The mechanism of wettability changes of carbonate rocks under supercritical CO2-water-rock interaction: Implications for CO2-enhanced gas recovery and geo-sequestration

Injecting CO 2 into carbonate hydrocarbon reservoirs can enhance gas recovery (EGR) and achieve carbon sequestration. However, the mechanism by which supercritical CO 2 -water-rock interactions alter carbonate wettability remains unclear. This study investigates Maokou Formation carbonates from the Wolonghe gas reservoir (Sichuan Basin, China) under reservoir conditions (85 °C, 55 MPa). Contact angle measurements, X-ray diffraction, scanning electron microscopy, and nuclear magnetic resonance were used to monitor wettability evolution. Numerical models were developed to assess the impact of wettability on gas recovery and CO 2 sequestration. Results show that CO 2 -water-rock reactions cause significant changes in surface minerals and formation water. Extensive dolomite dissolution and subsequent precipitation of calcium carbonate and calcium sulfate occur. The abundance of hydrophilic minerals decrease, leading to a transition from hydrophilic to hydrophobic reservoir wettability. Prolonged reaction further diminishes water-wetness, while increased pore size due to dominant mineral dissolution reduces water retention and hydrophilicity. Simulations indicate that reduced hydrophilicity enhances CO 2 structural trapping but decreases CO 2 mobility and CH 4 recovery efficiency. Residual, dissolution, and mineral trapping capacities also decline, increasing CO 2 leakage risks and reducing sequestration safety. Therefore, maintaining water wettability is crucial for efficient and safe CO 2 -EGR operations. This study offers theoretical insights into CO 2 -water-rock interaction mechanisms affecting carbonate wettability and informs the integrated evaluation of CO 2 -enhanced recovery and sequestration strategies. • The impact of CO 2 -water-rock reactions on the mineral composition and pore structure of carbonate rocks was analyzed. • The mechanism of the impact of CO 2 -water-rock reactions on the wettability of carbonate rocks was analyzed. • The effects of different wettability conditions on CO 2 gas displacement for improving gas reservoir recovery were discussed. • The effects of different wettability conditions on different sequestration mechanisms of CO 2 were compared. • Water wettability enhances gas recovery and the safety of CO 2 sequestration.