Stress-fracture-migration field coupling characteristic in gas-water bearing coal under the influence of mining activity

Coal damage and gas conductivity under mining conditions govern the gas drainage and mining safety, which concerns the coupling results of in situ stress, fracture, and fluid seepage. In this study, the evolution behaviors of damage and conductivity in coal ahead of the mining face were investigated using a novel true triaxial loading apparatus with a function of nuclear magnetic resonance monitoring. The multifield coupling mechanism and its influence on gas drainage were analyzed based on experimental data and field measurements. The results reveal that the stress-dependent coal undergoes four stages: a homogeneous compression stage with a large number of gas desorptions, anisotropic fracture growth stage with limited desorption gas, a homogeneous crack penetration stage with abundant desorption gas, and a mature pore-fracture stage with abundant gas channeling. Permeability varied from 0 to 0.17 mD throughout the compression-failure process as “X-shape” shear fracture formed with localized tension crack. Coal damage morphology transitioned from multi-winged to single-wing with increasing distance from the roadway. Elevated intermediate principal stress enhanced the crack compaction, with a mature state conductivity of pore-fracture (fractal dimension of 1.015) at the intermediate principal stress of 5 MPa. In contrast, a well-developed pore-fracture network with a fractal dimension of 0.998 was observed at 10 MPa. The gas migration was limited by the surface tension, capillary effect in micropore and mesopore, but promoted by the gas slipping effect in macropore. The maximum relative gas permeability occurred at the developed pore-fracture stage, and lower relative gas permeability was present at the compression and maturation pore-fracture stages. The desorption-migration zone, desorption zone, compression zone, and free gas zone were divided ahead of the mining face, and a relatively higher gas discharge of 0.12–1.15 m3/min and concentration of 12%–83% was present at the roadway side. Fracture development promoted the gas desorption, and efficient pore connectivity combined with low water saturation benefits gas migration. The findings provide significant insight into the coal mining of coal and gas in engineering practice.