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 the 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 a methane capture on advance of longwall coalfaces by boreholes drilled in front of the face and a 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 a removal of water from coal beds to achieve a drop in a bed pressure in the area of a given borehole, necessary for start of processes of desorption, 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 Coal Basin. Economic importance of CBM occurrences in the Lublin Coal Basin is still to be established. Concentration of natural gas in the 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 63 exploited hard coal deposits in the area of the Upper Silesian Coal Basin. These anticipated economic resources amounted in 2016 to 95,953.81 million m3 and increased by 5,180.97 million m3 in comparison with the previous year. There were four new deposits documented in 2016: 1 documented within the area of hard coal deposits exploitation – Chwałowice 1 deposit (with anticipated economic resources equal 986.54 million m3); 2 documented beyond the areas of hard coal deposits exploitation – Anna (+41.92 million m3) and Marcel 1 (+209.13 million m3) and 1 with the coal bed methane as the main raw material – Wilchwy (+57.17 million m3). There were new documentations with recalculated resources approved for: Rydułtowy (+144.60 million m3) and Szczygłowice (+4,251.25 million m3). There were two deposits crossed out from “The balance…”: Chwałowice (-319.44 million m3) and Wujek-część Stara Ligota (-12.77 million m3). Other resources drops were caused by the picking up by mines and the emission.
The majority of anticipated economic resources constitute the resources documented in C category (87,004.34 million m3 – 90.67%). Resources documented in A and B categories amount only to 8,949.47 million m3 and account for 9.33% of domestic anticipated economic resources. Anticipated sub-economic resources have been documented within 8 deposits and are equal 11,419.08 million m3, from which 11,323.98 million m3 are resources in C category (99.17% of the total anticipated sub-economic resources) and only 95.10 million m3 are resources in A and B categories (0.83%).
CBM output amounted to 357.09 million m3 in 2016. This figure covers the amount of CBM which is picked up by every hard coal mine in Poland and the amount of methane which is being exploited independently – as a self-outflow from the boreholes reaching the cavings of abandoned coal mines. In the case of several deposits (due to the technical capabilities) the output covers also the amount coming from the low-methane area – it is the part of coal deposit where the methane presence was proved but due to the low content the resources have not been documented. There is also CBM emitted from the mines airing systems presented in table 1. It amounted to 547.22 million m3 in 2016. The exploitation from Anna 1, Pawłowice 1 and Jankowice-Wschód deposits started in 2016.
Economic resources of CBM, established for 30 developed coal deposits, are equal 5,852.86 million m3 and increased by 34.08 million m3. In 2016 there were new mine management plans with recalculated resources approved for: Anna 1, Halemba II, Rydułtowy, Silesia, Staszic, Szczygłowice, Wieczorek and Ziemowit deposits. The resources decrease was caused by the output and the emission from airing systems.
Prognostic and perspective resources of coal bed methane in USCB amounted to 107 billion m3 as of 31.12.2009*. 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
*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.
Table 1. List of coal bed methane deposits (CBM) - million m3