Multiscale Mechanical Deterioration Mechanism of Shale Induced by Supercritical CO2–Water Interaction

Reliable evaluation of shale caprock integrity during long-term CO2 geological storage requires a quantitative understanding of how supercritical CO2 (scCO2)–water interaction degrades the mechanical properties of shale; however, the coupled mineral–pore–crack–energy mechanisms under reservoir-representative triaxial stress remain poorly understood. In this study, 30 day scCO2–water soaking experiments were conducted on organic-rich shale from the Longmaxi Formation in the Sichuan Basin across pressures of 0 (pure-water control), 5, 10, and 20 MPa at a constant 60 °C and temperatures of 30, 60, and 90 °C at a constant 10 MPa. Mineral composition was quantified by X-ray diffraction; pore structure was characterized by low-field nuclear magnetic resonance (NMR) calibrated with 4A molecular sieves and by scanning electron microscopy; and triaxial compression tests at a 15 MPa confining pressure were combined with crack volumetric strain and strain-energy analyses to resolve the deterioration mechanism. After soaking, carbonate and clay minerals decreased through differential dissolution, whereas the relative quartz content increased, accompanied by secondary quartz generation. Total porosity increased from 3.98% to 4.12%–4.95%, and the macropore proportion rose from 29.04% to 33.10%–35.59%. Triaxial compressive strength decreased from 325.41 MPa to 210.34–289.89 MPa and elastic modulus from 13.92 GPa to 10.71–12.73 GPa, while Poisson’s ratio increased from 0.117 to 0.120–0.181. The peak total strain energy decreased by up to 58.5%, and the dissipated energy ratio increased at every loading stage. The intensities of mineral alteration, pore expansion, and mechanical deterioration were positively correlated with soaking pressure and negatively correlated with temperature. These findings establish a multiscale deterioration chain─mineral dissolution, pore expansion, crack evolution, and energy dissipation─that explains the enhanced ductility and weakened brittleness of shale after scCO2–water exposure and provide a quantitative basis for caprock stability assessment in CO2 geological storage.