Modeling the Effect of Reaction Kinetics and Dispersion during Low-Salinity Waterflooding

Wettability alteration has been recognized as the primary mechanism responsible for improved oil recovery during low-salinity waterflooding (LSWF). A complex network of ionic reactions at the oil/brine/rock interfaces facilitates the alteration in wettability. The objective of this paper is to evaluate the effects of reaction kinetics and dispersion during LSWF. In this research, we construct a mechanistic binary model that has been implemented on carbonate reservoirs. We consider the impact of physical dispersion and reaction kinetics on recovery. The proposed model is based on the premise that the wetting species are known and can be lumped as either oil-wetting or water-wetting pseudocomponents. For the cases studied, the model was found to reproduce the experimental results well. Further, simulations show a significant impact of reaction kinetics on the rate of wettability alteration compared to assuming instantaneous equilibrium. To adequately represent field-scale response from the laboratory scale, one needs to ensure that comparable Damköhler numbers are used. Some laboratory corefloods for LSWF may underestimate the recovery because the Damköhler number is not representative of field scale. For the limiting case of a slow reaction rate [Damköhler number [(Da) ~ 0] that corresponds to laboratory scale, low-salinity injection does not alter wettability. For fast reactions (Da ~ 10⁵) that correspond to the field-scale behavior, the ultimate oil recovery is highly sensitive to the injected fluid salinity. The wettability alteration front is delayed compared to the injected fluid because of the excess salt desorbed from the rock surface into the aqueous solution. Such a delay in wettability alteration is important when considering an appropriate slug size for the low-salinity slug. Finally, we observed that dispersion had little effect on the ultimate oil recovery during wettability alteration as compared to reaction kinetics.