Electrokinetics at calcite-rich limestone surface: Understanding the role of ions in modified salinity waterflooding

Abstract Despite recent efforts to understand the wettability alteration process in limestone rocks during low salinity waterflooding, the findings related to wettability alteration due to changes in salinity and in the presence of a thin water film are still inconclusive. In this work, the effect of ions, temperature and solution pH on the rock and oil surface charges were explored by measuring zeta potential and predicting it using double layer surface complexation modelling (SCM). SCM fitted the trends of the measured zeta potential for both rock-brine and oil-brine interfaces by varying the surface equilibrium constants, particularly the equilibrium constant for the Ca2+ binding site. The SCM was used to predict 32 zeta potential values either measured experimentally or extracted from the literature. Zeta potential and isoelectric potential of rock-brine interface is dependent on brine salinity, sulfate concentrations and solution pH. The isoelectric point (IEP) for rock-brine interface decreased from pH of 8.5 for formation water to 7.8 for low salinity brine. Increasing the sulfate concentration in the seawater brine resulted in shifting the IEP to a lower pH value, indicating that sulfate ion shifts the zeta potential of rock-brine interface to negative over a wide pH range, due to the increased site density of >CaSO4− and the simultaneous reduction in the site densities of both CaOH2+ and CO3− as sulfate concentration increases. Bond product sum (BPS), together with total disjoining pressure calculations were used to predict wettability trends, supported by contact angle measurement, where the total BPS was observed to be the lowest and repulsive disjoining pressure generated at the COBR for LSW brines. This was observed to be significantly influenced by [COOCa+][CO3−] and [COOMg+][CO3−] bond linkages at low pH (below 8) whiles the bond linkages of [COOCa+][CaCO3−] and [COOMg+][CaCO3−] dominated the COBR interface at higher pH (above 8).