Oil Recovery by Low-Rate Waterflooding in Low-Permeability Water-Wet Sandstone Cores

Smart water or low-salinity (LS) water injection are environmentally friendly methods for efficient hydrocarbon recovery. Wettability alteration toward more water-wet conditions and increased spontaneous imbibition (SI) of water are responsible for enhanced oil production. Wettability alteration and SI to expel oil from the low-permeability matrix are time-dependent processes and both injection rate and oil viscosity are important factors affecting the contribution of capillary and viscous forces to oil production. Low flooding rate must be applied in laboratory corefloods to allow for SI and improved sweep to take place. Residual oil saturation by waterflooding and SI has previously been determined in low-permeability limestone and in higher permeability sands under various flooding rates, wetting conditions, and initial oil saturations. In this study, the effect of flooding rate on oil displacement from low-permeability, water-wet Bandera Brown outcrop sandstone cores has been examined. Viscous forces have been varied by injection at two different rates in addition to SI experiments and using mineral oils with different oil viscosities. The results showed small differences in oil recovery by SI and viscous flooding at high and low rates, indicating that capillary forces contribute significantly to the oil mobilization and production process from this low-permeability, water-wet rock. By varying the oil viscosity, the results indicated that capillary forces were especially important for oil displacement at higher oil viscosity as the ultimate oil recovered by low-rate injection was higher than that from the high-rate injection. Capillary number calculations indicated that viscous forces should be dominant in the recovery tests; however, the experiments showed that capillary forces were important for efficient oil displacement from the low-permeability, water-wet cores used in this study. There was no direct link observed between generated pressure drops at high and low injection rates, including SI, and the ultimate oil recovery. Thus, to simulate oil production in the middle of the reservoir, it was concluded that low-rate waterflooding is needed in laboratory tests to allow SI into the matrix to displace oil by positive capillary forces. The combination of using oils that differ in viscosity at different injection rates could add some additional information to the literature on how to increase the efficiency of waterflooding with a low injection rate.