Impact of Phase Transition and Reaction Fluids on Shale Mechanical Properties after CO ₂ Injection

The injection of CO2 into shale reservoirs can alter the mechanical properties of the shale. However, the influence mechanism of the CO2 phase transition induced by the change in temperature and pressure conditions under different burial depths, and the influence degree of different reaction fluids (gas–solid, liquid–solid, and gas–liquid–solid interaction) are still unclear. To address this issue, the uniaxial compression tests and real-time acoustic emission monitoring, combined with the interaction tests between different injected gases (N2, N2–water, CO2, CO2–water) and shale, were used to investigate the variations in the mechanical strength of shale under different exposure times (0, 10, 20, 30 days) and burial depths (500, 1000, 1500, 2000 m). Results indicate that after supercritical CO2 (ScCO2)–water–rock interaction, the elastic modulus and the stress threshold at different stages of the uniaxial compression process decreased, and all stress thresholds were observed to decrease exponentially with prolonged interaction time, with the most significant reduction occurring during the crack closure stage. As the reservoir burial depth increases, all stress thresholds exhibit an exponential increasing trend yet remain lower than those of raw shale, demonstrating that the phase transition of CO2 caused by greater burial depth can inhibit the deterioration of shale mechanical strength. In the ScCO2–water–shale system, the influence of the water–rock interaction on the deterioration of shale mechanical properties is greater than the adsorption-induced irreversible expansion and the pore pressure generated by high-pressure gas, while the cementation and filling effects from chemically precipitated secondary minerals may inhibit this degradation. This study systematically elucidates the influence of the CO2 phase transition caused by burial depth changes on the mechanical properties of shale and clarifies the influence degree of different mechanisms, which provides theoretical support for the stability evaluation of the CO2 storage process in shale reservoirs.