Molecular dynamics investigation of the aqueous film barrier to CO2 transport and miscibility with oil for geological storage and enhanced oil recovery processes

The aqueous phase in the porous medium separates CO2 and crude oil, significantly impacting CO2 flooding and underground storage. Molecular dynamics simulations were employed to investigate CO2–water–crude oil (modeled here by n-octane) systems. The influence of water film thickness ratio, temperature, and pressure on interfacial characteristics and CO2–oil miscibility was systematically analyzed. Simulations identified the water film thickness ratio as the critical factor governing CO2 breakthrough and miscibility. When this ratio reaches or exceeds 0.5, changes in interfacial parameters slow markedly, and CO2 essentially loses the ability to penetrate the water film. Consequently, the critical threshold for effective CO2 breakthrough and subsequent mixing with oil is a ratio below 0.5. Attributed to the dominant effect of molecular compression over the minor effect of increased CO2 accumulation at the interface, the increase in temperature (300–373 K) led to a decrease in interfacial tension (from 51.28 to 42.35 mN/m) and an increase in CO2 boundary area agglomeration density (0.092–0.164 g/cm3). The increase in pressure (10–35 MPa) led to a slight increase in interfacial tension (48.06–52.76 mN/m) and an increase in CO2 boundary area agglomeration density (0.113–0.167 g/cm3). Although the changes in temperature and pressure significantly change the interfacial properties, they do not change the ability of CO2 to break through the critical water film thickness ratio (0.5) or dominate the overall miscible process. This study provides vital theoretical insights into the microscopic phase equilibria of CO2–crude oil–water systems, offering valuable guidance for optimizing enhanced oil recovery (EOR) by CO2 and subsurface storage.