record fH'Series Bore Hole. Viewed Down Hole. Figure A-2 Sample impression record A-10 Axial lead Test fluid Injection. Core sample Figure A-3 Laboratory test arrangement Figure A-8 Depth Normalised Major Horiz. Stress Mag. Stress Orientation field orientation of Major v magnitude Horiz. Stress for WA55 (deg) and 1 BO WA58 I 5 : I 4 5 0 flT S 1 I 90 180 I Orientation of Major Horiz. Stress (deg) Figure A-13 Stress field orientation v magnitude for WA69 / 69R - nominal \ overburden SG>=X.5 Mag. of Minor Horiz. Stress (MPa) Figure A-14 Summary of minor horizontal stress magnitude for all data combined A-20 -200- -400- •500- ■ nominal \ overburden SG.=2.5 -600 Mag. of Major Horiz. Stress (MPa) Figure A-15 Summary of major horizontal stress magnitude for all data combined •500 — overburden S.dsiLS Mag. of Minor horiz. Sireea (MPa) Figure A-16 Summary of regional data - minor horizontal stress A-21 0 •100- 3v*ftvnd#n S.S =2 Mag. of Major Horix. Stress (MPa) Figure A-17 Summary of regional data - major horizontal stress* nominal overburden SG=2.5 -100- E -200- WA 58 * Coal Depth 69/R (m> A Coal Depth 55 (m) •300- WA 55 a Coal Depth 58 (m) 50% of overburden WA 69/ H Min. Stress (MPa) Figure A-18 Summary of step rate test results A-22 APPENDIX B GAS SORPTION TESTING B GAS SORPTION TESTING B.1 Summary This report presents results of the gas sorption testing aimed at defining the adsorption isotherm parameters for the Arrowfield and Bowfield seams in boreholes WA55 and WA58. Sorption capacities were determined for both C02 and CH4. Table B.1 Summary of Adsorption Isotherm Parameters Isotherm parameters Test (absolute pressure) Ash Borehole Sample Test Tem Seam Langmuir Langmuir Content No No Gas pera pressure volume (%) ture (abs) m3/t kPa WA55 AF Arrowfield 20.00 2,313.96 9.9 WA55 ch4 30 WA55 BF Bowfield 18.90 2,158.24 12.0 WA55 AF Arrowfield 54.43 1,721.63 9.9 WA55 C02 30 WA55 BF Bowfield 49.47 1,605.99 12.0 WA58 AF Arrowfield 20.92 2,162.29 5.8 WA58 ch4 30 WA58 BF Bowfield 20.35 2,239.77 13.2 WA58 AF Arrowfield 56.18 1,675.95 5.8 WA58 8 30 WA58 BF Bowfield 54.53 1,757.09 13.2 WA69 AF Arrowfield 22.47 2,264.34 5.4 WA69 ch4 40 WA69 BF Bowfield 20.37 2,303.6 9.8 WA69 AF Arrowfield 54.68 1,972.07 ' 5.4 WA69 C02 40 WA69 BF Bowfield 54.00 2,145.00 9.8 B-1 B.2 Introduction This report presents results of the gas sorption testing aimed at defining the adsorption isotherm parameters for the Arrowfield and Bowfield seams in boreholes WA55 and WA58. Sorption capacities were determined for both C02 and CH4. The scope covers: • Methane (CH4) adsorption isotherm determination; • Carbon Dioxide (C02) adsorption isotherm determination and; • The effect of ash content on sorption capacity. Test details are contained in the Appendix (at the end of this section). B-2 B.3 Background The gas sorption isotherm assesses the gas adsorption capacity as a function of the pressure at constant temperature. The adsorption of the gas by coal is applied to the evaluation of coal bed methane resources and modelling the amount of gas, which can be desorbed during underground mining. Coal samples came from exploration boreholes WA55 and WA58 (Table B.1). Table B.2 Borehole Data Gas GC composition Borehole Depth Ash (%)' Sample No Seam (Qm) Air & N2 Free No (m) (%) (m3/t)1 Basis O O ch4 WA55 AF Arrowfield 298.16 9.9 9.70 60 40 WA55 WA55 BF Bowfield 316.71 12.0 N/A N/A N/A WA58 AF Arrowfield 226.65 5.8 8.84 70 30 WA58 WA58 BF Bowfield 238.28 13.2 10.70 N/A N/A WA69 AF Arrowfield N/A 5.4 N/A N/A N/A WA69 WA69 BF Bowfield N/A 9.8 N/A N/A N/A B.3.1 Experimental Procedure A gravimetric method was used to measure the high pressure CH4/CO2 sorption isotherm. The total mass of gas adsorbed is defined by the difference between the mass of the sample with and without gas. Initially, the coal is crushed to 75% less than 125 jim and 100% less than 250 p.m, and then approximately 70 g of coal is placed into the high-pressure adsorption bomb. The equipment consists of a gas supply system, which is connected to a manifold through a pressure regulator valve. The system is thermostatically controlled in an air bath at the required temperature. An “as received" state of coal (ie, with the moisture content) is used for determination of the amount of adsorbed gas. The samples were evacuated at the required temperature over 3 hours with the isotherm determination starting at atmosphere pressure. For isotherm testing the volume of adsorbed gas on a known weight of coal is measured at gauge pressures of 1 Determined by Earth Data Pty. Ltd. (at 30deg C, 1 atm) B-3 about 150, 300, 700, 1300, 2500, 3500 and 4500 kPa. From 2 to 12 hours is required at each pressure