1. Development of a Methodology for Estimating Methane Emissions from Abandoned Coal Mines in the UK thinking beyond construction 1. DEVELOPMENT OF A METHODOLOGY FOR ESTIMATING METHANE EMISSIONS FROM ABANDONED COAL MINES IN THE UK May 2005 Reference: REPORT/D5559/SK/May 2005/EMISSIONS/V3 Issue Prepared by: Verified by: Steven Kershaw Keith Whitworth V3 BSc, PhD BSc Associate Associate File Ref: I:\Projects D0000 to D9699\D5559 DEFRA IMC White Young Green Environmental Newstead Court, Little Oak Drive, Sherwood Business Park, Annesley, Nottinghamshire, NG15 0DR. Telephone: 01623 684550 Facsimile: 01623 684551 E-Mail: [email protected] Environmental Consultancy WHITE YOUNG GREEN ENVIRONMENTAL This report has been prepared for and on behalf of DEFRA in response to their particular instructions, and any duty of care to another party is excluded. Any other party using or intending to use this information for any other purpose should seek the prior written consent of IMC White Young Green Environmental. The conclusions reached are those which can reasonably be determined from sources of information, referred to in the report and from our knowledge of current professional practice and standards. Any limitations resulting from the data are identified where possible but both these and our conclusions may require amendment should additional information become available. The report is only intended for use in the stated context and should not be used otherwise. Where information has been obtained from third parties, IMC White Young Green Environmental have made all reasonable efforts to ensure that the source is reputable and where appropriate, holds acceptable quality assurance accreditation. IMC White Young Green Environmental do not accept any liability for the accuracy of any information supplied by third parties. Copyright in this report is owned by IMC White Young Green Environmental and may not be published, reproduced or adapted without their consent. D5559\SK\DRAFT\May 2005\V3 Nottingham DEFRA i WHITE YOUNG GREEN ENVIRONMENTAL EXECUTIVE SUMMARY • The principal aim of the project was to produce a verifiable methodology for generating accurate and reliable estimates of methane emissions from abandoned mines in the UK, to be considered for inclusion in the UK greenhouse gas inventory. • The prime drivers for methane emission from abandoned mines are displacement by rising mine waters and the rate of emission of methane from the coal seams in the strata disturbed by mining. Rising mine water also serves to isolate methane reserves by cutting them off by flooding. The UK coalfields have been modelled to obtain estimates of water inflow and methane reserves within the coalfields. Measurements have also been made on methane emissions from mines, either from vents or from more general diffuse emissions from the surface. The general methodology has been to seek a relationship between the measured methane flows and parameters relating to the water and gas in the underlying abandoned workings. • Critical information was required for completion of this project. The main source of data on coal mine workings was the Coal Authority Mining Records Database System (MRDS), which provides data, inter alia, on the location, area and seam of all known workings in the UK in an electronic form. Mine water level data was obtained principally from Coal Authority measurements at shafts, boreholes pumping stations and discharge points, but data was also obtained from private coal producers and coal mine methane operators. Seam gas content data were obtained principally from a gas content database, established during the previous three decades using data from exploration boreholes and underground sampling. Geological maps and memoirs of the British Geological Survey (BGS) were used to identify seam sequences (with MRDS data), surface cover and aquifers overlying the mine workings. • Water level data from open shafts and boreholes indicates that water recovery after mine abandonment follows a predictable exponential curve similar to the recovery in any aquifer following pumping. This allows the future inflow rate of water into a mine to be calculated for use in the controlled pumping of mine water or to estimate the potential surface discharge rate. In this project, the data have been used to model the flooding of mines and the effect of cutting off methane from abandoned mine workings. To allow a detailed assessment of mine water recovery, and hence methane emissions over time, some coalfields required sub- division. A total of 34 coalfield areas were assessed divided into 142 areas for calculation. • Mine water recovery has only been monitored in a few coalfields in the UK. To determine the impact of mine water recovery on total UK methane emissions between 1990 and 2004, an assessment of mine water levels in all abandoned UK coal mines was essential. Data from the MRDS were used to identify the residual