Impacts of mining-induced stress concentration on coal-bed methane drainage in boreholes parallel with bedding

The self-developed multi-field coupling test system is applied in the physical simulation of coal-bed methane (CBM) drainage in boreholes along seam, considering different stress concentration conditions caused by mining activities. The evolution of coal-bed gas pressure, gas drainage rate at boreholes, stress sensitivity coefficient, and permeability at stress released and concentrated zones during coal-bed methane drainage process, and the initial stress regions are analyzed. The experiment results show that (1) In the drainage process, the coal seam gas pressure decreased rapidly at the early stage, and then declines steadily. The gas drainage flow rate is much larger in regions closer to the borehole than far from it. In the periphery of the borehole, the gas pressure gradient increases at first and then decreases a certain period later. (2) During the mining process, the coefficient of stress concentration increases, while the permeability of coal seam and coal-bed methane flow decrease. The minimum drainage amount of coal-bed methane is located at Region 1 characterized with maximum stress concentration, while the maximum drainage amount is located at the stress released zones with minimum stress state. (3) The stress sensitivity coefficient in the stress concentration zone, which is characterized as the slowest decline rate of dimensionless permeability, is higher than that of stress released zones and initial stress regions.