Molecular dynamics simulation of enhancing surfactant flooding performance by using SiO2 nanoparticles

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In this study, four kinds of commonly used surfactants in oil fields, namely anionic surfactant sodium dodecyl sulfate (SDS), cationic surfactant hexadecyl trimethyl ammonium bromide (CTAB), nonionic surfactant primary alcobol ethoxylate (AEO-9) and zwitterionic surfactant dodecyl dimethyl carboxyl betaine (BS-12), were used as the research objects. The oil–water interfacial behaviors at different concentrations were studied by molecular dynamics simulation. Then, silica (SiO2) nanoparticles were combined with surfactant to observe the influence of nanoparticles on the configuration of surfactant monolayer, and to explore the optimal conditions for the synergistic flooding scheme of these nanoparticles and surfactant. The thickness and density of the oil–water interface layer, the radial distribution function (RDF) between the surfactant head group and water molecules, the mean square displacement (MSD) of water molecules around the surfactant head group, the oil–water interfacial tension (IFT) and the surfactant interface formation energy (IFE) were investigated. Surfactant flooding performance of different types and the effect of nanoparticles on surfactants were discussed. The results show that the surfactant with different ionic types have the best concentration in reducing the oil–water interfacial tension, and the interfacial tension is significantly reduced to 7.33 mN/m after adding nano-SiO2 particles in BS-12. After kinetic equilibrium, SDS and CTAB are arranged in an approximately regular manner, non-polar groups are inserted into the oil phase, polar groups extend into the aqueous phase, the long-chain head groups of AEO-9 molecules spread and stack at the interface, and BS-12 is irregularly distributed and some molecules are immersed in the oil phase. After adding nano-SiO2 particles, SDS, BS-12 and nano-SiO2 particles can be observed mutually repulsive in the interface morphology, while CTAB, AEO-9 and nano-SiO2 particles exhibit mutual attraction.