to reach sorption equilibrium. Free space (i.e., the space between the coal particles) is calculated from the weight and specific gravity of the coal and the bomb volume. The calculated equivalent volume of adsorbed gas is reported to 20°C and 101.325 kPa. The most commonly used adsorption isotherm model at elevated temperature and pressure is the Langmuir model: P + b where: V - Volume of gas adsorbed (m3/t) P - Absolute or the gauge pressure (kPa) a - Langmuir volume (m3/t) b - Langmuir pressure (kPa) Coefficient a (Langmuir volume) represents the maximum amount of gas, of a particular type that will be absorbed on the surface of the coal. Coefficient b (Langmuir pressure) represents the pressure at which half the gas of a particular type that can adsorb on the surface of the coal will be adsorbed. The Langmuir isotherm curve was made to fit the methane and carbon dioxide experimental data through the following: A linear regression was performed for P/V versus P chart The Langmuir constants a and b were calculated from the slope and intercept. Gas Adsorption Properties The values of isotherm parameters are calculated both in terms of absolute and gauge pressures. The summary and adsorption measurements are presented in Table B.3, Figure B-1, Figure B-2, Figure B-3, Figure B-4 and the Appendix, with proximate analysis and sampling location. B-4 Table B.3 Summary of Adsorption Isotherm Parameters — Isotherm parameters Test (absolute pressure) Ash Desorption Bore Tem Langmuir Sample Test Seam Langmuir Content Pressure hole pera pressure No Gas volume (%) (abs), kPa* No ture (abs) m3/t kPa ch4 20.00 2313.96 WA55 WA55 AF 30 Arrowfield 9.9 N/A C02 54.43 1721.63 ch4 18.9 2158.24 WA55 WA55 BF 30 Bowfield 12.0 N/A o ° 49.47 1605.99 ch4 20.92 2162.29 WA58 WA58 AF 30 Arrowfield 5.8 1271.5 co 2 56.18 1675.95 ch4 20.35 2239.77 WA58 WA58 BF 30 Bowfield 13.2 1831.5 0 0 54.53 1757.09 ch4 22.47 2264.34 WA69 WA69 AF 40 Arrowfield 5.4 N/A 8 CM 54.68 1972.07 ch4 20.37 2303.60 N/A I WA69 WA69 BF 40 Bowfield 9.8 co 2 54.00 2145.00 * Desorption pressure calculated from isotherm test data, sample gas content and gas composition B-5 B.3.2 Proximate Analysis Proximate analyses were carried out on all Wambo samples (Table B.4). Table B.4Proximate Analyses of Adsorption Samples. Sample No WA55 AF WA55 BF WA58 AF WA58 BF WA69 AF WA69 BF Moisture, % 3.4 3.0 3.6 4.0 2.7 2.2 Ash, % 9.9 12.0 5.8 13.2 5.4 9.8 Volatiles, % 34.8 31.0 34.8 29.4 32.0 31.8 B.4 Appendix Test Details APPENDIX C PERMEABILITY REPORT C PERMEABILITY REPORT C.1 INTRODUCTION During the period 21st November to 8 th December 2000 a series of production buildup, drawdown buildup, and step-rate tests were conducted on the Arrowfield and Bowfield Seams in boreholes WA55, WA58 and WA69R at the Wambo Coal Mine near Warkworth New South Wales for CSIRO and Sumitomo Coal Mining Company Limited. Test borehole locations are marked on Figure C-1. The step rate results are included in the sediment stress report for the same three holes. Multiphase production testing was also undertaken with the results also covered in detail in a separate report. C.1.1 Description of Testing Tool A special purpose test string of O-ringed AW drill rods is used to locate the tool with the upper and lower test packers spanning the test interval. Inflation of hydraulic packers isolates the coal seam or test interval from the rest of the hole. The spacing between the upper and lower test packers may be varied as required to seat packers in competent areas of the wellbore. Packer seats are chosen from examination of the core (where possible) and calliper log. Pressure transducers mounted inside the tool allow real-time monitoring of the down hole pressure responses in: • the test interval, below the control valve, and if necessary, • the borehole (annulus), immediately above the test interval. Surface recording equipment receives data signals from the tool via cable strapped to the outside of the test string. The data is recorded and displayed on a computer screen for real-time evaluation of test progress. A digital data logger periodically records time, pressure and flow information for later computer processing and data analysis. C.1.2 Operational Procedures Production / Buildup Test • space the packers and position the tool to isolate the target coal seam; • inflate tighthead to seal rod string; • pressurise the rod string to displace water from the string (approximately 100 metres); • open the control valve and inflate the packers to set them in the rod string and the test interval; • close the control valve to allow the seam to stabilise at its static reservoir pressure; C-1 • vent