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.