Mechanism Insights into the Miscibility Behavior of CO ₂ /N ₂ -Multicomponent Hydrocarbon–Surfactant Systems under Reservoir Conditions

The high miscibility of oil with CO₂ significantly enhances its mobility and thus improves oil recovery efficiency during CO₂ flooding. Many factors have strong influence on miscibility, including the chain length of oil, the component of injected gas, reservoir conditions, etc. Despite their critical role, the mechanisms underlying these influences remain poorly understood due to challenges in analyzing pore-scale mixing dynamics. Therefore, the molecular dynamics method was employed, and mixed gas-surfactant-mixed oil models were established to investigate the influence of different factors on the miscibility behavior during the multicomponent gas flooding process. Nonionic CO₂ friendly polyoxypropylene alkyl ether (POP) was chosen as a surfactant, and its influence on interfacial tension was also explored. The research results indicate a strong hindering effect of long-chain alkanes toward CO₂-oil miscibility. When its content is reduced to less than 30%, the hindering effect started to diminish. As for the gas component, less than 15% N₂ is beneficial for the movement of CO₂ and accelerates the miscibility process. In addition, as the chain length of POP increases, its ability to improve the miscibility of CO₂ and oil first increases and then decreases. The results also indicate that an optimized structure of POP with 8 PO groups has the highest ability to assist the CO₂-oil mixing process. Compared to the system without POP, the IFT can be reduced by approximately 46.78%. The simulation results can provide useful guidance for enhancing oil recovery in deep, low-permeability, and tight reservoirs.