the rod string to relieve the air pressure in the rod string thereby creating a pressure differential between the rod string and test interval; • open the control valve after pressure in the test interval has stabilised near the static pressure; • monitor the flow of water from the test interval into the rod string; • after a suitable period of production, close the control valve to begin a pressure buildup in the test interval. Drawdown / Buildup Test • space the packers and position the tool to isolate the target coal seam; • open the control valve and inflate the packers to isolate the test interval; • close the control valve to allow the seam to stabilise at its static reservoir pressure; • switch to pump configuration; • open the control valve and begin pumping water from the test interval with downhole pump; • simultaneously stop pumping and close the control valve after a pre-determined period of drawdown or once the desorption pressure limit is reached, i.e. a pressure above the desorption pressure; • monitor the pressure buildup as the seam returns to its static condition. Injection / Falloff Test • space the packers and position the tool to isolate the target coal seam; • open the control valve and inflate the packers to isolate the test interval • close the control valve to allow the seam to stabilise at its static reservoir pressure; • fill test string with water; • open the control valve and begin injecting water; • simultaneously stop injecting, close the control valve and well head valve after a pre-determined period of injection or once the pressure buildup limit is reached. The well head and hence test string is isolated to maintain fall-off integrity should the control valve fail to close; • monitor the pressure fall-off as the seam returns to its static condition; Step Rate Injection Test • similar to an injection test except that the injection rate is increased (approximately doubled) over a period of equal time increments. C-2 C.3 INTERPRETATION PROCEDURES C.3.1 Well Testing Analysis is undertaken using “Saphir” a well test interpretation software programme developed by Kappa Engineering. In addition to conventional semi-log, type curve matching and pressure-time simulation of the test response is also undertaken using various well, reservoir and boundary model combinations. As more than one reservoir model may equally match the data, it is often not possible without additional detailed reservoir (coal seam) information, to define which is more likely, and in some cases more than one reservoir model may be presented. C.3.2 Step-rate Testing Step-rate analysis plots pressure against a series of increasing injection rates. As fracture dilation or breakdown occurs the rate of pressure increase slows for successive rate increases. The intersection of the two (or more) straight lines are considered to represent an opening pressure which in coal is likely to be coincident with the minimum stress. C-6 C.4 RESULTS SUMMARY Table C.1 Arrowfield Seam Permeability Test Results Summary Test Interval Flow Analysis kh Permeability Water Skin Investigation Depth (m) Period Model (md.m) (md) Level (m) Radius (m) WA58 DST RC 25.7 8.35 -23.7 2.15 20.0 221.56- Buildup Semi 25.8 8.38 -23.8 10.1 229.0 2 log h = 3.07 WA55 DST1 RC 2.40 0.59 -11.2 1.16 6.97 293.5 - Buildup Semi 2.29 0.56 -11.1 3.67 300.94 2 log h = 4.1m DST2 RC 2.49 0.61 -12.5 -1.58 12.5 Buildup Semi 2.45 0.60 -12.4 -2.21 2 log WA69R DST RH 0.017 0.005 -19.9 0.17 1.9 486.5 - Buildup RH 0.048 0.014 -25.0 4.96 3.22 493.32 2 h = 3.41m RH - Radial Homogeneous : The reservoir is homogeneous, isotropic and of constant thickness. RC - Radial Composite : The reservoir has two homogeneous zones with different reservoir characteristics (in this instance permeability) extending radially out from the wellbore. C-7 Table C.2 Bowfield Seam Permeability Test Results Summary Test Interval Flow Analysis kh Permeability Water Skin Investigation Depth (m) Period Model (md.m) (md) Level (m) Radius (m) WA58 DST RCCP 54.3 14.7 -19.2 -0.01 24.1 233.7 - Buildup Semi 53.9 14.6 -19.3 6.38 241.14 2 log h = 3.68m WA55 DST RC 0.161 0.050 -10.9 -1.09 4.63 311.06- Buildup Semi 0.21 0.064 -11.8 -2.14 318.5 2 log h = 3.3m RC - Radial Composite : The reservoir has two homogeneous zones with different reservoir characteristics (in this instance permeability) extending radially out from the wellbore. RCCP - Radial Composite, Constant Pressure Boundary : The reservoir has two homogeneous zones with different reservoir characteristics (in this instance permeability) extending radially out from the wellbore, limited by a constant pressure circular boundary. Notes : Two