Effect of Salinity and Water Ions on Electrokinetic Interactions in Carbonate Reservoir Cores at Elevated Temperatures

Summary Smartwater flooding through tailoring of injection-water salinity and ionic composition is becoming an attractive proposition for improved oil recovery in carbonate reservoirs. Most recent studies suggest that surface-charge change induced by lower salinity and certain water ions on carbonate surfaces is the main mechanism responsible for favorable wettability alteration and, consequently, higher oil recovery in smartwater flooding. Unfortunately, these studies determined ζ-potentials using the technique of electrophoretic-mobility (EPM) measurement with powdered-crushed-core samples, which may not reflect the natural conditions existing in the subsurface reservoirs. In this study we used an experimental technique dependent on streaming-potential (SP) measurements to determine ζ-potentials in intact carbonate reservoir core samples saturated with different brine salinities and individual ion compositions at both ambient and elevated temperatures. The results indicated a favorable effect of sulfate ions in carbonate reservoir cores to alter ζ-potentials toward more negative, and the reactivity of these ions increased significantly by nearly one order of magnitude at higher temperatures. Among the positive ions, calcium showed the highest reactivity to shift the ζ-potentials to slightly positive. Both magnesium and sodium ions showed almost similar behavior to change the ζ-potentials toward less negative. In addition, only minor to moderate changes in ζ-potentials were observed with the positive ions when the temperature is increased. In view of considering the reported negative ζ-potentials at the carbonate crude-oil/brine interfaces, such a change in ζ-potential to an extreme negative obtained in carbonate reservoir cores upon exposure to injection waters containing sulfates could have a favorable effect on wettability alteration because of the resulting same polarity at both the interfaces. The dynamic time-dependent effects on ζ-potentials measured during the displacement of seawater by smartwater (10-times-reduced ionic strength seawater) in reservoir cores showed an immediate shift in the ζ-potential value from positive to negative. This instantaneous change to negative values observed in the ζ-potentials confirms the beneficial effect of smartwater to activate favorable electrokinetic interactions in carbonate reservoir cores.