Molecular Dynamics Characterizations of the Supercritical CO2–Mediated Hexane–Brine Interface

Abstract

In the carbon dioxide (CO2) enhanced oil recovery (EOR) process and subsequent geological CO2 sequestration, a ternary system consisting of CO2, crude oil, and brine exists in the reservoir due to the common practice of injecting CO2 together with brine. In this paper, we carried out molecular dynamics simulations to study the interfacial properties of the ternary CO2, hexane, and 1.52 mol/L sodium chloride (NaCl) solution system under 330 K and 20 MPa with different CO2 compositions at the supercritical state, which are very important for the efficiency of the EOR and CO2 sequestration processes. We observed that CO2 mixes well with hexane and a clear interface separates the CO2–hexane mixture with the NaCl solution. The interfacial roughness increases with the CO2 composition, indicating deeper molecular penetrations and shorter capillary wavelengths, which leads to the reduced interfacial tension. Interestingly, the surface excess of CO2 reaches maximum at a CO2 molar fraction of 62.5% (or a weight fraction of 46%), which implies the amphiphilic feature of CO2, acting like surfactants, toward the hexane–brine interface. The orientational preferences of CO2, hexane, and water molecules at the interface are more random at higher CO2 compositions, as a result of the increased absolute amount of CO2 and the absence of hexane at the interface.