Study on the Stress–Damage–Seepage Coupling Mechanism for Permeability Enhancement by Grid-Pattern Static Fracturing in Low-Permeability Coal Seams and Optimization of Borehole Layout Parameters

To address limited permeability enhancement caused by insufficient free faces during static fracturing in low-permeability coal seams, this study investigated the No. 2 coal seam of the 32203 working face in Hanzui Coal Mine. A coupled stress–damage–seepage fluid–solid model was developed, and COMSOL Multiphysics 6.0 was used to compare stress, plastic zone, and gas pressure evolution under single-borehole and grid-pattern multi-borehole fracturing. Simulations showed that single-borehole fracturing was constrained by the free face, with limited lateral plastic-zone expansion (0.45 m) and non-uniform distribution due to in situ stress and coal anisotropy. In grid-pattern fracturing, drainage boreholes provided multiple free faces; superimposed expansion stresses connected plastic failure zones around fracturing boreholes, enlarging pressure relief. Field tests with six fracturing and six observation boreholes, using stable flow after secondary sealing as baseline and excluding sealing improvement, showed mixed drainage flow increased by 2.5–3.1 times and pure-gas flow by 2.1–2.5 times; pure flow at 0.6 m from a fracturing borehole rose from 0.0046 to 0.018 m3/min. Permeability enhancement was significant at spacing ≤1.6 m, consistent with simulations. Grid-pattern static fracturing expanded the fracture network via stress–damage–seepage coupling, providing a theoretical basis and parameter reference for efficient permeability enhancement in low-permeability coal seams.