Brine salt ion valency and concentration can dramatically affect the functionalities of surfactant formulas at brine-oil interfaces during chemical flooding processes which are an important enhanced oil recovery (EOR) approach by altering their structural and thermodynamic properties. In general, surfactant efficiency deteriorates under high salinity and/or hard water conditions in contrast to the low salinity and/or soft water conditions. On the other hand, cosurfactants are usually blended into surfactant formulas to improve chemical flooding efficiencies. It is imperative to study the effect of salt valency and concentration on the structural and thermodynamics properties of surfactant formulas at brine-oil interface. Though there have been numerous works that provided important insights into salt ion effect on the efficacy of surfactant formulas, studies about its effect on the systems containing cosurfactants are scarce. Herein, we report a molecular dynamics simulation study to investigate the salt ion valency and concentration effect on structural and thermodynamic properties of anionic surfactant (SDS) and cosurfactant (propanol) at brine-oil interfaces under typical reservoir conditions (353 K and 200 bar). We use two different cations (Na+ and Ca2+) and a wide range of ion concentrations (up to 3.96 M) to simulate brine. We demonstrate that the ion valency has a significant effect on the molecular configurations of SDS and propanol, which further influence the thermodynamic properties of brine-oil interfaces. Ca2+ ions can stimulate the formation of pentagon-like SDS-Ca2+ complexes through cation bridging, which renders a non-uniform distribution of SDS at the interface. This non-uniform distribution further affects interfacial tension (IFT). On the other hand, ion concentration has a negligible effect on the SDS and propanol functional group distributions in the direction perpendicular to the oil-brine interface. Overall, the presence of propanol lowers IFT, as it can form hydrogen bonds with water. Under low salinity conditions with Ca2+ ions, propanol can also melt the pentagon-like SDS-Ca2+ complexes which can potentially prevent surfactant precipitations. This work should provide important insights into the structural and thermodynamic properties of brine-oil interfaces with anionic surfactant and cosurfactant which can aid the optimization of chemical flooding processes for EOR.

Effect of salt ion valency and concentration on the structural and thermodynamic properties of SDS and propanol at brine-oil interfaces from molecular perspectives

ACS Fall 2021

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