In CO 2 sequestration projects involving abandoned mines, the sealing depth and initial CO 2 injection pressure significantly affect both storage safety and efficiency. To investigate the rock–CO 2 interaction mechanisms during deep mine CO 2 sequestration, this study conducted CO 2 triaxial seepage tests and numerical simulations using granite specimens. Confining pressure was varied to represent different burial depths. The mechanical property evolution and energy characteristics of granite under different confining pressures and CO 2 injection pressures were systematically analyzed. Extended numerical simulations were conducted to determine the optimal injection pressure at various depths. The experimental results indicate that (a) the CO 2 triaxial seepage test demonstrated a strong correspondence between permeability evolution and stress–strain stages. The permeability curve exhibited a U-shaped trend with 3 stages: slow decline, slow rise, and rapid rise—corresponding closely to elastic deformation, plastic deformation, and failure stages. Given the minimum permeability observed in the elastic stage (minimum of 0.34 × 10 −16 m 2 ), engineering applications should aim to maintain the caprock within the elastic deformation stage to suppress CO 2 leakage. (b) CO 2 pressure and confining pressure have opposite effects on the mechanical and seepage properties of granite. Increased CO 2 pressure reduces peak strength and increases permeability. Under 12-MPa confining pressure, the maximum permeability sensitivity coefficient ( β ) reached 0.82. Conversely, increased confining pressure enhances peak strength and decreases permeability. Under 5-MPa CO 2 pressure, the maximum β with increasing confining pressure was 0.35. (c) Higher confining pressure significantly increases total energy ( U total ), dissipated energy ( U d ), and elastic energy ( U e ). For example, under 5-MPa CO 2 pressure, increasing confining pressure from 12 MPa to 27 MPa increased U total , U e , and U d by 102%, 165%, and 49%, respectively. In contrast, increased CO 2 pressure decreases U total and U e . Under 12-MPa confining pressure, increasing CO 2 pressure from 5 MPa to 10 MPa decreased U total and U e by 6.1% and 20%, respectively. (d) A heterogeneous rock model was constructed using a Weibull distribution on the COMSOL platform. The simulated shear failure characteristics closely aligned with laboratory results, effectively reproducing the coupling relationships among damage evolution, crack propagation, and permeability variation. Further analysis indicated that the optimal CO 2 injection pressure should be ≤68.75% of the confining pressure at the target depth. These findings provide valuable guidance for improving the safety and efficiency of CO 2 sequestration in abandoned mines.
Tian et al. (Thu,) studied this question.