Wettability alteration in carbonates: Interaction between cationic surfactant and carboxylates as a key factor in wettability alteration from oil-wet to water-wet conditions

The chemical interaction between cationic surfactants of the type R-N(CH3)3Br, termed CnTAB, and carboxylates is the key factor in changing the wettability from oil-wet chalk to more water-wet conditions. Oil can then be displaced from the chalk by spontaneous imbibition of water. Carboxylates from the crude oil are the most strongly adsorbed material onto the chalk surface, and they may act as “anchor” molecules for other surface-active components present in the crude oil. This paper focuses on the ion-pair interaction between the cationic surfactant and the carboxylates present in crude oil and model oil systems made by dissolving fatty acids (octanoic, lauric and stearic) in heptane and crude oil. Partitioning of the cationic surfactant between the oil and the water phase was studied as a function of the type and amount of acid present, pH, and composition of the brine. By means of static contact angle measurements, it is verified that the concentration of surfactant is very important in desorbing carboxylates from the calcite surface. The process nearly stops at surfactant concentrations below the critical micelle concentration, indicating that the desorbed carboxylates must be stored in micelles or extracted into the oil phase in order to maintain a dynamic wettability alteration process in a porous medium. Dynamic experiments, using model oil systems, containing different types of fatty acids and C12TAB dissolved in brine, showed that the surfactant solution imbibed spontaneously into the oil-wet material in a counter-current flow regime governed by mainly capillary forces, indicating that a wettability alteration process had taken place.