Multiphysics Coupled Response of Stress and Seepage in Coal Under Borehole Disturbance and Optimization of Borehole Parameters

To rationally determine the borehole radius and borehole spacing for gas drainage using along‐seam boreholes, a fluid‐solid coupling framework incorporating coal deformation, fracture seepage, and matrix diffusion was adopted. A one‐way coupled numerical simulation method based on FLAC3D and COMSOL was employed to systematically reveal the evolution of the plastic zone, stress field, permeability field, and gas pressure field in the coal surrounding boreholes under different borehole‐radius conditions. Results show that, according to the spacing criterion 2r ≤ L ≤ R, the reasonable spacing range is 2–8 m, and L = 5 m was selected as the unified borehole spacing. Under this condition, the plastic‐zone area increased with borehole radius, while stress redistribution and permeability enhancement both showed nonlinear characteristics. Among the tested schemes, the 150 mm borehole radius produced more uniform plastic‐zone expansion, more effective stress relief, and greater permeability enhancement. Gas pressure decreased nonlinearly with drainage time, and the low‐pressure zone progressively expanded outward from the borehole vicinity; the 150 mm scheme showed the fastest pressure attenuation. Field tests further confirmed that the combination of a 150 mm borehole radius and 5 m spacing increased the pure gas drainage rate by 35.4% relative to the conventional scheme.