mine volumes for each modelled area as a function of depth; usually divided into volumes within 10m intervals. Water can enter mines via mine entries from aquifers and the surface, although strata water enters a mine laterally through the strata by permeable flow, the rate of water flow decreasing as the water level rises. The rate of water inflow into the mine was modelled using a permeability function that decreases the inflow of the workings into the mine depending on the volume of the workings that remain unflooded. • Results of the mine water recovery modelling fall in to four broad categories: Category 1. Areas where mine water has fully recovered, but workings remain above the recovered water level (82 areas). Category 2. Areas where mine water has fully recovered, with no mine workings above the water level (13 areas) Category 3. Areas where mine water is continuing to recover (31 areas) Category 4. Areas where mine water recovery is controlled by pumping (16 areas) Category three and four type recoveries contain the majority of mines closed in the 1990’s. D5559\SK\DRAFT\May 2005\V3 Nottingham DEFRA ii WHITE YOUNG GREEN ENVIRONMENTAL • Methane from abandoned mines originates in the coal seams within the strata disturbed by the mining process. In the UK coal seams generally contain methane produced during coalification, at values from close to zero to 20m3/tonne. During mining the seams release gas into the workings, and continue to do so for many years albeit at a much lower rate. Methane from abandoned mines escapes to atmosphere either through installed vents, designed to prevent the build up of pressure, or in a diffuse manner through old mine entries, broken strata, faults etc. • Vents at 16 locations around England were monitored from November 2002 to December 2004. The sites were chosen to provide a cross section of the types of abandoned mines found in the UK. Sites were sampled with workings at shallow, intermediate and deep levels and those where water levels had recovered and those where water was still rising. Methane flows ranged from zero to 153 litres per second (l/s), with over half the sites having flows less than 20 l/s. • Levels of diffuse emissions were made using long term gas samplers taking atmospheric samples over a period of a month. The sampling and measurement using very accurate tunable diode lasers was carried out by the Centre for Ecology and Hydrology (CEH) Edinburgh. As with the vent monitoring, sites were chosen to reflect a range of underlying mine conditions, although sites were chosen not to be close to gas vents. • Average measured enhancements lay between zero and almost 2,000ppb (parts per billion) above the national background of 2,017ppb. However, at sites where no other potential source of gas was present the maximum enhancement was about 1,300ppb. At these sites methane emissions were estimated at up to 360 tonnes methane per year. By using estimates of the surface area from which the emission might derive, flux estimates were calculated of up to 69 tonnes methane per square kilometre per year (t km-2y-1), with a mean of 10.5 t km-2y-1 and a standard deviation of 22 t km-2y-1. Multiplying the average flux value by the area of shallow workings provides an estimate of 37 kilotonne per year (kt y-1) for the emission from shallow workings. However, the large standard deviation of the flux, suggests this value should be treated with caution. • Methane reserve modelling was used to determine the potential source size of methane in abandoned workings. For major coalfields and those where detailed information was available, a full method of calculation was used. This method calculates a degassing area around workings, calculates the total gas in place before mining and subtracts an estimate of the methane released during mining. Only unflooded seams are assumed to contribute towards flow, so combination of the gas reserve model and water level data from the water model enables calculation of a methane reserve against time. For minor coalfields (representing, in total, less than 10% of the total UK gas reserve) the relationship between gas reserves and water level, established for the major coalfields, was used to provide an estimate of gas reserve using known or estimated water levels. • The estimate of reserves in 1990 is 10.0 billion m3 methane. Due to the pattern of colliery closure during the early 1990’s the estimate of reserves associated with closed mines first rose to 19.5 billion m3 methane in 1993 then fell, due to mine flooding, to 8.6 billion m3 methane in 2004. • No suitable relationship was found between vent methane flow data and the water flow in the underlying abandoned workings. However, vent methane flow data did show an increase with the size of the underlying methane reserve in the abandoned workings. Flux data from the diffuse monitoring was converted into flows by multiplying by the area of underlying workings.