The Role of Supercritical/Dense CO2 Gas in Altering Aqueous/Oil Interfacial Properties: A Molecular Dynamics Study

One of the main mechanisms contributing to enhanced oil recovery processes using compressed (supercritical) carbon dioxide (sc-CO2) is alterations in the oil water interfacial properties. However, it has been a challenge to experimentally investigate such effects. In our investigation presented here, we performed molecular dynamics simulations to explore these changes. We studied the role of sc-CO2 in changing the interfacial and transport properties of systems composed of water and pure hydrocarbons, namely, hexane, octane, benzene, and xylene. The simulations were performed at 100 bar and 350 K. It was observed that sc-CO2 accumulates at the interface, which leads to a reduction in the interfacial tension (IFT) of water oil systems. Our further analysis of such accumulation showed that the ratio of sc-CO2 density at the interface to sc-CO2 bulk density decreases as the sc-CO2 mole fraction increases. This interesting behavior is owed to the difference in the interaction between CO2 and water and between CO2 and hydrocarbon, which diverges as the CO2 mole fraction increases in the system. Moreover, our investigation indicated that sc-CO2 forms a film between the two phases, which displaces oil molecules away from the interface. This film was stabilized by hydrogen bonds between water and CO2. We also found that, as the CO2 content increases, the interfacial width increases, which contributes negatively to the IFT. Furthermore, it was found that, as the sc-CO2 mole fraction increases, the hydrocarbon diffusion coefficients increase. The diffusivity response to CO2 addition was determined by the molecular weight and polarity of the hydrocarbon.