Borehole pressure response and rock fracture evolution during pipe-assisted high-pressure gas-impact under true-triaxial stress

Water-lean rock stimulation and non-explosive excavation approaches are increasingly pursued to reduce freshwater use and improve operational safety. Among these options, high-temperature, high-pressure gas-impact fracturing provides a waterless means to rapidly pressurize boreholes and trigger fracture initiation and growth, yet its near-field transient borehole loading remains insufficiently quantified. Here, true-triaxial gas-impact fracturing tests were conducted on cubic rock specimens containing a horizontal borehole using a biomass-driven gas generator and a vessel-pipe-borehole system. The results reveal a dual-frequency borehole loading process: a low frequency pressurization envelope whose peak loading rate increases with release pressure, and moderately high frequency pulsations excited by pipe-borehole acoustic resonance, with pulse amplitude increasing with the vessel depressurization rate. Correspondingly, the number of radial cracks on the borehole wall increases from two to four and surface fractures evolve from Y-shaped to wide-angle Y/H-shaped patterns as pressure increases. Mechanistically, anisotropic stress favors initial vertical radial cracking, higher low frequency loading rates activate more defects before stress redistribution, and resonance-driven pulsations superimpose instantaneous crack-driving-force peaks and mixed mode conditions near free surfaces, promoting branching. These findings indicate that both the low frequency loading rate and resonance-induced pulsations should be jointly controlled when designing efficient and safer high-temperature gas-impact fracturing.