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Understanding crude oil/brine interface chemistry is essential to elucidating the effect of low-salinity waterflooding (LSWF) on enhanced oil recovery (EOR). The acid and base functional groups in crude oil result in an electrostatic interaction with the rock’s surface, thereby affecting wettability conditions. Moreover, the content of carboxyl acid components is a key factor influencing electrostatic interaction during LSWF. In this study, the number of carboxyl groups in four different crude oils with varying acid number (AN) was estimated using a combination of zeta potential experiments and a triple-layer surface complexation model. In addition, the surface complexation modeling parameters for the dissociation of carboxyl groups and the adsorption of calcium and magnesium ions were also determined. The experimentally determined parameters and carboxyl groups sufficiently predicted the crude oil/brine interface at high and low salinities of seawater and formation water. The density of carboxyl groups (expressed in sites/nm2) is logarithmically related to the AN of crude oil, and it is revealed that the effect of AN on the density is lower for high-AN crude oil. Further, for crude oils with high AN, divalent cations exhibit higher adsorption ability than those with low-AN crude oil. The percentage of resin components in crude oil has a linear relationship with the number of carboxyl sites, thus indicating the importance of resin components in crude oil/brine interface chemistry. The study discusses the influence of AN on potential distribution and possible wettability alteration by LSWF in sandstone and carbonate reservoirs.

Several mechanisms have been proposed for enhanced oil recovery (EOR) in low salinity waterflooding (LSWF). Coupling of the significant processes affecting crude oil-brine-rock system is necessary to understand the LSWF effect. In this study, mineral thermodynamic equilibrium and surface complexation reactions at crude oil/brine and calcite/brine interfaces were coupled with solute transport to simulate LSWF in carbonate reservoir. The dissolution and precipitation of minerals were considered thorough thermodynamic phase-equilibrium model, and the triple-layer surface complexation model was developed to predict the interface reactions and the associated surface and zeta potentials. These models were coupled with solute transport model to predict ionic profiles and oil recovery during LSWF. In the integrated geochemical model, the crude oil was considered as colloids and the ionic adsorbed/ionized and un-ionized surface groups of oil were transported via advective and dispersive transport. These sub-models were coupled in a geochemical code PHREEQC. The coupled model was first used to predict Ca2+ and Mg2+ profiles in chalk saturated with NaCl without crude oil. The agreement between published experimental data and simulation results validate the proposed model. A nearly equal equilibrium constant in the surface complexation model provides a similar breakthrough composition for Ca2+ and Mg2+ ions. The model was further validated in chalk core aged with the crude oil. Both model and experimental results show an earlier breakthrough composition of sulphate in oil-aged core. The model was then used to predict ionic profiles and oil recovery in two-phase flow experiment. The modelling results reproduces the experimental data on relative concentration of ionic species and pH increase with dilution of injecting water, however additional mechanism should be incorporated in the model for better prediction of oil recovery.

Wettability alteration (from oil-wet to mixed-or water-wet condition) is the most prominent mechanism in lowsalinity water flooding (LSWF) for enhanced oil recovery (EOR) in sandstone reservoirs. Although several factors influence the wettability alteration, many efforts have been made to find the main controlling factor. In this study, the influence of interface properties of sandstone/brine and thermodynamic equilibrium of sandstone minerals were evaluated to understand the wettability alteration during LSWF. A triple-layer surface complexation model built-in PHREEQC was applied to a quartz/brine interface, and the modeling results were verified with zeta potential experimental data. This model was combined with that of kaolinite/brine to predict sandstone/brine interface properties. The measured and predicted sandstone zeta potentials were between those obtained for quartz and kaolinite in the diluted seawater. The predicted surface potential of sandstone together with that of crude oil was used in extended Derjaguin−Landau−Verwey−Overbeek theory to estimate the attractive or repulsive force. Consideration of thermodynamic equilibrium between minerals and solution significantly increased the pH and hence resulted in an increase in negative surface potential in the surface complexation. This provided a strong repulsive force between crude oil and sandstone, thus resulting in a more water-wet condition.