Experimental study on gas fracturing behavior of damaged basalt in compressed air energy storage caverns

Compressed air energy storage (CAES) caverns excavated in hard rock experience repeated mechanical disturbance during construction and cyclic gas pressurization during operation, which may progressively degrade surrounding rock and influence fracture stability. To investigate gas-fracture initiation in pre-damaged hard rock, basalt from a CAES site was subjected to mechanical cyclic loading–unloading, and the initial damage states were characterized using axial P-wave measurements. The specimens were subsequently machined into thick-walled cylinders and pressurized with nitrogen under confining pressures of 3, 5, and 7 MPa. The effects of confining pressure, gas injection–release cycling, and prescribed initial damage (0–0.20) on fracture initiation pressure and failure morphology were evaluated. Representative CT slices were quantified using crack-trace length and trace-length density. Within the tested conditions, increasing the confining pressure from 3 to 7 MPa raised initiation pressure by approximately 1.5–3 MPa, whereas repeated gas injection–release cycling reduced it. At 3 MPa confinement, increasing initial damage from 0 to 0.20 reduced initiation pressure by approximately 46%, from 9.6 to 5.1 MPa. CT analysis showed spatial variation in crack-trace length and density within the partially scanned fractured specimen. A condition-specific damage-corrected pressure relationship, anchored to the measured intact-specimen baseline at each confining pressure, was empirically calibrated and reproduced the observed damage-induced pressure decrease. These results provide rock-scale experimental evidence relevant to gas-fracture initiation in damaged basalt for CAES-related applications. Their quantitative interpretation is limited to the tested material, specimen geometry, nitrogen-injection procedure, confining-pressure range, and initial-damage range.