2016 International Conference on Power Engineering & Energy, Environment (PEEE 2016) ISBN: 978-1-60595-376-2

Research on Forming Conditions and Enrichment Rules of Coal Bed Methane in West

Junmin Zhang1, Yaming Yao2, Jianjun Chen3, Juanjuan Zhao2, Fei Zhao2

1Department of Mining Engineering Xinjiang Institute of Engineering, Wulumuqi 830023 2School of Resources & Environment University of Water Resources and Electric Power, 450011 China 3School of Earth Sciences and Engineering Xi’an Shiyou University, Xi’an 710065 China [email protected] , [email protected]

Key words: West Henan, coal bed methane, forming condition, resource evaluation, favorable area choosing

Abstract. Based on the analysis of coalfield geological characteristics, forming conditions and distribution law in such coal aggregation areas as , and Yima in West Henan, this paper studies macroscopic lithotypes of coal, microcal rock components and features of thermal evolution focusing on air content and influential factors, especially features of thermal evolution and influence of fault development on coal bed methane content. Main physical and chemical properties for coal bed methane, including gas saturation, desorption rate, crustal stress and critical pressure received special attention, including a contrast with main coal bed methane aggregation area in North China. After calculation, coal bed methane content in main blocks exceeds 12000×108m3, and thus Pingdingshan and Jiaozuo are ideal mining areas for coal bed methane in the future.

Foundation items: Xinjiang science and technology projects of Xinjiang Uygur Autonomous Region（201491105） “coal bed methane causing low coal rank enrichment regularity, fracturing technology, upper and lower joint exploration well comprehensive application technology development and demonstration”, key points of scientific research in colleges and universities in Xinjiang Uygur Autonomous Region (20130523135425562) of project funding of “Fukang City of Xinjiang coal bed methane efficient mining technology research”.

1. Introduction Being unconventional natural gas, coal bed methane is generated from coal bed and mainly accumulated in coal bed in an absorbed state. With methane as the main component, coal mine becomes mashgas. On one hand, it is listed among one of calamities for coal mine safety production and excessive emission will cause climate warming; on the other hand, it is an efficient and clean energy. Exploiting coal bed methane will help make full use of the clean energy, improve conditions for coal mine safety production and protect atmospheric environment human beings depending on, killing three birds with one stone. Favorable block for coal bed methane development is a key groundwork, since its research results directly determine success or failure. Increase in coal bed methane exploration practices prove coal reservoir in China which is featured by low permeability, low pressure, low saturation, strong heterogeneity and high coal rank, resulting from common and strong stimulation in late period, with active deep process and strong activity as the root. Medium-high metamorphic coal among coal resources in China is mainly distributed in North China coal-accumulation area Carboniferous—Permian and Southern China coal-accumulation area Permian coal-bearing strata. Coal-bearing basin in these periods belongs to reformed basin, and thus it is difficult to choose favorable block for coal bed methane development and to conduct the related research urgently.

2. West Henan Coal Bed Methane Geological and Geochemical Characteristics 2.1 Geological and geochemical characteristics for coal bed methane in West Henan Covering , , Jiaozuo, Zhengzhou, Pingdingshan and Yima west of - Railway, West Henan coal-bearing area is the main coal production base for Henan. Upper Palaeozoic Carboniferous-Permian coal-bearing series has stable sediment, great thickness of coal bed and wide distribution, with coal and coal bed methane reaching one trillion cubic meters, which lays foundation for coal bed methane development and utilization. Currently the focus lies in Pingdingshan in the middle and Jiaozuo in north-central area. However, there are obvious differences in coal bed methane content, coal bed fracture distribution rule and coal texture, as well as coal bed methane adsorption/desorption due to low coal rank for Pingdingshan and high coal rank for Jiaozuo. As a result, different block choosing method will be adopted to offer data on recourses potential truly and objectively. 2.2 Stratum and Sediment West Henan Carboniferous-Permian is mainly coal-bearing clastic deposit against rivers-coastal wetland environment. Composed of gray, grey-green and grey-white sandstone, gray and dark grey mudstone as well as limestone with coal bed in between. With sediment depth of 800-1200m, the stratum is divided into two systems, four series and six formations (refer to chart 1). Coal-bearing formations cover Carboniferous System Upper Series Formation, Permian System Lower Series Shanxi Formation, Xiashihezi Formation and Permian System Upper Series Shangshihezi Formation.

