Sorption capacity and natural gas content of coal beds of Donbas

Purpose. To determine the general patterns in the formation of the sorption capacity of coal and the natural regional methane content of coal beds using a quantitative indicator – the relative gradient of gas content. Methodology. To determine the sorption capacity of coal, the authors used the well-known “volumetric method” and the method of EPR-spectroscopy (electron paramagnetic resonance). We used the results obtained during geological exploration to analyze the natural gas content of coal beds. The determination of natural gas content was carried out using special gas-core-samplers and formation testers. Methods of mathematical statistics were used to process experimental data. Findings. According to experimental data regarding the sorption methane-bearing capacity and natural gas content of Donbas coal beds, we carried out the analysis, statistical processing, and generalization of the obtained results. A regular change in the relative gradient of coal beds’ methane content has been established for the entire Donbas as a whole. The values of relative gradients for coals of various grades naturally decrease with increasing stratification depth and also naturally decrease in each depth interval from low-metamorphosed (grade D) coal to highly-metamorphosed one. Based on the approximation results, the corresponding equations were obtained for each grade of coal metamorphism from gas to-anthracite (A). Originality. New indicators have been proposed – the relative gradient of sorption methane-bearing capacity and the relative gradient of gas content, which allow comparison of these characteristics measured in absolute values (cm3/g, m3/t) for individual coal beds of different grades of metamorphism, at different depths and lying in different geological conditions. It has been proven that the sorption capacity of coal matter determines the natural regional (background) methane content of coal beds, and naturally, according to a hyperbolic dependence, decreases with increasing stratification depth and also naturally decreases in each of the depth intervals from low-metamorphosed coal (grade D) to highly metamorphosed one (grade A), with a relative gradient that asymptotically approaches to 1 at pressures above 6 MPa. Practical values. The obtained dependences of the relative gradient of gas content on depth and gas pressure for various grades of metamorphism can be used to predict the natural regional (background) gas content of coal beds by determining the maximum sorption capacity and calculating the desired depth or pressure.