Background Supercritical carbon dioxide (ScCO₂ )-enhanced shale oil and gas recovery serves as an effective approach to geologic CO₂ sequestration. Injecting ScCO₂ into shale reservoirs can trigger complex ScCO₂-water-rock interactions, thereby altering the material composition, nanopore structure, and adsorption capacity of shales. Examining their evolutionary processes represents a critical scientific issue in the evaluation of CO₂ sequestration efficiency.

Methods  This study investigated the shales of the Longmaxi Formation in the southeastern Chongqing area. Through simulation experiments on ScCO₂-water-rock interactions and by integrating X-ray diffraction (XRD), low-pressure CO₂/N₂ adsorption, fractal theory, and isothermal adsorption experiments, this study explored the evolution mechanisms behind the material composition, nanopore structure, fractal heterogeneity, and adsorption capacity of shales under ScCO₂-water-rock interactions. By controlling the time, temperature, and pressure of ScCO₂-water-rock interactions, this study conducted in-depth analyses of factors affecting CO₂ adsorption and sequestration efficiency.

Results and Conclusions  The ScCO₂-water-rock interactions consisted primarily of organic matter extraction and dissolution, mineral dissolution, and ion-exchange reactions, which significantly reduced the mass fractions of the total organic carbon (TOC), carbonates, and clay minerals in shales. With an increase in the reaction time and pressure, the mass fractions of the organic and inorganic components in shales varied more significantly. Organic matter extraction and mineral dissolution led to increases in the volume and specific surface area of nanopores in shales. Elevated temperature primarily promoted the development of micropores measuring 0.9–1.5 nm in size. In contrast, a rise in pressure led to a significant increase in the volumes of both micropores (<2 nm) and mesopores (2–50 nm) while also enhancing the heterogeneity of pore structures. However, as the reaction time increased, secondary minerals precipitated and blocked pores. Notably, micropores and their heterogeneity were particularly susceptible to precipitation-induced volume reduction. Compared to subcritical temperature and pressure conditions, shales after CO₂-water-rock interactions exhibited a surge in CO₂ adsorption capacity and rate under supercritical conditions. Therefore, maintaining the temperature and pressure conditions for ScCO₂ is crucial to improvements in the CO₂ adsorption capacity and sequestration efficiency of shale reservoirs. Notably, in the long-term CO₂ sequestration process, the degradation of storage efficiency induced by secondary mineral precipitation-associated pore blocking warrants more attention. Overall, the results of this study provide a theoretical foundation for both the application of ScCO₂-enhanced shale oil and gas recovery and the scientific optimization of geologic CO₂ sequestration efficiency.