DST tests were undertaken on the Arrowfield Seam in WA55 because it became apparent during multiphase (production) testing that a thick mud (most likely associated with the numerous tuff layers) had settled in the bottom of the hole. Although the weight of the mud reduced the flow rate and consequently the radius of investigation the permeability remained unchanged. The presence of the mud also obscured a near wellbore region of enhanced permeability not seen in the first test. The fact that a similar response was also seen in the Bowfield Seam test may suggest the presence of fractures or some other form of near wellbore stimulation associated with permeability enhancement that is unique to this hole. C-8 Due to transducer signal instability two hours into the buildup for Arrowfield Seam WA69R two sets of results are supplied that represent a likely range based on simulation of the overall test trend. Skin is a measure of the permeability of the immediate wellbore region compared to the rest of the formation. If the wellbore is damaged or stimulated, the skin will be positive, or negative respectively. With the exception of borehole WA55 the radial composite models have lower skin values and therefore less wellbore damage than other reservoir models, and semi log analysis in particular, because the near wellbore region of the reservoir in this instance is modelled as having a lower permeability, rather than the pressure response of this zone being considered representative or indicative of wellbore damage. In WA55 the semi-log skin estimates are more negative because the higher permeability near wellbore area is interpreted as wellbore stimulation using this technique. Table C.3 Arrowfield Seam Step-rate Test Results Summary Borehole Opening Pressure Reservoir Pressure Effective Stress (KPa) (KPa) (KPa) WA58 3400 1926 1474 WA55 4400 2751 1649 WA69R 10600 4507 to 4556 6093 to 6044 Table C.4 Bowfield Seam Step-rate Test Results Summary Borehole Opening Reservoir Effective Pressure (KPa) Pressure Stress (KPa) (KPa) WA58 3800 2088 1712 WA55 4900 2917 1983 C-9 Based on opening pressures, the minimum stress within the Arrowfield Seam is around 60% of overburden pressure (assuming average SG = 2.5) in boreholes WA55 and WA58 to as high as 87% in WA69R. The Bowfield Seam is around 63% to 65% in both holes tested. C-10 APPENDIX D MULTIPHASE TEST REPORT D MULTIPHASE TEST REPORT D.1 INTRODUCTION During the period 21st November to 8 th December 2000 a series of multiphase drawdown tests were conducted on the Arrowfield and Bowfield Seams in boreholes WA55, WA58 and WA69R at the Wambo Coal Mine near Warkworth New South Wales for CSIRO and Sumitomo Coal Mining Company Limited. Test borehole locations are marked on Figure D-1. DST type production buildup, drawdown buildup and step-rate testing was also undertaken and is covered in detail in a separate report. For completeness a copy of the conventional analysis results are included in Appendix I. D.1.1 Description of Testing Tool A special purpose test string of O-ringed AW drill rods is used to locate the tool with the upper and lower test packers spanning the test interval. Inflation of hydraulic packers isolates the coal seam or test interval from the rest of the hole. The spacing between the upper and lower test packers may be varied as required to seat packers in competent areas of the wellbore. Packer seats are chosen from examination of the core (where possible) and caliper log. Pressure transducers mounted inside the tool allow real-time monitoring of the down hole pressure responses in: • the test interval, below the control valve, and if necessary, • the borehole (annulus), immediately above the test interval. Surface recording equipment receives data signals from the tool via cable strapped to the outside of the test string. The data is recorded and displayed on a computer screen for real-time evaluation of test progress. A digital data logger periodically records time, pressure, temperature and gas and water production information for later computer processing and data analysis. Operational Procedures Multiphase test • space the packers and position the tool to isolate the target coal seam, • prime return line to purge any air, • open the control valve and inflate the packers to isolate the test interval, • close the control valve to allow the seam to stabilise close to its static reservoir pressure, • open the control valve and begin pumping water to surface, • monitor and record pressure response and water and gas production, • simultaneously stop pumping and close the control valve after a pre-determined period of production. • monitor the