Smart Water Flooding in Berea Sandstone at Low Temperature: Is Wettability Alteration the Sole Mechanism at Play?
Authors: Mohamed Mohamed Ibrahim , Alvarado Vladimir 
Published: October 2017

Smart waterflooding in hydrocarbon reservoirs has been a subject of intense speculation. Several recovery mechanisms have been proposed, including wettability alteration toward water-wetness. In contrast, the role of fluid-fluid interactions has been overlooked. In this study, we evaluate rock-fluid interactions in connection with the observed increased oil recovery under smart waterflooding conditions. Focus is placed on fluid-fluid interfacial rheological effects resulting from adjusting the injection brine ionic profile. Berea sandstone cores were aged at high oil saturation and subsequently allowed imbibed spontaneously with a number of selected brines to investigate the impact of sulfate concentration and to compare responses to low- vs. high-salinity brines. For each sample, a sister end trim was aged and exposed to the respective fluids to measure oil-water contact angle. Supplementary, we measured zeta potential at the rock-fluids interfaces to examine the effect of low-salinity brine on the electrical double-layer expansion. The spinning drop and pendant drop methods were used for interfacial rheological measurements, to obtain interfacial visco-elastic moduli and interfacial tension, respectively. Spontaneous imbibition results showed significantly higher oil recoveries for brines with greater sulfate concentrations or lower salinities. However, the rates of imbibition, which reflect the wettability of the rock surfaces, did not differ significantly. Contact angle measurements also correlated with the results of spontaneous imbibition. Measurements showed a slight change from strongly oil-wet to intermediate-wet. Zeta potential results indicate a possible expansion of the electrical double layer at the rock-brine interface, and thus a minor change of wettability. It was noticed that even at intermediate-wet conditions, oil droplets were still attached to surface. We also observed that when the relatively small oil droplets coalesce on the surface, the newly formed oil droplet detached more readily. Changes in wettability cannot sufficiently explain the observed increase in oil recovery. It is believed that other factors such as interfacial visco-elasticity could affect capillary-driven interactions in the pore space. The spinning drop measurements show that by increasing the sulfate concentrations or decreasing salinity, the crude oil-water visco-elasticity increases, as previously shown in our group through interfacial shear rheological measurements. The wettability observations along with the interfacial rheology findings provide a more satisfactory explanation of the recovery trends observed. Our findings indicate that during smart waterflooding processes, wettability alteration is unlikely the sole mechanism at play. After wettability alteration has taken place, the higher crude oil-water visco-elasticity due to presence sulfate ions or low-salinity brine, allowed oil droplets to coalesce forming a continuous oil banks and contributing to more oil recovery.