Stress and permeability evolution of high-gassy coal seams for repeated mining

Coal bed methane reservoirs in China are generally characterized by high stress and low gas permeability coefficient. Protected layer mining has the effect of unloading pressure and increasing gas permeability coefficient. There are few studies in the case of repeated mining with an orthogonal arrangement of the protected layer. Based on the engineering practice of the high gas coal seams group in Xinjing coal mine, a combination of numerical simulation and field test was used. The distribution law of plastic zone, stress-strain and gas permeability coefficient of the protected layer under the two states of single mining and repeated mining were obtained. The stress and gas permeability coefficient distribution of the protected layer was divided into 5 and 6 zones in plane for single and repeated mining cases. The field test tested the distribution law of stress, gas pressure, deformation, and gas permeability coefficient, and verified the reliability of the numerical simulation results. According to the characteristics of gas permeability coefficient distribution under the condition of repeated mining, a method for accurate gas extraction is proposed. The results of the study have important implications for the design of gas extraction schemes in orthogonal repeated mining situations.