pressure buildup as the seam returns to its static condition. D-1 D.3 INTERPRETATION PROCEDURES D.3.1 Well Testing Conventional single phase analysis is undertaken using “Saphir” a well test interpretation software programme developed by Kappa Engineering. In addition to conventional semi-log, type curve matching and pressure-time simulation of the test response is also undertaken using various well, reservoir and boundary model combinations. As more than one reservoir model may equally match the data, it is often not possible without additional detailed reservoir (coal seam) information, to define which is more likely, and in some cases more than one reservoir model may be presented. D.3.2 Multiphase Testing The multiphase gas production data is corrected to standard temperature (20° Celsius) and pressure conditions and time shifted to coincide with the onset of gas production and not the time of first measurement at the surface. Initial or starting parameter estimates are provided for SIMED relative permeability modelling. This is an iterative process and some parameters will need to be varied slightly. The data to be modelled is supplied in a time, pressure and cumulative gas production format in Appendix II. DA RESULTS SUMMARY D.4.1 Borehole WA58 - Arrowfield Seam Table D.1 Initial SIMED Input Parameters Arrowfield Seam Parameters Arrowfield Units Seam Seam thickness 3.075 metre Borehole radius 0.048 metre Permeability 1 8.37 md Skin 1 10 Reservoir pressure2 1925 kPa Desorption pressure2 = 1000 to 920 kPa D-6 Langmuir pressure 3 CH4 2162.19 kPa C02 1675.95 Langmuir volume 3 CH4 20.92 m3/t C02 56.18 Gas Content 3 8.84 m^/t Gas Composition 3 To be supplied CH4, CO; % Desorption time To be supplied days constant 1 Permeability, skin and other reservoir and wellbore variations noted in conventional single-phase analysis need to be included in SIMED modelling. 2 All pressures are recorded 1.2 metres above the top of the test interval (see Appendix I - Conventional Analysis, for test intervals). 3 Provided by Geogas. Langmuir volumes and pressures are absolute, however, bottomhole (and estimated i.e. reservoir etc) pressures are gauge and must be corrected if using absolute. Notes : The Arrowfield Seam in WA58 is moderately undersaturated in methane, retaining around 63% of its maximum possible sorptive capacity when the carbon dioxide component is taken into account, i.e. 37% of the total adsorbable amount of methane has been lost. Although not characterised by a definitive pressure response, desorption appears to have occurred between 920 and 1000 kPa (gauge pressure) with in excess of 220 litres of (desorbed and solution) gas produced to the surface. The red and yellow cross on Figure D-4 represents the theoretical desorption pressure based on the measured reservoir pressure and gas content. There is good agreement between this and the desorption pressure estimated from the drawdown pressure record and water production profile. The reservoir parameters, namely permeability and skin supplied in the above table represent a radial homogenous reservoir. While this should form the basis or starting point for modelling and history matching it may be necessary to more accurately characterise the permeability distribution around the borehole. Analysis of all the production buildup data (DST, Drawdown and multiphase) suggests a somewhat more complex reservoir response with at least two different (concentric) permeability regions around the borehole. Assuming a radial composite reservoir model, i.e. for simplicity, two different permeability zones extending radially out from the borehole axis, a near wellbore permeability of 3.1 md is indicated for the first 0.67 m before increasing to 8.35 md thereafter. While in reality this is most likely a gradual or progressive change, assuming a discrete D-7 D.4.2 Borehole WA 58 - Bowfield Seam Table D.2 Initial SIMED Input Parameters Bowfield Seam Parameters Arrowfield Units Seam Seam thickness 3.68 metre Borehole radius 0.048 metre Permeability 1 14.8 md Skin 1 6.49 Reservoir pressure12 2088 kPa Desorption pressure2 * 625 to 505 kPa Langmuir pressure 3 CH4 2239.77 kPa C02 1757.09 Langmuir volume 3 CH4 20.35 m3/t C02 54.53 Gas Content3 10.7 rri3/t Gas Composition 3 To be supplied ch 4, co 2 % Desorption time To be supplied days constant 1 Permeability, skin and other reservoir and wellbore variations noted in conventional single-phase analysis need to be included in SIMED modelling. 