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

Abstract

Smart Water or low salinity water injection are environmentally friendly methods for efficient hydrocarbon recovery. Wettability alteration towards more water-wet conditions and generation of positive capillary forces and spontaneous imbibition are responsible for the increased oil production. Spontaneous imbibition to expel oil from the low permeable matrix is a time-dependent process and both injection rate and oil viscosity are important factors affecting the contribution of capillary and viscous forces to the oil production. It is hypothesized that when capillary forces and spontaneous imbibition are important for oil production, low flooding rate must be applied in laboratory corefloods to allow for wettability alteration. In this study the effect of flooding rate on oil displacement from low permeable sandstone cores has been examined. Viscous forces have been varied by injection at two different rates and performing spontaneous imbibition experiments, in addition to varying the oil viscosity. Low permeable, water-wet Bandera Brown outcrop sandstone cores were used as the porous medium, and synthetic oil and formation water were used to avoid any wettability alteration during fluid restoration and oil displacement. The results showed only small differences in oil recovery by spontaneous imbibition and viscous flooding at high and low rate, proving that capillary forces and spontaneous imbibition were major contributors to the oil mobilization and production process. By varying the oil viscosity, the results indicated that capillary forces were especially important for oil displacement at higher oil viscosity, since the ultimate oil recovered by low-rate injection was higher than that from high-rate injection. As expected, capillary number calculations indicated that capillary forces were important for efficient oil displacement from the low permeable, water-wet cores used in this study. However, there was no direct link observed between generated pressure drops at high and low injection rate, including spontaneous imbibition, 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 spontaneous imbibition into the matrix to displace oil by positive capillary forces. The combination of using oils that differ in viscosity in different injection rates could add some additional information in the literature on how to increase the efficiency of waterflooding by a low injection rate.