Study on coal damage and pore-fracture evolution ahead of the mining face under the disturbance of coal mining

Abundant coal and coalbed methane resources are endowed in the vast basins of China. Mining disturbance alters the primary stress and fracture fields, thereby impacting the transportation and distribution of coalbed methane as well as gas extraction. This study conducted a true triaxial stress loading experiment on coal samples to investigate the evolution of the stress-fracture-damage field induced by mining stress, utilizing a self-developed low-field nuclear magnetic resonance (NMR) true triaxial test system. The porosity, compressibility, connectivity, damage characteristics, and spatial distribution of pore-fracture structure were quantitatively characterized in different regions through T2 distribution curves and MRIs. Subsequently, the relationship between dynamic pore-fracture and gas extraction was further discussed, combined with practical engineering. The results show coal’s pore-fracture was identified as adsorption pore (0-0.1 μm，60%-74%), seepage pore (0.1-1 μm，26%-29%), and migration pore (>1 μm, 0%-13%). The coal experienced elastic, elastic-plastic, plastic, and damage stages. Corresponding to elastic, plastic, and damage stages, the porosity of 4.9%, 2.62%, and 3.1%, the porosity of 2.46%, 1.45% and1.8%, and porosity of 2.57%, 1.03% and 1.95% occurred in in-situ stress zone, in peak-stress zone, and working face, respectively. Before damage, the adsorption pore gradually transforms into seepage and migration pores, in contrast, the adsorption and migration pores gradually transform into seepage pores during damage, which results in enhanced pore connectivity. The heterogeneity of the adsorption and migration pores enhanced while presenting an opposite trend in the seepage pore, accompanied by increased compressibility and significantly developed pore-fracture during the range from the in-situ stress zone to the working face. Adsorption pore, seepage pore, and seepage-migration pores dominate pore-fracture evolution within the in-situ stress zone, peak stress zone, and working face, respectively. In the in-situ stress zone, compression and expansion of pore-fracture form adsorption-type gas desorption-seepage channels, while developed shear and tensile fractures in the peak zone generate adsorption-permeation-migration type gas distribution channels, with gas seepage-migration channels in the working face due to large-scale shear fissures and enhanced connectivity between tension fractures. During the gas extraction, two distinct stages were identified: a stable stage characterized by gas adsorption-desorption in the in-situ stress zone and a peak stage involving gas desorption-migration in the damage zone. The damage of coal in the peak zone significantly impacts the underground gas extraction efficiency. These findings provide theoretical support for the safe and efficient exploitation of coal and associated gas resources.