Understanding the chemical mechanisms for low salinity waterflooding

Low salinity water (LSW) is reported to improve oil recovery (IOR) significantly in sandstone and carbonate core experiments. Ranges of IOR vary significantly depending on the chemical composition of brine, oil and cores. We previously developed a process-based and predictive model that explicitly includes the chemical interactions between crude oil, brine, and the carbonate surface that alter rock wettability. In this research, we improve the developed model to include explicitly the acid/base interaction and ion-binding interaction of crude oil adsorption. The wettability alteration is quantified by the surface concentrations of adsorbed carboxylic acids, which is a result of aqueous and surface reactions. The total concentrations of aqueous and surface species are varied individually and together over a large range while precipitation constraints are also included. The wettability is for a variety of brine compositions used in experiments. The wettability depends strongly on the concentration of Ca²⁺, Mg²⁺ and SO₄^2-, as well as the total salinity. Including the acid/base interaction can explain the wettability alteration trend when Ca²⁺, Mg²⁺ and SO₄^2- concentrations are significant. Including the ion-binding through Ca2+ can better explain the wettability alteration trend when diluted formation water or seawater is injected. We can reproduce the wettability alteration trend reported in experiments from different sources by combining the acid/base and ion-binding mechanisms and considering the irreversibility of the carboxylic adsorption reactions.