2AII pressures are recorded 1.2 metres above the top of the test interval (see Appendix I - Conventional Analysis, for test intervals). ^Provided by Geogas. Langmuir volumes and pressures are absolute, however, bottomhole (and estimated i.e. reservoir etc) pressures are gauge and must be corrected if using absolute. Notes : The Bowfield Seam in WA58 is slightly less undersaturated in methane than the Arrowfield Seam, retaining around 77% of its maximum possible sorptive capacity. A larger pump was required to draw the pressure down sufficiently to initiate desorption in this seam, which is characterised by D-11 the first increase in pressure around 2 hours as the water saturation is reduced. This puts the desorption pressure at somewhere between 505 and 625 kPa which is significantly higher than that predicted by the measured gas content and calculated isotherm. This will need to be resolved prior to relative permeability modelling. As with the Arrowfield Seam the reservoir parameters, supplied in the above table represent a radial homogenous reservoir. Radial composite analysis of the production/drawdown buildup data has yielded an inner zone or near wellbore permeability of 4.42 md increasing to 14.7 md after approximately 1.0 metres with a slightly negative to zero skin factor. D-12 D.4.3 Borehole WA55 - Bowfield Seam Table D.3 Initial SIMED Input Parameters Bowfield Seam Parameters Bowfield Units Seam Seam thickness 3.3 metre Borehole radius 0.048 metre Permeability 1 0.050 md Skin 1 -2.25 Reservoir pressure12 2926 kPa Desorption pressure2 = 1075 kPa Langmuir pressure3 CH4 2158.24 kPa C02 1605.99 Langmuir volume 3 CH4 18.90 m3/t C02 49.47 Gas Content3 11.85 nrAt Gas Composition 3 To be supplied ch 4, co 2 % Desorption time To be supplied days constant 1 Permeability, skin and other reservoir and wellbore variations noted in conventional single-phase analysis need to be included in SIMED modelling. 2AII pressures are recorded 1.2 metres above the top of the test interval (see Appendix I - Conventional Analysis, for test intervals). ^Provided by Geogas. Langmuir volumes and pressures are absolute, however, bottomhole (and estimated i.e. reservoir etc) pressures are gauge and must be corrected if using absolute. Notes : In WA55 the Bowfield Seam is undersaturated in methane, by about 34%. Desorption is characterised by an initial increase in pressure shortly after 3 hours at around 1075 kPa. With D-15 D.4.4 Borehole WA55 - Arrowfield Seam Table D.4 Initial SIMED Input Parameters Arrowfield Seam Parameters Arrowfield Units Seam Seam thickness 4.1 metre Borehole radius 0.048 metre Permeability 1 0.60 md Skin 1 -2.21 Reservoir pressure12 2750 kPa Desorption pressure2 Not Initiated kPa Langmuir pressure3 CH4 2313.96 kPa CG2 1721.63 Langmuir volume 3 CH4 20.00 m3/t C02 54.43 Gas Content3 9.7 m3/t Gas Composition 3 To be supplied ch 4, co 2 % Desorption time To be supplied days constant 1 Permeability, skin and other reservoir and wellbore variations noted in conventional single-phase analysis need to be included in SIMED modelling. 2AII pressures are recorded 1.2 metres above the top of the test interval (see Appendix I - Conventional Analysis, for test intervals). ^Provided by Geogas. Langmuir volumes and pressures are absolute, however, bottomhole (and estimated i.e. reservoir etc) pressures are gauge and must be corrected if using absolute. Notes : The Arrowfield Seam in WA55 is slightly more undersaturated in methane, retaining around 55% of its maximum possible sorptive capacity. This test was cut short before desorption could be initiated, because of a thick mud, (most likely associated with the numerous tuff layers) which had settled towards the bottom of the hole fouling the pump filters. D-19 APPENDIX I - CONVENTIONAL ANALYSIS RESULTS CONVENTIONAL ANALYSIS APPENDIX I A1.1 Arrowfield Seam - Permeability Test Results Test Flow Analysis kh Permeability Water Skin Investigation Interval Period Model (md.m) (md) Level Radius (m) Depth (m) (m) WA58 DST RC 25.7 8.35 -23.7 2.15 20.0 221.56- Buildup2 Semi-log 25.8 8.38 -23.8 10.1 229.0 h = 3.07 WA55 DST1 RC 2.40 0.59 -11.2 1.16 6.97 293.5 - Buildup2 Semi-log 2.29 0.56 -11.1 3.67 300.94 h = 4.1m DST2 RC 2.49 0.61 -12.5 12.5 Buildup2 Semi-log 2.45 0.60 -12.4 WA69R DST RH 0.017 0.005 -19.9 0.17 1.9 486.5 - Buildup2 RH 0.048 0.014 -25.0 4.96 3.22 493.32 h = 3.41m RH - Radial Homogeneous : The reservoir is homogeneous, isotropic and of constant thickness. RC - Radial Composite : The reservoir has two homogeneous zones with different reservoir characteristics (in this instance permeability) extending radially out from the wellbore. D-21 A1.2 Bowfield Seam - Permeability Test Results Test Flow Analysis kh Permeability Water Skin Investigation Interval Period Model (md.m) (md) Level Radius (m) Depth (m) (m) WA58 DST RCCP 54.3 14.7 -19.2 