The evolution of permeability and gas composition during remote protective longwall mining and stress-relief gas drainage: a case study of the underground Haishiwan Coal Mine

The mining of protective coal seams can cause changes in geostress, leading to changes in the permeability of coal rock and creating favorable conditions for gas extraction from coal seams. At the Haishiwan Coal Mine, field tests using remote protective coal seam mining were performed in the protected layer, which is rich in CO2 gas. In remote protective longwall mining, the permeability and composition of extracted stress-relief gas can vary. Under the conditions of remote protective longwall mining, the permeability of a protected coal seam can be generally described by the Liu model. During protective layer mining, the permeability of the protective layer increases rapidly with the release of stress, then decreases gradually with the recovery of the geostress. However, matrix shrinkage and decreased pore pressure caused by CO2 desorption from coal seams also cannot be ignored when considering the factors that affect the permeability. Thus, it is necessary to appropriately configure the cross-measure boreholes in advance to drain the stress-relief gas during remote protective layer mining. Stressrelief CO2 gas extraction presents multiple consecutive peaks. The No. 2 coal seam has different trap pressure systems as CO2 migrates into the coal seam. The protected seam experiences different effective stresses during protective layer mining, and the permeabilities appear to periodically increase due to differences in the original permeability. The various permeability and diffusion coefficients for CO2 and CH4 in coal induce CO2 and CH4 fractionation in the roof and floor of the No. 2 coal seam.