Table 1. List of Main Coal Bed Depth in West Henan Shanxi Formation. coal bed no. Ⅱ１ Ⅱ２ (m)

coal mine Min Max Average Min Max Average Shanmian 0.80 34.43 2.60 0.00 2.50 0.62 Xin’an 0.00 18.88 4.32 Yiluo 0.00 33.20 6.50 Yanlong 0.00 27.50 4.16 0.00 4.02 1.88 Xinggong 0.00 23.80 4.15 0.50 37.18 7.00 0.00 4.45 1.80 0.18 17.13 4.60 Yuxian 0.15 17.18 5.50 Linru 0.00 22.17 4.52 Pingdingshan 1.49 31.10 5.50 Jiaozuo 2.20 38.2 6.4

2.3 Coal Lithotypes 2.3.1 Macroscopic Lithotypes of Coal According to underground observation for over 10 coal mines in Pingdingshan and Jiaozuo, coal petrography is mainly composed of bright coal and dull coal, followed by vitrain and fusain. As viewed from the longitudinal changes, the coal beds of Taiyuan and Shanxi Formation have similarities, with bright coal and semibright coal as the mainstay. Seen features of coal macroscopic lithotypes of various Carboniferous-Permian section as a whole, Shanxi Formation Ⅱ1 coal bed enjoys better coal lithotypes, with a simpler structure and being more favorable for coal bed methane generation and aggregation. 2.3.2 Microcal Rock Components After analyzing samples of Carboniferous-Permian main coal mines in West Henan, vitrinite content ranges from 44.6% to 99.67％, 60-80％ being on average; fusinite content ranges from 0% to 52.8％, 10-30％ being on average; exinite content is smaller than 2.0％ on average. There is obvious regularity for vitrinite content changes, being high up and low down, the highest for Taiyuan Formation, and then Shanxi Formation, Xiashihezi Formation and Shangshihezi Formation respectively. 2.4 Features of Thermal Evolution There are gas coal, fat coal, coking coal, lean coal, meager coal and anthracite from low to high for main coal ranks, with regularity for space distribution. Coal rank decreases gradually from north to south for Ⅱ1 coal, Shanxi Formation. Jiaozuo, Yanlong-Xinggong falls into anthracite area, Xin’an, Dengfeng, and Xinmi meager coal area, Shanmian, Yiluo, north Linru and Yuxian lean coal area, west Shanmian, south Linru and middle Pingdingshan coking coal and south Pingdingshan gas coal and fat coal. There is also a regularity for coal rank in a longitudinal direction. It takes a relatively decreasing trend for Taiyuan Formation, Shanxi Formation, Shangshihezi and Xiashihezi Formation from bottom to top. As for a single coal mine, coal rank for Taiyuan Formation is higher than Shanxi Formation, with a difference of less than one coal rank, and higher than Shangshihezi and Xiashihezi Formation with 1-2 coal rank (Fig. 2).

Table 2. Change Features of West Henan Carboniferous-Permian Coal Rank.

formation Taiyuan Formation Shanxi Formation Xiashihezi Formation

index volatile Coal volatile volatile Coal rank Coal rank Mine lot （%） rank （%） (%) Pingdingshan Fat & coking coal 18.96 lean coal 20.05 Coking coal 29.04 No.13 Mine

Jiaozuo Encun 7.83 anthracit 9.42 anthracite e Yuxian Liangbei 16.44 22.30 Coking coal Mine Xinmi meager 12.92 13.45 meager coal 25.00 Coking coal Exploration Area coal

Xinggong Jihe 4.33 anthracite 6.05 anthracite Mine Field

Xin’an Mine Lot 16.02 lean coal 18.03 lean coal 20.05 Coking coal 3. Coal Bed Methane and Influential Factors

