Behavior of coal failure and permeability characteristics caused by liquid CO₂ phase change fracturing

Low methane extraction efficiency and prolonged treatment duration in deep, low-permeability coal seams constitute bottlenecks constraining safe and efficient coal mining. Liquid CO₂ phase-change fracturing, as a water-free permeability enhancement measure, simultaneously avoids the water-lock effect in coal seams and displaces methane within coal, thereby intensifying extraction efficacy. To elucidate the mechanism by which liquid CO₂ phase-change fracturing enhances coal permeability, coal samples from Shenmu Ningtiaota and Pingmei No. 10 Mine were studied. Through Hopkinson bar dynamic impact tests, high-pressure CO₂ fracturing tests, and triaxial compression-seepage tests, the fracture behavior and permeability characteristics of coal at different stages were investigated. Results indicate: As impact velocity increases, peak strain rate and dynamic peak strength of coal samples exhibit an upward trend. The failure mode of Shenmu coal shifts from splitting to crushing, with stress-strain curves lacking a secondary compression phase. Pingmei coal primarily exhibits splitting failure accompanied by pronounced strain hardening. A significant linear relationship exists between the vertical principal stress and the coal’s initiation pressure. For every 1 MPa increase in vertical principal stress, the initiation pressure increases by approximately 0.6 MPa. For the Shenmu coal sample with highly developed fractures, the fracturing effect primarily manifests as fracture connection and expansion. In contrast, for the structurally dense Pingmei coal sample, the effect mainly involves the formation of new fractures. Inclined fractures significantly degrade the mechanical strength of coal, substantially reducing the stress required for failure. Under confining pressure, peak stress in coal samples with inclined fractures decreased by approximately 50% compared to pristine coal, while permeability increased to 12.7−14.9 times that of pristine coal. The research reveals the three-stage synergistic permeability enhancement mechanism of “dynamic crushing-static expansion-geostress extrusion” of liquid CO₂ fracturing, which can provide theoretical reference for the popularization and application of liquid CO₂ fracturing permeability enhancement technology.