Study of the mechanism of enhanced gas extraction by pressure relief and permeability enhancement in protective mining in a coal seam group

Protective coal seam mining is an important technical method used to strengthen the effect of gas extraction and prevent accidents caused by gases. However, it is necessary to thoroughly study the selection of the first coal seam to be mined and its effects related to pressure relief and permeability enhancement under the complex conditions of a coal seam group. This includes studying the relationships that govern pressure relief and permeability enhancement and the mechanism of the extraction of gas. In this study, a certain mine was used as engineering background. A damage-permeability model of a coal rock body undergoing mining disturbance was constructed based on experimental results and related theories. COMSOL Multiphysics software was used to perform numerical calculations with the model. The distribution of gas pressure in a coal seam was simulated under different mining conditions for the first mining seam. The first mining layer for protective coal seam mining of a coal seam group was identified. On this basis, the changes in parameters such as coal seam gas content, vertical stress and seepage velocity were studied during the advancement of the working face of the first mining seam. Furthermore, the numerical simulation was used to analyze the arrangement of boreholes for gas extraction from the bottom extraction roadway and the effect of extraction on pressure relief and permeability enhancement during protective coal seam mining. The results of the study showed the following: (1) Selecting 6# coal seam as the first mining seam had the best pressure relief effect on the adjacent seam, followed by 4# coal seam, and 8# coal seam had the worst pressure relief effect. (2) With the advancement of the 6# coal seam working face, the range of the pressure relief of the protected seam gradually expanded, the maximum stress relief degree of 4# coal seam was 25%, and the maximum decrease in gas content was 28.1%. The maximum stress relief degree of 8# coal seam was 60%, and the maximum decrease in gas content was 45.9%. (3) The gas content of coal seams decreased rapidly after extraction using boreholes; the gas contents of 4# coal seam and 8# coal seam decreased 81.25% and 99%, respectively, compared with the original values after 50 days of extraction. The efficiency of extraction by drill holes was related to the degree of decompression in the coal seam.