Effect of supercritical CO2-water-shale interaction on mechanical properties of shale and its implication for carbon sequestration

The mechanical behaviors of shale are important parameters influencing the CO2 storage security in shale reservoirs. Thus, understanding the mechanical properties alteration of shale induced by the injected CO2 is essential for evaluating the stability of shale reservoir rock and caprock. In this study, the impact of supercritical CO2(ScCO2)-water soak pressure and temperature (P = 0, 10, 15, 20 MPa, T = 308, 323, 338, 353 K) on the mechanical properties of shale were investigated. Mineral, pore and mechanical alterations in shale triggered by ScCO2-water soak were obtained by multiple characterization methods. The results indicated that ScCO2-water soak caused the decrease in clay and carbonate mineral contents, and the increase in macropore volume and porosity. Then, the average triaxial compressive strength and elastic modulus of shale decreased from 286.65 MPa to 156.90–247.20 MPa, and 13.62 GPa to 10.52–12.99 GPa, respectively, while Poisson's ratio v in shale increased from 0.1147 to 0.1179–0.1780. ScCO2-water soak induced mechanical weakening in shale is positively correlated with the soak pressure and negatively associated with soak temperature. The dissipated energy ratio decreased first and then increased during the stress loading process in all the tested shales, ScCO2-water soaked shale has a larger proportion of energy dissipation in each stage of stress loading. The results inferred that ScCO2-water soak caused an enhanced plastic deformation behavior in shale. The mechanical degradation in shale may increase the failure risk of reservoir rock or caprock, which should be concerned in CO2 geological storage engineering.