Stress and permeability evolution characteristics of a long distance upper protected coal seam based on hydromechanical coupling and gas extraction technology

To address the ambiguity in the coupling mechanism between stress and gas seepage in the upper protected coal seam during deep and long-distance protective layer mining, this study takes the V 8 coal seam (protective layer) and IV 5-6 coal seam (upper protected coal seam) of Pingmei No.6 Mine as the research objects. Based on the theories of gas adsorption–desorption, rock mechanics and seepage mechanics, a fluid–solid coupling model for gas-bearing coal was constructed. The FLAC3D numerical simulation software was adopted to investigate the stress variation characteristics of the long-distance upper protected coal seam under different mining distances of the protective layer, while the COMSOL Multiphysics software was used to analyze the permeability evolution law of the aforementioned coal seam under the same mining conditions. The reliability of the established model was verified by field measurement data. The results show that with the advance of the protective layer working face, the pressure relief zone of the upper protected coal seam expands continuously, and the stress distribution gradually transitions from a "V" shape to a "U" shape. The permeability presents a "W"-shaped evolutionary trend with the characteristics of "reduction-increase-reduction-stabilization". After pressure relief, the simulated permeability value of the coal seam reaches 18.53 × 10⁻ 1 ⁷ m 2 , with a relative error of only 9.83% compared with the field measurement results. During gas extraction, the gas pressure in the coal seam shows an elliptical distribution pattern centered on the drilling hole, and the effective extraction radius increases with the extension of extraction time. The average error between the simulation results and actual measurement data of the effective extraction radius is 4.81%. The fluid–solid coupling model established in this study and the revealed evolution laws of stress, permeability and gas seepage provide accurate theoretical support and a solid technical foundation for the optimal design of pressure relief gas extraction in long-distance protected coal seams.