The Effect of Monovalent Ions on the Oil/Brine Electrical Behavior Using a Novel Surface Complexation Model

Oil/brine electrical behavior could control the wettability of the oil/brine/rock system during low salinity water flooding. However, there is a lack of understanding of chemical reactions occurring on the oil surface. There are a few surface complexation models (SCMs) for the oil/brine interface, and all of them assume that the surface site density is a fixed number, like the mineral surface. The current study assessed two existing models on this subject (Model A and B). These models ignore the dynamic nature of the oil/brine interface. Therefore, they failed to capture experimentally measured ζ potentials appropriately. Therefore, this study constructed a novel diffuse layer SCM (Model C) considering the interfacial concentration of surface carboxylic acid as a function of brine salinity for each salt, including NaCl, Na2SO4, CaCl2, MgCl2, and NaHCO3. Model C matches the experimental data of the literature far better than A and B. Based on Model C, Na+, Cl-, and SO42- cannot be adsorbed on the oil/brine interface; however, the role of these ions in the electrical behavior of crude oil/brine is only to affect the interfacial concentration of -COOH. For example, an increase in Na+ reduced the oil/brine IFT. Therefore, more carboxylic groups would be available at the oil/brine interface. As a result, -COO- concentration increases, and the crude oil surface becomes more negatively charged. The current study indicated that Ca2+ and Mg2+ are not the only factors that make the interface more positively charged. However, an increase in IFT (in this study by salinity reduction) significantly makes the oil/brine interface more positive, too.