General information and occurrence
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 years witnessed a development of a technology of 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.
Deposits of CBM have 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 technologies.
Resources and output
The prospecting made it possible to evaluate CBM resources and show the presence of important CBM resources in 65 hard coal deposits in the area of the Upper Silesian Coal Basin. These anticipated economic resources amounted in 2020 to 107,229.25 million m3 and decreased by 2,319.28 million m3 (that is by 2.12%) in comparison with 2019. In 2020, there was not any single CBM deposit documented. Within the areas of hard coal exploitation, there was 1 deposit allocated - Śmiłowice from which the exploitation started. There were taken into account new geological documentations of deposits of CBM as the raw material accompanying hard coal deposits: Marcel (-107.80 million m3) and Rydułtowy (-437.66 million m3). These documentations were elaborated as a result of Marcel 1 and Rydułtowy 1 deposits assignation. In the hard coal resources remained in Marcel and Rydułtowy deposits methane resources have not been documented, therefore the deposits were crossed out from “The balance…”. In the case of Marcel deposit it was stated that coal beds are not methane-bearing and the coal resources are located above the methane zone top. In the light of the applicable rules, methane occurring in this part of the deposit did not match the balancing criteria so could not have been considered as the accompanying raw material. Speaking of Rydułtowy deposit, there were two main reasons for not documenting the methane resources as the accompanying raw material: - the minor amount of methane; - the irregular spread in the deposit of the methane-bearing values exceeding 2.5 m3 CH4/Mg csw. Aside from the documentations mentioned above, the anticipated economic resources drops were caused by the picking up the methane by mines (-315.09 million m3), the methane emission through the mine’s airing systems (-449.48 million m3) and the resources recalculations. The resources growth were the result of the detailed exploration and resources recalculations.
The majority of anticipated economic resources constitute the resources documented in the C category (98,561.34 million m3 – 91.92%). Resources documented in A and B categories amount only to 8,667.91 million m3 and account for only 8.08% of domestic anticipated economic resources. Anticipated sub-economic resources have been documented within 8 deposits and are equal 9,411.45 million m3, from which 9,316.35 million m3 are resources in C category (98.99% of the total anticipated sub-economic resources) and only 95.10 million m3 are resources in A and B categories (1.01%).
CBM output amounted in 2020 to 315.09 millionm3. 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 451.54 million m3 in 2020.
Economic resources of CBM, established for 33 deposits, are equal 11,352.90 million m3 and increased by 921.42 million m3 (that means by 8.83%) in comparison with the previous year. In the balance covering 2020, there were the new deposit development plans and supplements to existing plans taking into account for the following deposits: Budryk (+116.20 million m3), Halemba (-3.80 million m3), Halemba II (+173.11 million m3), Murcki (+194.17 million m3), Pniówek (+304.44 million m3), Sośnica (-31.48 million m3), Staszic (+278.12 million m3), Szczygłowice (-56.42 million m3), Śmiłowice (+121.72 mln m3), Wesoła (-22.58 million m3), Zabrze-Bielszowice (-1.33 million m3). In the case of Ziemowit deposit, the deposit development plan did not affect the resources volume. The resources decreases were caused by the output and the emission from the airing systems.
List of coal bed methane deposits is presented in Table 1.
In the latest edition of The balance of prospective mineral resources of Poland, there was the assessment of exploitable anticipated economic resources of coal bed methane in Poland updated*). The assessment was prepared on the basis of the current obligatory criteria for determining the coal bed methane deposits from the hard coal beds – both as the main raw material and as the accompanying raw material. There were two resources types distinguished: - prognostic resources of the coal bed methane as the accompanying raw material; to this type of resources there were included the resources documented in hard coal deposits in category D, which are not reported in the resources register; - prospective resources of the coal bed methane as the main raw material; to this type of resources all the remaining resources which have not been documented so far were included, without determining the prognostic resources due to the relatively high uncertainty of the resources quantity assessment. The both types of resources were calculated assuming the following parameters: - the minimum coal beds thickness of 0.6 m; - within the borders of deposit area where the coal beds methane-content is ≥4.5 m3/tonne of pure carbon substance; - to the depth of the coal deposit documentation in the case of the accompanying raw material or the depth of 1,500 m in the case of the main raw material. The prospective resources of coal bed methane as the main raw material were assessed below the deposits in which coal bed methane is the accompanying raw material, in the deposits where coal bed methane had not been documented as the accompanying raw material or within the areas which had not been documented. The total prognostic resources of coal bed methane in Poland as of 31.12.2018 amounted to 1.69 billion m3 (only within USCB area), whereas the prospective resources amounted to 111.27 billion m3 – including 1.75 billion m3 within LSCB (Lower Silesian Coal Basin) area, 94.33 billion m3 within USCB area and 15.19 billion m3 within LCB (Lublin Coal Basin) area.
Table 1. List of coal bed methane deposits (CBM) - million m3