Experimental Investigation of Shale Wettability and Its Alteration Mechanisms in Supercritical CO2–Brine–Oil Systems: Implications for CO2 Storage and Enhanced Oil Recovery

CO₂ is highly effective at enhancing shale oil recovery while facilitating geological sequestration. The interaction between supercritical CO₂ (ScCO₂) and shale significantly alters the wettability, a key factor influencing both the oil recovery efficiency and the CO₂ storage capacity. A novel investigation was carried out to explore the dynamic characteristics of the CO₂–brine–oil–shale multiphase system and the mechanisms underlying wettability alteration using samples from the Yanchang shale formation in China. The wettability of shale samples was characterized by high-temperature and high-pressure contact angle tests, while quantitative analysis of the mineral composition was conducted through ScCO₂-shale reaction experiments combined with XRD. FTIR and zeta potential tests determined the types and amounts of surface charged groups, and 3D surface morphology scanning reflected structural changes. Results showed that ScCO₂ injection significantly weakened the water-wetness of shale. Temperature and pressure were key external factors: increasing temperature shifted the wettability from water-wet toward neutral-wet, while higher pressure drove it closer to oil-wet. After ScCO₂ treatment, significant alterations in the physicochemical properties of shale were observed, which fundamentally influenced its wettability. The content of clay minerals, encompassing both hydrophilic and hydrophobic phases, was reduced, whereas the proportion of hydrophilic quartz increased. The decrease in hydrophilic hydroxyl groups (OH), increase in oleophilic oxygen-containing groups (C–O–C), and reduction in the zeta potential collectively altered the multiphase interfacial forces, thereby impacting the spreading behavior of liquids on shale surfaces. ScCO₂ also increased shale surface roughness at the nanoscale, which can alter fluid distribution by providing additional adsorption sites and modifying the spreading behavior of fluids. These findings provide theoretical support for artificial wettability regulation, aiding shale oil recovery and CO₂ storage.