Insight into mass transfer mechanism of CO2–N2 mixed gas with oil

Abstract

The appropriate selection of CO₂–N₂ mixed gas is critical for cost-effective and efficient gas flooding in enhanced oil recovery. Optimizing this process requires a molecular-level understanding of the interfacial mass transfer mechanisms and their governing factors. This study employs molecular dynamics simulations to reinvestigate the classical surface renewal theory from a molecular perspective, develop a quantitative characterization method for the renewal frequency of surface voids, and elucidate the underlying micro-mechanisms of interfacial mass transfer. It was found that the renewal frequency of surface voids serves as the dominant factor influencing mass transfer, governed by the competition between the gas–oil interaction and the oil–oil interaction. The crude oil extraction efficiency improves with a higher CO₂ mole fraction in the mixed gas. A critical mole fraction (CMF) of CO₂ was identified for different alkane systems, beyond which the mass transfer performance improves significantly. Alkane systems with longer chain lengths are associated with higher CMF values. These results are attributed to the higher molecular density and stronger gas–oil interactions in CO₂-rich mixed gases. Therefore, optimizing the CO₂–N₂ mixed gases with a CO₂ fraction above the system-specific CMF can achieve high mass transfer efficiency while enhancing economic feasibility by reducing reliance on pure CO₂ with high economic costs, providing a foundation and molecular-scale insights for designing gas injection processes.