There are many factors restricting coal bed methane. Factors restricting coal bed methane content in the oilfields such as Jiaozuo, Pingdingshan, Xinggong and Xin’an include evolution of coal metamorphism, effective overlying strata thickness over coal bed, structure conditions, macerals and ash yield after analysis and research. 3.1 Coal Evolution Coal in this area ranges from gas coal to anthracite. The higher coal metamorphism is, the more coal bed methane content there will be. Content of gas coal bed methane is four times of fat coal and coking coal. On one hand, as coal metamorphism is intensified, internal pore surface area gradually increases, leading to a greater adsorption to methane; on the other hand, being mainly tectonic coal, high-rank coal in the area has poor permeability, and thus coal bed methane is not easily desorbed, leading to a greater coal bed methane.

Table 3. Statistics of Coal Bed Methane Content for Different Coal Rank in West Henan Shanxi Formation Ⅱ1 Coal (according to Jiang Yongfu). Coal bed methane content（m3/ t） Volatile Mine lot Coal rank Maximum Minimum Average flux（%） Deep Pingdingshan 8.73 1.95 4.41(20)※ Fat coal No.1, 4 and 6 Mine Yuxian Wangying Coking 6.20 2.51 4.36(8) Mine coal Linru Bishan No.2 Coking 3.02 0.28 1.31(8) Mine coal Pingdingshan No. Coking 11.06 1.55 5.03(20) 18-20 13 Mine coal Yuxina Lilou Mine 7.50 1.95 4.26(7) 11-15 Lean coal Yuxian Liangbei 11.91 2.22 6.37(19) Lean coal Mine Lot Dengfeng Merger Gaocheng Mine 8.74 1.36 4.95(6) coal Lot Xinmi Quliang Merger 15.86 1.75 6.80(8) Mine Lot coal Xinmi Peigou Merger 10.07 8.60 9.34(4) Mine Lot coal Xinggong Jihe 27.54 6.06 17.54(16) Anthracite Mine Field Xinggong Huangzhuang 31.20 7.20 18.26(11) Anthracite Mine Field Jiaozuo Encun 33.43 12.01 19.88(15) Anthracite ※ Average (sample number) 3.2 Effective Overlying Strata Thickness over Coal Bed After west Henan Carboniferous-Permian coal measures deposition, uplift and denudation process due to multiphase tectonic movement of Indo-Chinese, Yanshan and Himalayan caused a decreasing coal bed pressure and temperature, which destroyed the original absorption balance, formerly-generated coal bed methane began to desorb and resulted in lower methane content. According to coal bed methane reservoir mechanism, coal bed adsorbing capacity kept a dynamic balance with strata pressure. Despite settling and deposition again in later period, there is hardly any possibility for secondary hydrocarbon generation and another coal bed methane adsorption within an area with a burial depth less than 2000m. Consequently, Cenozoic sedimentary thickness is ineffective strata depth, namely strata depth without sedimentary thickness, rather than current coal bed burial depth, equals to effective overlying strata thickness over coal bed. 3.3 Faults and Fissures Development Faults and fissures also influence coal bed methane content. Northwest by west, northwest and northeast-trending faults cross cut the coal bed, derive many fissures and change coal bed permeability on one hand, and cause coal bed methane loss near faults or fissure-development zone, change the normal adsorption status dependent on effective overlying strata thickness over coal bed and make coal bed methane distribution complicated.

3 4 8 12 CH4 (m /t) 200

300

400

500

600

700

800

900

1000 H(m) fault zone Figure 1. Relation between Pingdingshan Coal Bed Methane Content and Effective Overlying Strata Thickness.

Size of fracture and distance from the fault also influence coal bed methane to a different extent. Generally speaking, a large fracture or short distance means a low content, and vice versa.

3 16

(m /t) 12

gas content

8

4 0

5 10 15 20 25 30 35 40 ash content（） % Figure 2. Relation between Carboniferous-Permian Coal Ash Yield and Methane Content in part of North China.

Research on West Henan and neighboring area shows that as vitrinite content increases, coal bed methane content also increases; meanwhile as ash yield increases, coal bed methane content will obviously decrease. In conclusion, coal bed methane is featured by the following: ① Effective overlying strata thickness over coal bed is a main geological factor influencing coal bed methane; ② Open fault and fracture cause great loss of coal bed methane and make the distribution complicated; ③Syncline structure is good for coal bed methane occurrence and its enrichment.

