A triple-layer based surface complexation model for oil-brine interface in low-salinity waterflooding: Effect of sulphate interaction with carboxylic and basic groups, pH and temperature

The determination of the electrokinetic properties of oil-brine interface is necessary to understand and evaluate the low-salinity waterflooding (LSWF) effect on enhanced oil recovery (EOR). Among the least understood aspects are the effect of SO42- interaction with the polar oil components, and temperature on the zeta-potential of the oil-brine interface. Therefore, a novel triple-layer based surface complexation model (SCM) was developed to capture the effect of SO42- interaction with both carboxylic and amine sites. One of the key uncertainties in a SCM is the surface site density of the functional groups. In this regard a Monte-Carlo based optimization algorithm for determination of these parameters as a function of pH and brine composition was devised. The SCM was successfully validated with the published zeta-potential data and was further used to evaluate the electrokinetic properties of oil-brine interface in a wide range of temperature and pH and to unravel the reason behind positive oil-brine zeta-potential at high ionic strength and pH, as reported for some crude oils in the literature. A sensitivity analysis was conducted to determine the relative contribution of site densities of carboxylic and amine groups as well as the capacitances to the oil-brine zeta-potential. The results clearly show that the interaction of SO42- ions with both carboxylic and particularly amine groups affects the surface potential significantly, thus should not be underestimated. Presence of SO42- in Na2SO4 and MgSO4 brines results in an incremental negative zeta-potential compared to that in NaCl and MgCl2 brines; confirming its role in changing charge distribution in the Stern layer. If these interactions are ignored, it should be compensated for by artificially adjusting concentration of positively and neutrally charged surface species. One important finding is that, despite the common perceptions, the site density of the oil polar groups is not constant and should be changed as pH and brine composition are changed and linear correlations between acid and base number and site densities are not justifiable. It is also found that zeta-potential tends to shift to neutral values as temperature is elevated due to the increased adsorption of divalent cations to the interface. The findings of this paper can help predict the zeta-potential of oil-brine interface more accurately and explain the observed trends in the experimental data.