Quantitative assessment of coal seepage characteristics and pore–fracture structures using micro-computed tomography under stress–seepage coupling effects

The study of coal permeability is essential for ensuring coal mine safety and the efficient extraction of coalbed methane (CBM). This research investigated coal permeability evolution and pore–fracture structural dynamics under stress–permeability coupling using a flow–solid coupling evaluation system and high-resolution micro-computed tomography (Micro-CT). Results reveal that coal permeability demonstrates nonlinear “suppression-activation” behavior controlled by effective stress. Under low confining pressure (<3 MPa), osmotic pressure triggers fracture expansion, yielding a 366.4% permeability surge. Conversely, high confining pressure (>3 MPa) intensifies pore closure, constraining permeability enhancement. Hydraulic treatments prove most effective at low effective stress (<3 MPa), where crack volume increases by 429.2% in volume growth rate, accompanied by 230.8% and 428.6% rises in fractal dimension and connected porosity, respectively. Large pores expand to 6.2%, forming interconnected flow channels. Under high effective stress, homogeneous pore–throat structures develop, compressing flow paths and restoring near-original permeability levels. Finite element simulations demonstrate enhanced coal anisotropy post-treatment, with axial permeability increasing 217.5%, improving directional flow capacity. These findings highlight stress-dependent permeability regulation mechanisms, suggesting optimal hydraulic fracturing parameters should prioritize low effective stress conditions to maximize fracture network connectivity in deep CBM reservoirs. This study elucidates the regulatory mechanisms of stress–permeability coupling on the microstructure of coal, providing a theoretical foundation for optimizing hydraulic fracturing parameters in deep CBM extraction.