Reactivity of Wet scCO₂ toward Reservoir and Caprock Formations under Elevated Pressure and Temperature Conditions:...

Carbon capture and storage (CCS) and CO₂-based geothermal energy are promising technologies for reducing CO₂ emissions and mitigating climate change. Safe implementation of these technologies requires an understanding of how CO₂ interacts with fluids and rocks at depth, particularly under elevated pressure and temperature. While CO₂-bearing aqueous solutions in geological reservoirs have been extensively studied, the chemical behavior of water-bearing supercritical CO₂ remains largely overlooked by academics and practitioners alike. We address this knowledge gap by conducting core-scale laboratory experiments, focusing on the chemical reactivity of water-bearing supercritical CO₂ (wet scCO₂) with reservoir and caprock lithologies and simulating deep reservoir conditions (35 MPa, 150 °C). Employing a suite of high-resolution analytical techniques, we characterize the evolution of morphological and compositional properties, shedding light on the ion transport and mineral dissolution processes, caused by both the aqueous and nonaqueous phases. Our results show that fluid-mineral interactions involving wet scCO₂ are significantly less severe than those caused by equivalent CO₂-bearing aqueous solutions. Nonetheless, our experiments reveal that wet scCO₂ can induce mineral dissolution reactions upon contact with dolomite. This dissolution appears limited, incongruent, and self-sealing, characterized by preferential leaching of calcium over magnesium ions, leading to supersaturation of the scCO₂ phase and reprecipitation of secondary carbonates. The markedly differing quantities of Ca²⁺ and Mg²⁺ ions transported by wet scCO₂ streams provide clear evidence of the nonstoichiometric dissolution of dolomite. More importantly, this finding represents the first reported observation of ion transport processes driven by water continuously dissolved in the scCO₂ phase, which challenges prevailing views on the chemical reactivity of this fluid and highlights the need for further investigation. A comprehensive understanding of the chemical behavior of CO₂-rich supercritical fluids is critical for ensuring the feasibility and security of deep geological CO₂ storage and CO₂-based geothermal energy.