Coupled Simulation of the Effective Radius Evolution for Gas Extraction in the Pressure-Relief Zone under Mining Disturbance

Abstract

Coal mining remains a high-risk industry, with gas disasters among the most severe safety threats. Effective gas management requires accurate understanding of drainage dynamics under complex stress conditions. However, nearly all prior studies assume uniform in-situ stress, an assumption that does not hold in the pressure-relief zones induced by protective-seam mining. To address this critical gap, we develop a sequentially coupled FLAC3D–COMSOL Multiphysics framework that faithfully reproduces field conditions. FLAC3D first quantifies stress redistribution triggered by extracting the upper and lower protective seams, delivering a realistic heterogeneous overburden profile. This profile is then imposed as the boundary condition in COMSOL, where a fully coupled fluid-solid model that integrates gas diffusion, coal deformation, and dynamic permeability tracks the spatiotemporal evolution of the effective drainage radius of in-seam boreholes. Results show a significant ‘O-shaped’ pressure-relief ring in the protected seam, featuring a clear central low-stress basin flanked by high marginal stresses, with vertical stress reduced from approximately 14 MPa to between 6 and 8 MPa. The effective radius expands dramatically with time, reaching approximately 5 m after 300 days, growing 10–18% faster in deeply relieved zones than in the central high-stress zone. Therefore, in this non-uniform stress field, traditional uniformly close-spaced boreholes are no longer suitable; instead, borehole spacing should be increased in low-stress zones and reduced in high-stress zones. This study provides a field-validated approach to model gas drainage under realistic mining-induced stresses, delivering practical guidance for safer, more efficient gas control in multi-seam operations.