Permeability evolution of deep coal under the coupling damage of mining-adsorption

It is essential to investigate the permeability variation in deep coal seam to prevent gas disasters, which threaten the safety of the mine. The primary inducement for the variation of coal permeability in deep coal seam is determined by analyzing the coal geological environment. The inducement of these changes is thought to be the coupling damage of mining-adsorption effect. The mining damage variable and adsorption damage variable are constructed. Subsequently, the coupling damage variable of mining-adsorption is proposed. Furthermore, the permeability model of deep coal under the coupling damage of mining-adsorption is developed relying on the coupling damage variable of mining-adsorption. According to the laboratory permeability tests of deep coal under mining stress and adsorption, the developed permeability model is validated. The variation of permeability and damage variable of deep coal under different mining paths and gas pressure distributions are analyzed. The results show that the mining damage variable in laboratory tests indicate three stages: slow increase, rapid increase, and approximately constant. The coal permeability under mining can be divided into three stages: slow increase, rapid increase, and slow increase. As the gas pressure increases, the adsorption damage variable increases; however, the increasing rate decreases. Under different mining paths and different distribution forms of gas pressure, both the coupling damage variable and permeability in front of the deep working face present three stages. The relationships of coupling damage variable at the peak stress and the permeability ratio at the working face are protective layer < top coal caving < no coal pillar. In different gas pressure distributions, the relationships of coupling damage variable and permeability ratio at the peak stress are distribution form 1 < distribution form 3 < distribution form 2, while the relationships of coupling damage variable and permeability at the working face are distribution form 2 < distribution form 1 < distribution form 3.