Study on Wettability Reversal Evolution and Wax Removal: Enhancement Mechanism Based on the CO₂ Synergistic Effect

CO2 flooding holds significant potential for the EOR in high-wax reservoirs. Wax crystals precipitated from high-wax oil adhere to pore channels, reducing productivity. The mechanisms of wettability reversal driven by wax/CO2 interactions and CO2-induced stripping of wax deposits are critical. This study employs molecular dynamics simulations and NMR experiments to reveal the wettability reversal mechanism under CO2-wax coupling and investigates the wax removal behavior enhanced by CO2. Results demonstrate that wax coverage on sandstone surfaces masks hydrophilic groups, increasing the water contact angle from 3.3 to 62.2° and shifting wettability from strongly hydrophilic to weakly hydrophilic. Notably, high-density CO2 forms an isolation layer on the wax surface, hindering water–rock contact and increasing the contact angle to 137.14°, achieving wettability reversal through a CO2 synergy. Additionally, postwax deposition NMR wettability index reverses from +0.27 to −0.13, while the wax deposition increases MMP by 8.5% and reduces recovery by 5.2%. Owing to high diffusivity and low interfacial tension, scCO2 demonstrates exceptional efficiency, far surpassing water flooding, with optimal efficiency at 8–12 MPa, though its advantage diminishes above 70 °C. This research provides theoretical support for CO2 flooding in high-wax reservoirs for EOR.