4. Main Physical and Chemical Properties for Coal Bed Methane and Distribution 4.1 Gas Saturation Isothermal adsorption experiment date for minable coal bed in various coal mines in West Henan indicates that Langmuir volume is 15.2-50.93 m3/t and Langmuir pressure is 1.32-2.89Mpa (Fig. 4), among which Yanlong and Xinggong coal mine Ⅱ1 coal bed has the largest Langmuir volume, being more than 45 m3 /t, Pingdingshan coal mine has relatively low volume, being less than 22 m3/t in general and others keep at 25 m3/t. Isothermal adsorption takes a middle-low curve (Fig. 3). Coal bed gas saturation for West Henan coal-bearing area ranges from 26% to 91%, being 40-63% in general, which falls into less saturated coal bed. Pingdingshan coal bed, with well-preserved coal body structure, has a gas saturation of 51.1-63.8%, being higher than that in Liulin and Jincheng coal bed methane test area in North China. Thus based on this single parameter, Pingdingshan coal mine enjoys a better workability. Table. 4. List of Coal Bed Adsorbability in West Henan (according to Jiang Yongfu).

Langmuir Coal Langmuir Equilibrium Mine lot pressure bed volume (m3/t) humidity (%) (Mpa)

Yuxian No. 5 Mine Ⅱ1 22.83 1.81 1.71

Dengfeng Xinxin Mine Ⅱ1 27.39 1.85 2.32

Bishan No.1 Mine Ⅱ1 26.93 1.88 2.64 Shanmian Caoyao Mine Ⅱ1 24.17 1.72 2.15 Xin’an Mine Ⅱ1 24.01 1.61 3.39

Yanlong Jiaocun Mine Ⅱ1 47.51 2.56 7.09

Xinggong Sanhe Mine Ⅱ1 50.93 2.89 5.44

Pingdingshan No.8 Mine Ⅱ1 20.28 1.59

Pingdingshan No.8 Mine Ⅱ1 21.79 1.37

Pingdingshan No.8 Mine Ⅳ2 17.01 1.57

Pingdingshan No.811Mine Ⅱ1 15.20 1.32

Yuxian Linru Pin ding shan Dengfeng Shanmian Xinan coalfield coalfield coalfield coalfield coalfield coalfi eld Figure 3. Isothermal Adsorption Curve for Main Coal Fields in West Henan (according to Fan Yongjie).

4.2 Rock Stress Determination Previous study shows that as effective rock stress increases, coal bed permeability usually takes an index decrease. As Fig. 5 shows, coal bed permeability decreases from 10×10-3μm2 to 0.007×10-3μm2 as effective rock stress increases from 2Mpa to 12Mpam, namely four orders of magnitude decrease of coal bed permeability corresponding to a 10Mpa increase of effective rock stress. Jiaozuo: with a burial depth of 738.77-815.47m, the maximum principal stress is 10.90-26.40MPa, and the minimum principal stress is 9.10-18.5 MPa, with a difference of 0.3-8.4 Mpa. Qinshui Basin: with a depth of 627.31m for middle and deep coal bed of No. 15 coal, the minimum in-site horizontal principal stress is 7.45 Mpa, and with a depth of 1021.9m for middle coal bed of Shanxi Formation, the minimum in-site horizontal principal stress is 15.5 Mpa.

10.0

Permeability(md)1.0

0.1

0.0

0 2 4 6 8 10 12 14 effective stress(MPa) Figure 4. Relation between Coal Bed Permeability and Effective Rock Stress in North China.

4.3 Desorption ratio Desorption rate means the percentage lost gas quantity plus desorption quantity accounts for the overall methane. Changes in desorption rate reflect coal permeability, diffusivity and workability of coal bed methane. Based on more than 80 coal bed methane content materials by over ten Carboniferous-Permian coal mine exploration areas according to desorption standard set up by coal departments, desorption changes from 7% to 80%, being a large scope. It is mainly related to evolution of coal metamorphism (Fig. 5). Desorption rate is high at coking coal, with better permeability and pore connectivity, being favorable for coal bed methane depressurization, desorption and diffusion and thus a better workability.

