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 55 exploited coal fields in area of the Upper Silesian Coal Basin. Recoverable resources of CBM, that is resources for which there are economic incentives for production, were estimated at 89.1 billion m3. That amount comprises 34.3 billion m3 of CBM in areas of exploited coal fields and 54.8 billion m3 in hitherto undeveloped coal deposits of so-called reserve coal mine fields or in deposits situated at depths over 1,000 m.
In 2011, CBM resources decreased by 0.86 billion m3.
CBM output amounted to 244.8 million m3 in 2011. 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 489.5 million m3 in 2011.
Economic resources of CBM, established for 22 developed coal fields, are equal 5.601 billion m3.
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) J. Kwarciński, 2011 – "Metan pokładów węgla" w "Bilans perspektywicznych zasobów kopalin Polski wg stanu na 31 XII 2009 r. " pod red. S. Wołkowicza, T. Smakowskiego, S. Speczika. PIG-PIB Warszawa.