Justification of the prospects for developing gas-bearing sites in the Karaganda coal basin

The relevance of this study is due to the current need to meet energy resources demand. A promising option worth considering is the production of methane gas from unconventional sources, whose reserves significantly exceed those of conventional deposits. Objective: to evaluate gas-bearing sites and justify the principles for rating their prospects based on an analysis of geological and technological factors affecting the process of gas release from coal seams. To achieve this goal, the study examined the Karaganda coal basin as a promising district for methane gas extraction, divided into five sites: Tenteksky, Saransky, Promyshlenny, Sherubainurinsky, Taldykuduksky. The most gas-bearing seams (reservoirs), with depth of occurrence ranging from 420 to 635 m, were studied. As a result of the study, first- and second-level coal seam zones were identified for methane extraction from coal seams. The parameters used to identify these zones were: depth of occurrence, permeability, and desorption of coal in a seam. At a depth of 250–300 m, gas permeability of the seams was recorded within the range of 10–15 millidarcy (mD). It was established that gas permeability decreases with increasing depth, and at a depth of 600–700 m, it amounts to hundredths and thousandths of a millidarcy. To determine the gas content of coal seams, representative samples were taken to characterize the gas content of coal seams from K20 to K1 across the entire area of the mining allotment under investigation. Sites in fault zones were not considered. They are classified as unrepresentative due to gas losses exceeding 30%. It has been established that the increase in gas content (methane content) in the Karaganda basin follows the ascending branch of a S-shaped curve and is described by the Langmuir equation. Methane resources in all host rocks have been determined based on a minimum methane content of 1 m3/t, and in claystones and siltstones with disseminated coal inclusions, based on a methane content of 4–5 m3/t of rock. At the same time, it has been established that in order to make a rough estimate of the resources, it is necessary to have data on total and effective porosity, water saturation of sandstones, and other factors. For the first time in the Karaganda coal basin, an integrated system for assessment of sites in terms of the potential of methane extraction has been implemented, incorporating more than 10 factors. Unlike previous studies, where assessments were made based on individual characteristics (depth, gas content, permeability), this paper proposes a generalized scoring method that allows for quantitative comparison of sites and prediction of methane production performance. Besides, the study novelty comprises the dependencies of Langmuir coefficients on fusinite content and temperature established in the study, which had not previously been applied to local coals. During the study, first- and second-order zones were identified in terms of methane production prospects; quantitative dependencies of gas content on depth, coal rank (stage of maturation), and petrographic composition were established; sites with the greatest commercial production potential were identified, and a methodology for the criterion-based assessment of sites was developed.