A novel oil/brine surface complexation model: Capturing the dynamic nature of the interface using IFT

Abstract

The electrical properties of the oil/brine interface play a principal role in wettability alteration during low salinity/water flooding. There are few existing surface complexation models (SCMs) for the oil/brine interface. Similar to the mineral surface, all models assume that the surface site density of oil/brine is always constant. Assessment of the existing models shows that they ignore the dynamic nature of the crude oil/brine interface and fail to capture experimentally measured ζ potentials appropriately. The current study proposes a novel diffuse layer SCM considering the interfacial concentration of surface carboxylic acid as a function of brine pH, salinity, and composition. The oil/brine interfacial tension (IFT) was utilized to describe the change in the interfacial concentration of > COOH sites vs. pH and ionic strength. The developed model matches the experimental data of the literature far better than previous models successfully. Based on the model, 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. The novel model surpasses the previous model at high and low pH conditions. At a high pH range, due to the significant transport of acid groups to the interface, the oil surface becomes more negatively charged, which was accurately captured by the proposed model (while the previous models are malfunctioning at high pH conditions). Assuming maximum > COOH density at the whole pH range, previous models use a high > NH density to compensate for the extra negative surface charge due to > COOH dissociation. Therefore, these models overestimate the zeta potential significantly at low pH conditions, where > NH protonation is the dominant reaction. One of the main features of the developed model in this study is to predict a critical salinity for the extremum of zeta potential. All previous models state that salinity reduction must result in a decrease in the zeta potential. These models cannot capture the observed rise in zeta potential with further brine dilution after a specified salinity. However, the presented model of this study could predict the increase in the zeta potential at lower salinities.