Effects of salts and silica nanoparticles on oil-brine interfacial properties under hydrocarbon reservoir conditions: A molecular dynamics simulation study

Nanoparticles (NPs) have been extensively studied for enhanced oil recovery (EOR), in which the oil/brine/NPs interfacial properties play a critical role. However, the effects of NPs on oil-brine interfacial tension (IFT) have been much of debate from both experiment and simulation perspectives. In addition, the knowledge about salinity effects in oil/brine/NPs systems is still very limited. In this work, we use molecular dynamics (MD) simulations to study the effects of NPs (hydrophilic, hydrophobic, and Janus NPs, respectively) and salt concentration (NaCl and CaCl2) up to ~11 wt% on the oil-brine IFT and other interfacial properties under typical oil reservoir conditions. We find that NPs are adsorbed at the oil-brine interface, while salt ions are depleted from the interface. Ca2+ exhibits accumulations adjacent to the NP surfaces and the oil-brine interfaces before depletion, which is not observed in the cases containing Na+. All three NPs make water less ordered at the interfaces, but have negligible effects on oil-brine IFT. Divalent ions have a more pronounced layering structure than monovalent ions. While oil-brine IFT increases as the salinity increases, the three-phase contact angle is independent upon the salinity and cation types.