Coal bed methane
Coal-bed methane (CBM) is natural gas trapped in coal and occurring in the form of gas particles adsorbed at coal grains. A drop in bed pressure along with mining activities is followed by an increase in coal bed methane desorption and its release from coal and surrounding rocks to work areas of a coal mine. The release of methane is a serious safety concern as it can create an explosive hazard. Therefore, much attention is paid to draining methane from coal beds before and in the course of coal mining. This is achieved by methane capture on advance of longwall coalfaces by boreholes drilled in front of the face and reduction of concentration of methane to acceptable level by ventilation of work areas.
The last decades witnessed development of a technology of draining methane from coal beds by multiple boreholes drilled from the surface. The technology of drainage involves hydrofracturing of coal beds and surrounding rock and filling up fissures with permeable medium (usually sand) to facilitate migration of CBM released by desorption. The next step is removal of water from coal beds to achieve a drop in bed pressure in the area of a given borehole, necessary for start of processes of desorption and emission and migration of CBM. Draining of CBM by production wells is treated as natural gas production from unconventional source.
CBM occurs in coal deposits of the Upper Silesian Coal Basin, especially those from its southern and western parts. CBM concentrations in coal deposits of the Lower Silesian Coal Basin appear to be much smaller than in the Upper Silesian Basin. Economic importance of CBM occurrences in the Lublin Coal Basin is still to be established. Concentration of natural gas in areas of planned exploitation of coal deposits appears to be of negligible economic value whereas CBM accumulations matching economic criteria are expected in the case of deeper-seated coal fields as for example in the Dorohucza syncline.
The prospecting made it possible to evaluate CBM resources and show the presence of important CBM resources in 58 exploited coal deposits in the area of the Upper Silesian Coal Basin. These anticipated economic resources amounted in 2014 to 86.8 billion m3 and increased by 1,367 million m3 in comparison with the previous year. There were five new coal and methan deposits documented in 2014: Barbara-Chorzów 2 (+25.33 million m3), Brzezinka 3 (+134.05 million m3), Bzie Dębina 2 (+645.72 million m3) – all of them with CBM as co-occuring raw material; Jankowice-Wschód and Mszana (+57.40 million m3) – with CBM as the main raw material. The first three deposits were allocated from Barbara-Chorzów, Brzezinka-2 and Bzie Dębina 1 deposits, therefore the resources if these deposits decreased. Barbara-Chorzów deposit was crossed out of the “Balance…”. There were also new calculations of resources accepted for Jankowice and Knurów deposits - with resources increase for the latter one to 1,768.28 million m3.
CBM output amounted to 293.4 million m3 in 2014. This figure covers the amount of CBM which is picked up by every hard coal mine in Poland. There is also CBM emitted from the mines airing systems. It amounted to 471.17 million m3 in 2014. The exploitation from Knurów deposit started in 2014 whereas the output was stopped in Silesia Głęboka deposit.
Economic resources of CBM, established for 26 developed coal deposits, are equal 5,611.35 million m3. The resources decreased mainly due to the output and ambition from airing systems.
Prognostic and perspective resources of coal bed methane in USCB amounted to 107 billion m3 as of 31.12.2009(1). Perspective resources in LCB and LSCB are much lower and amounted to about 15 billion m3 and 1.75 billion m3 respectively.
List of coal bed methane deposits is presented in table 1.
Prepared by: Agnieszka Malon, Marcin Tymiński
(1)Kwarciński J., 2011 – Metan z pokładów węgla kamiennego. In: Bilans perspektywicznych zasobów kopalin Polski wg stanu na 31 XII 2009 r. (ed. S. Wołkowicz, T. Smakowski, S. Speczik): 63-70. PIG-PIB, Warszawa.