24.1 233.7 - Buildup2 Semi-log 53.9 14.6 -19.3 6.38 241.14 h = 3.68m WA55 DST RC 0.161 0.050 -10.9 4.63 311.06- Buildup2 Semi-log 0.21 0.064 -11.8 318.5 h = 3.3m RC - Radial Composite : The reservoir has two homogeneous zones with different reservoir characteristics (in this instance permeability) extending radially out from the wellbore. RCCP - Radial Composite, Constant Pressure Boundary : The reservoir has two homogeneous zones with different reservoir characteristics (in this instance permeability) extending radially out from the wellbore, limited by a constant pressure circular boundary. D-22 APPENDIX E RELATIVE PERMABILITY TESTS E RELATIVE PERMABILITY TESTS E.1 Summary This report presents results of SIMED curve matching simulations aimed at defining relative permeability curves and related SIMED input parameters. It is based on isotherm data presented in GeoGAS Report 00-141, gas content and composition results supplied by Earth Data Pty.Ltd. and the results of Multiphase well testing in borehole WA58. The curve matching results are generally poor. It is intended to investigate the reasons for this as part of a GeoGAS ACARP project. The results of the borehole multiphase production test curve matching suggest the first approach to gas emission/production prediction should use the following average parameters: porosity of 2.0%, coal compressibility of 72.5*10"® kPa" 1 and relative permeability “curve set No 2", desorption time constant of 40 days and 32 days for gas content of 8.84 m3/t and 10.7 m3/t for the Arrowfield and Bowfield seams, respectively. The relative permeability curve matching exercise was probably made difficult by the short multiphase test time of 8 hours. The sensitivity of relative permeability curve sets shows minor effect on cumulative gas and water rate. The history match for gas and water production for the Arrowfield seam was poor whereas the history match for the Bowfield seam was “reasonable ”. E.2 Introduction This report presents results of SIMED curve matching simulations aimed at defining relative permeability curves and related SIMED input parameters. It is based on isotherm data presented in GeoGAS Report 00-141, gas content and composition results supplied by Earth Data Pty.Ltd. and the results of Multiphase well testing in borehole WA58. The scope covers determination of the basic SIMED parameters for gas emission/production prediction by curve matching the well test results. Definition of Relative Permeability Curves and Other Basic Reservoir Parameters E.2.1 Background The following gas reservoir properties for the Arrowfield and Bowfield seams are subjected to sensitivity analyses by curve matching the multiphase well test results using SIMED II: • Relative permeability; • Coal porosity; E-1 • Coal cleat compressibility and; • Desorption time constant. The approach covers: • Sensitivity analysis of basic SIMED parameters • Matching the multiphase tests from borehole WA58 for Arrowfield seam • Matching the multiphase tests from borehole WA58 for Bowfield seam. The permeability used in the SIMED simulations for borehole flow was determined from tests in borehole WA58. Data used in this work comprises: • Adsorption isotherm data for CH42 • Multiphase Test Report from boreholes 41870 and 41884 3 • Permeability Report from boreholes 41870 and 41884 4 • Gas content, composition and desorption rate data from borehole WA58 5 Definition of SIMED and gas reservoir terms is given in the Appendix 1 and 2. Borehole WA58 Multiphase Test Results Multiphase test results are shown in Figure E-1 and Figure E-2. 2 Gas Sorption Isotherm Testing Wambo Mine - GeoGAS Report 00-141 31st January 2001 3 Multiphase test report Wambo Mine Borehole WA58 Multiphase Technologies Pty. Ltd. 4 Permeability test report Wambo Mine Borehole WA58 Multiphase Technologies Pty. Ltd. 5 Gas content and composition results - Email from S.Xue 29 th January 2001 E-2 E.2.3 Borehole WA58 Arrowfield seam Initial SIMED input parameters, obtained from multiphase well testing and gas content measurement, are: • Seam thickness 3.075 m • Depth to top of the seam 226.66 m • Borehole diameter 0.096 m • Gas composition CH4 70%, C02 30% • Langmuir volume 21.63/58.10 m3/t (CH4/C02) • Langmuir pressure 2162.29/1675.95 kPa (CH4/C02) • Gas content 8.84 m3/t • Desorption time constant 40 days • Borehole skin -2.15 • Near borehole permeability of 3.1 mD, increasing to 8.35 mD after 0.67 metres. Sensitivity analyses were carried out on: • Borehole skin • Relative permeability curves • Coal porosity • Coal cleat compressibility • Desorption time constant. Borehole Skin Borehole skin sensitivity analysis, performed