Plastic Damage Evolution Around Deep Coal-Seam Boreholes and Its Effect on Gas Drainage Efficiency

Efficient gas drainage in deep coal seams is critical for safe mining, yet the coupling between plastic damage evolution in borehole surrounding rock and seepage characteristics remains a key barrier to improving drainage efficiency. This study established a dual-porosity model that couples gas diffusion–seepage with elastoplastic coal deformation and conducted numerical simulations under various stress states. Triaxial tests were conducted to support the stress–deformation–permeability trends used in the numerical analysis. The simulation results showed a strongly nonlinear positive correlation between plastic damage and in situ stress, and the damage scale under uniform stress was well described by an empirical quadratic fit. The lowest and most symmetric damage occurred at a lateral pressure coefficient of 1.0, whereas deviations from this value changed the damage morphology, produced uneven gas pressure distributions, and formed high-velocity seepage zones favorable for directional drainage. Plastic damage exerted dual effects on drainage, with moderate damage enhancing permeability and high stress suppressing far-field seepage. Experiments revealed that confining pressure was the dominant factor affecting permeability and that it suppressed both deformation and seepage, whereas gas pressure was kept constant and was not treated as an independent variable in the experimental design. These findings provide support for optimizing gas drainage parameters in deep coal seams.