Impact of Salinity and Water Ions on Surface Charge Alteration in Arab D Reservoir Cores at Elevated Temperatures

Abstract

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 of the 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 surface charges based on the electrophoretic mobility (EPM) measurement technique using powdered crushed core samples, which may not reflect the natural conditions existing in the subsurface reservoirs. In this study we used a state-of-the-art experimental technique based on streaming potential measurements to determine surface charge in intact Arab-D reservoir core samples saturated with different brine salinities and individual ion compositions. We also believe that this is the first time such a measurement technique has been used to measure surface charges in reservoir cores at elevated temperatures. The results indicated a favorable effect of sulfate ions in Arab-D rocks to alter the surface charge to more negative and the reactivity of these ions increased significantly by almost one order of magnitude at higher temperatures. Such a surface charge alteration to an extreme negative obtained upon exposure to injection waters containing sulfates would release the oil droplets from the carbonate surface. Among the positive ions, calcium showed the highest reactivity to shift the surface charge to slightly positive. Both magnesium and sodium ions showed almost similar behavior to change the surface charge toward less negative. In addition, only minor to moderate changes in surface charge were observed with the positive ions when the temperature is increased. The dynamic time-dependent effects on surface charge measured during the displacement of seawater by SmartWater (10 times diluted seawater) in reservoir cores showed an immediate shift in the surface charge from positive to negative. This instantaneous change observed in the surface charge confirms the beneficial effect of SmartWater on wettability alteration. All of these novel findings from this study will provide several major fundamental insights to better understand the dynamic role of surface charge alteration mechanism on oil recovery in SmartWater flooding.