Microscale mineral dissolution and mechanical degradation in lacustrine shale during supercritical CO2–water interaction

Shale formations are considered suitable reservoir and caprock candidates for CO 2 geologic sequestration. Upon CO 2 injection, fluid–rock reactions may weaken host rocks. Lacustrine shales, with rapid lithofacies shifts and pronounced compositional heterogeneity, warrant close investigation of the mechanisms of CO 2 –induced chemo-mechanical alteration. In this study, rock–fluid interaction experiments were conducted on three representative types of lacustrine shale – carbonate-rich, felsic, and mixed lithology– at 10 MPa and 40 °C. The results revealed that under supercritical CO 2 (SCCO 2 )–water–shale interactions, carbonate minerals dissolved at significantly higher rates than feldspar, with the carbonate-rich shale showing the greatest carbonate mineral loss (18.53%). Scanning electron microscopy (SEM) imaging revealed marked increases in post-reaction surface porosity and pore connectivity across all samples, with the trend becoming more pronounced at higher carbonate content. Dolomite dissolution was found to initiate at high-energy defect sites and propagate inward into the grain interiors, offering new insight into early-stage weakening under SCCO 2 exposure. Following reaction, all shale samples exhibited significant mechanical degradation: Young's modulus decreased by 14.3%–20.5%, hardness by 14.3%–25.0%, and fracture toughness by 11.8%–55.2%. Among them, the carbonate-rich shale exhibited the most substantial reductions, whereas the felsic shale – dominated by feldspar (albite, orthoclase) and quartz – showed the least deterioration. These findings suggest that, in the short term following CO 2 injection, the evolution of physical properties in reservoir rocks is primarily governed by the initial carbonate mineral content, establishing a predictive link between mineralogy and short-term storage integrity.