80

60 Desorptionrate(%)

40

20

0 4 8 12 16 20 24 28 32 36 V(%) Figure 5. Relation between Coal Metamorphism and Methane Desorption Rate. The above analysis displays parameters influence coal bed methane workability in different ways and to a different extent, among which coal bed permeability is of the greatest significance. As it is known to all that the key for break-through in coal bed methane exploration in the areas like Liulin lies in its favorable permeability. 4.4 Critical Pressure Critical pressure refers to the pressure under critical state, namely the minimum pressure for gas liquefaction under critical temperature, and also the saturated vapor pressure of the liquor under critical temperature.

Table 5. List of Critical Desorption Pressure for Coal Bed Methane Well in North China. Critical Theoretical Coal Measured gas desorption Area R0（%） recovery bed no. content（%） pressure ratio（%） （MPa） 3 1.33 8.36 0.86 12.64 4 1.37 8.74 1.16 27.08 Ordos 5 1.43 7.83 0.82 10.13 Basin 8 1.41 10.27 1.19 28.60 9+10 1.68 12.48 1.00 19.93 3 2.14 8.91 0.81 11.50 Shanxi Yangquan 9 2.08 11.03 1.17 32.38 15 2.17 11.60 1.00 22.95 3 2.22 7.10 0.63 12.62 Lu’an 15 2.12 11.37 1.84 47.66 3 2.99 19.63 2.46 58.26 Jincheng 15 3.09 18.63 2.30 57.17 3 1.74 10.37 1.2 31.22 Shaanxi Hancheng 5 1.80 4.72 0.8 10.24 11 1.80 14.97 3.17 45.92 Henan Jiaozuo Ⅱ1 4.63 21.25 2—3.27

5. Resources Calculation and Favorable Block Evaluation 5.1 Coal bed methane distribution Coal bed methane date for various coal mines and coal beds show that the methane content ranges from 1.36 to 33.63m3/t (Fig. 6). Form the graph, we can see that the coal bed methane content for Shanxi Formation ranks the highest, with a scope of 7.23-17.33m3/t for various coal mines, more than 12m3/t for Jiaozuo and Xinggong, 7.15-11.18m3/t for Pingdingshan, Dengfeng and Xinmi coal mine and less than 8m3/t for other mines. As for Shangshihezi and Xiashihezi Formation, coal bed methane contents ranges from 1.36 m3/t to 12.74m3/t, being 6.22m3/t on average. Table 6. List for West Henan Carboniferous-Permian Coal Bed Methane Content.

Coal bed methane content（m3/ t）

Shangshihezi and Notes Coal field Taiyuan Formation Shanxi Formation Xiashihezi Formation Shanmian 2.82-8.34/4.97(14) Coal bed Xionggong 16.98-32.30/24.84(26) methane Xinmi 2.23-18.18/9.15(25) content for Shanxi Dengfeng 3.22-10.18/6.77(15) 10.12-29.56/18.08(32) Formation is Linru 2.08-8.79/5.32(17) 2.67-14.23/8.96(24) 1.36-5.67/3.43(12) that for Ⅱ 1 Yuxian 4.34-22.19/14.81(29) 2.45-11.04/6.52(16) coal bed

Pingdingshan 8.26-20.94/13.16(37) 3.65-12.74/8.04(19)

12.84-33.63/23.25 Jiaozuo （26）

Limited coal bed distribution for Taiyuan Formation causes lower methane content. Shanxi Formation, with stable coal bed deposition, wide distribution and great depth, has the highest methane content, being the main group for coal bed methane evaluation and exploration, and Ⅱ1 coal as the target coal bed for research. Thus this project conducts an in-depth research on its coal bed methane distribution. 1. Coal bed methane more than 20m3/t: mainly in Encun, Jiulishan in Jiaozuo, east of Xin’an, north of Yanlong and Xinggong coal mine; 2. Coal bed methane ranging from 5 m3/t to 20m3/t: south of Shanmian, middle of Xin’an, middle of Dengfeng and middle and north of Pingdingshan coal mine; 3. Coal bed methane less than 5 m3/t: north of Shanmian coal mine, west of Xin’an, southern shallow buried area of Yanlong and Xinggong, south and northwest of Pingdingshan coal mine;

