Study on Gas Emission Characteristics in Lower Stress-relief Coal Seam for Improving Mining Safety of Extremely-close Upper Seam: Implementation of Coupled Permeability Model

Abstract In two extremely close coal seams, when the upper coal seam being first mined, large amount of gas normally emits into the mining area from lower stress-relief seam. This is a significant safety issue which may lead to gas-related incidents. In this paper, a coupled permeability model was developed. Then COMSOL Multiphysics numerical simulation was conducted to study multi-factor coupling gas emission flow characteristics. Furthermore, effects of coal stress, gas pressure and initial coal permeability on gas pressure change in lower stress-relief coal seam was analysed. The numerical model established is based on a real case of Ji15 coal seam and Ji16-17 coal seam in Pingdingshan No. 8 coalmine, China. Results show that after upper seam (Ji15 coal seam) being mined, the closer to bottom of working face, the lower the gas pressure because of increasing gas emission performance. As the distance from floor of working face increases, gas pressure becomes closer to its original value. Meanwhile, due to gas emission, gas pressure drop in lower stress-relief coal seam (Ji16-17 coal seam) is symmetrical from the middle of mining face to two ends. Along the parallel direction of mining face, gas pressure curve is approximately in “W”-shape, with gas pressure at both ends tends to be original. From two working face ends to the middle, gas pressure first decreases before increasing, with a maximum decrease of 9.36%. Furthermore, coal stress, gas pressure and initial coal permeability are all positively correlated with gas pressure decrease. Significance of initial coal permeability’s effect is higher than the other two factors. Above results could provide references on drainage borehole design for related scholars and on-site engineers, which is of significance for improving mining safety.