for skin values of “0”, “-1.5” and “-2.15” shows considerable effect on cumulative gas and water (Figure E-5). E-5 E.2.5 Borehole WA58 Bowfield Seam Initial SIMED input parameters, obtained from Multiphase well testing and gas content measurement, are: • Seam thickness 3.68 m • Depth to top of the seam 238.28 m • Borehole diameter 0.096 m • Gas composition CH4 70%, C02 30% • Langmuir volume 21.04/56.39 m3/t (CH4/C02) • Langmuir pressure 2239.77/1757.09 kPa (CH4/C02) • Gas content 10.7 m3/t • Desorption time constant 32 days • Borehole skin 0.0 • Near borehole permeability of 4.42 mD with increasing to 14.7 mD after 1.0 metres. Sensitivity analyses were carried out on: • Relative permeability curves • Coal porosity • Coal cleat compressibility • Desorption time constant. Relative Permeability Curves The same three sets of relative permeability curves were used to assess their influence on cumulative gas and water rate (Figure E-6). Relative permeability curve sets No 1 and 2 sensitivity analysis shows minor effect on cumulative gas and water rate. Relative permeability curve set No 3 has a reduced rate of gas production compared to sets No 1 and 2, due to its having a lower relative permeability (Figure E-13). E-12 E.3 Appendices E.3.1 Appendix 1 - SIMED Simulations SIMED II is a two phase (gas and water), three-dimensional, multi gas component (ie C02, CH4 N2 etc simultaneous modelling capabilities), single or dual porosity reservoir simulator. The dual porosity capability is used for coal seams, simulating the slow, concentration gradient driven, desorption from the coal matrix, and the pressure gradient flow of gas through the fracture network. Significant features of SIMED relevant to the work are: Dual Porosity Model: The coal matrix is considered to be the low permeability, high storage capacity primary porosity system. The cleat system is analogous to the high permeability, low storage secondary porosity system. In SIMED, the transfer of material between the two systems is modelled using a characteristic desorption time constant (tau). Multicomponent Adsorption: Use of multiple gas components with different sorption properties for each component. For the case of a single component gas, the Langmuir Isotherm for that gas gives the amount adsorbed at any given pressure. Matrix Shrinkage Option: It has been proposed that matrix shrinkage resulting from the loss of adsorbed gas from the coal matrix may result in an increase in porosity. This may counter the effect of decreasing porosity with increasing stress during the course of pressure depletion. Although the importance of matrix shrinkage is not yet clearly established, the effect is included in SIMED Rib Drainage Option: A rib drainage option allows for gas to drain to atmospheric pressure through grid block faces. This option is used to simulate the times required for pillars or longwall panels to de-gas sufficiently to comply with mining safety requirements. Gas Drainage: The model allows flexible, multi-block completions. This enables the user to simulate in-seam drainage patterns routinely used to de-gas coal mine panels. E.3.2 Appendix 2 - Terminology A brief explanation of some of the terms used in this report is: • Langmuir Pressure - Pressure at which half of the gas will be adsorbed on the surface of the coal. • Langmuir Volume - Maximum amount of gas that will be absorbed on the surface of the coal. E-18 • Desorption time constant (tau) - Time taken for gas to desorb from the surface of the coal and then diffuse to flow paths where pressure gradient controls the flow rather than the concentration gradient. • Permeability - A property of reservoir rocks or coal. It is a measure of how easy it is for fluids to flow through the reservoir media. • Porosity - Volume of the pore, cleat and other natural fractures as a fraction of the bulk volume of the coal • Skin Factor - A property of the borehole surface. It is a measure of how easy it is for fluids to flow through the borehole perimeter • Relative permeability - The permeability of gas relative to water. It is used in SIMED to model the change in permeability with decreasing water content in the coal. When water has a relative permeability of 1, there is no gas permeability. As the relative permeability of water declines from 1 to 0, the relative permeability of gas increases from 0 to 1 according to the shape of the relative permeability curves. • Coal Cleat Compressibility - Relates the change in effective stress to the change in porosity. E-19 REPORT ON RELATIVE PERMEABILITY AND RELATED PARAMETERS MODELLING WAMBO MINE E. Yurakov 21st February 2001 GeoGAS Report No.: 01-146 E-20