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 the 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 the acceptable level by the ventilation of work areas.
The last decades witnessed a development of a technology of a draining methane from coal beds by multiple boreholes drilled from the surface. The technology of drainage involves the hydrofracturing of coal beds and surrounding rocks and filling up fissures with a 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 the start of processes of desorption, emission and migration of CBM. Draining of CBM by production wells is treated as the natural gas production from unconventional source.
CBM has been documented only in coal deposits of the Upper Silesian Coal Basin. CBM concentrations in coal deposits of the Lower Silesian Coal Basin and the Lublin Coal Basin appear to be much smaller than in the Upper Silesian Basin. Their economic importance is still to be established.
The CBM usage is determined on one hand by the safety issues and on the other hand is treated as collecting the gas from the unconventional sources – due to its form of occurring which demands the application of the special recovery desorptive technology.
The prospecting made it possible to evaluate CBM resources and show the presence of important CBM resources in 62 hard coal deposits in the area of the Upper Silesian Coal Basin. These anticipated economic resources amounted in 2017 to 96,947.67 million m3 and increased by 993.86 million m3 in comparison with 2016. There was 1 new deposit documented in 2017, beyond the areas of hard coal deposits exploitation – Rydułtowy 1 (+510.51 million m3). There were new documentations with recalculated resources approved for: Budryk (-763.34 million m3) and Zofiówka (+1,341.81 million m3) deposits. The resources drops were also caused by the picking up by mines (-332.14 million m3) and the emissions (-545.79 million m3), whereas the growths were the result of the more detailed exploration and resources recalculation.
The majority of anticipated economic resources constitute the resources documented in a C category (87,201.06 million m3 – 89.95%). Resources documented in A and B categories amount only to 9,746.61 million m3 and account for 10.05% of domestic anticipated economic resources. Anticipated sub-economic resources have been documented in 8 deposits and are equal 11,411.53 million m3, from which 11,316.43 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 332.14 million m3 in 2017. 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 (in documented deposits). It amounted to 543.04 million m3 in 2017.
Economic resources of CBM, established for 29 coal deposits, are equal 5,691.32 million m3 and decreased by 161.54 million m3. In 2017 there were new mine management plans with recalculated resources prepared for: Borynia, Chudów-Paniowy 1, Halemba, Jankowice, Knurów, Marcel, Staszic, Szczygłowice, Wesoła and Zabrze-Bielszowice deposits. The resources decrease was caused by the output and the emission from the airing systems.
The USCB is characterized by the highest potential of CBM deposits concentrations. 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.