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

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 zeta-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 zeta-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 zeta-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 zeta-potentials to slightly positive. Both magnesium and sodium ions showed almost similar behavior to change the zeta-potentials toward less negative. In addition, only minor to moderate changes in zeta-potentials were observed with the positive ions when the temperature is increased. In view of considering the reported negative zeta-potentials at the carbonate crude-oil/brine interfaces, such a change in zeta-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 zeta-potentials measured during the displacement of seawater by smartwater (10-times-reduced ionic strength seawater) in reservoir cores showed an immediate shift in the zeta-potential value from positive to negative. This instantaneous change to negative values observed in the zeta-potentials confirms the beneficial effect of smartwater to activate favorable electrokinetic interactions in carbonate reservoir cores.