4. Ⅱ1 Coal bed methane takes a decreasing trend in a north-south and east-west direction. 5.2 Resources Calculation Coal bed methane content mainly depends on coal resources and gas in coal bed. Based on hosting geological conditions for coal bed and change features of methane content, a precise estimation has been conducted for area with a burial depth less than 2000m. The result shows that coal bed methane content for weathered coal bed zone, with a burial depth of less than 2000m is 12000×108m3, including 1200×108m3 for methane weathered zone, with a burial depth less than 500m, 4200×108m3 for 500-1000m, 3200×108m3 for 1000-1500m and 3500×108m3 for 1500-2000m. 5.3 Favorable Block Evaluation Viewed form resources in each block, it ranks Jiaozuo, Xinmi, Dengfeng, Xinggong, Linru, Pingdingshan, Shanmian and Xin’an coal mine from high to low (Fig. 6).

Figure 6. Coal Bed Methane Content Distribution for Coal Fields in West Henan.

There is a large change in resources abundance for West Henan Carboniferous-Permian, Jiaozuo coal field as the highest, being 3.63×108m3/km2, and Shanmian coal field the lowest, being only 0.73×108m3/km2 (Fig. 7).

Figure 7. Coal Bed Methane Resources Abundance for Coal Fields in West Henan (500-1500m).

6. Conclusion and suggestions

1. Carboniferous-Permian coal-bearing rock in West Henan enjoys superior coal-forming environment and contains large amount of coal bed methane resources, especially Shanxi Formation

Ⅱ1 with large thickness, wide distribution, ideal coal lithotypes and high methane content, being the main target interval for coal bed methane. 2. Among many factors influencing West Henan Carboniferous-Permian coal bed methane, coal rank comes the first, followed by effective overlying strata thickness over coal bed and faults. 3. Among many factors influencing workability of West Henan Carboniferous-Permian coal bed methane, there are permeability, strata pressure and desorption rate, permeability being the key. 4. Based on current economic and technological conditions for coal bed methane exploration, and after calculation for each block and depth interval, coal bed methane content for coal-bearing West Henan Carboniferous-Permian with a burial depth less than 2000m is 12000×108m3.

References

[1] Jiang Yongfu, Tian Yun, Huang Jinzhen and etc. On Workability of Coal Bed Methane in West Henan Oil-bearing Area [J]. Henan Oil, 2001(4). [2] Yao Yaming, Zhang Liansheng, Qiao Guilin and etc. On Formation Conditions of Immature-Low Mature Oil in Depression [J]. Nature Gas Geoscience, 2010(5). [3] Qin Yong. Progess and Review of Chinese Coal Bed Methane Geological Study [J]. Geological Journal of China Universities, 2003(3). [4] Yao Yaming, Zhao Hongge, Yang Binyi and etc. Geological Features of Oil and Gas in Zhoukou Depression [J]. Journal of Northwest University (Natural Science Edition), 2004(3). [5] Zhao Qingbo. Geological Characteristics of Chinese Coal Bed Methane and New Exploration Field [J]. Natural Gas Industry, 2004(5). [6] Wang Dewei, Li Yuanjian, Fan Yunxia compiled. Exploration Potential of Coal Bed Methane in Henan province and Research on Mining [M]. Wu Han: China University of Geosciences Press, 2010. [7] Ye Jianping, Qin Yong, Lin Dayang as Chief Editor. China Coal Bed Methane Resource [M]. Xu Zhou: China University of Mining and Technology Press, 1998. [8] Liu Chenglin, Che Changbo, Fan Mingzhu and etc. Chinese Coal Bed Methane Geology and Resource Evaluation [J]. China Coal Bed Methane, 2009(3). [9] Wumanjiang. Eli, Yao Yaming, Zhang Junmin Complied. An Introduction to Energy Science [M]. Xi’an: Shaanxi Science and Technology Press, 2013.