Quantitative evaluation of stress-relief and permeability-increasing effects of overlying coal seams for coal mine methane drainage in Wulan coal mine

Abstract

To eliminate the gas outburst hazards at Wulan Coal Mine, China, a methane and coal exploitation strategy is proposed for highly outburst-prone coal seams, in which the exploitation of the lower protective coal seams and pressure-relief methane drainage using surface boreholes are combined. This study investigates the stress-relief and permeability-increasing effects of overlying coal seams to test the effectiveness of this coal and methane exploitation strategy at Wulan mine. According to a numerical simulation of the protected seam extraction process in the Wulan coal mine, the stress-relief effect of the overlying coal seams was analyzed from two aspects: evolution regularities of the relief angle and stress evolution regularities. Due to the exploitation of the lower protective seams, the stress of the overlying coal seams can be fully relieved, which is beneficial for gas drainage.A permeability evolution model of a damaged coal seam was proposed in this study, in which the effect of damage on the permeability was considered, and the reliability of this model was demonstrated through a comparison with the permeability model of undamaged coal and an experimental data matching. Then, based on this permeability model of an undamaged coal seam, along with the stress evolution characteristics of the protected coal seam, the permeability distribution characteristics of the overlying coal seams were obtained. After mining coal seam 7# and coal seam 8#, the permeability maximum of coal seam 3# is 2054 times the value of the initial permeability, and the permeability maximum of coal seam 2# is 1781 times the value of the initial permeability.A field test at Wulan mine showed that this methane and coal exploitation strategy effectively eliminates the outburst hazards of the overlying coal seams and thereby creates suitable conditions for coal mine methane drainage. Over more than 2 years of degasification, the methane content of coal seam 3# decreased from 9.81 m3/t to approximately 3.14 m3/t, and that of coal seam 2# decreased from 10.6 m3/t to approximately 3.63 m3/t. The outburst hazards of coal seams 2# and 3# are eliminated completely, and a safe mining condition could be achieved by executing this strategy at Wulan mine.