LAUSR

Abstract In recent years, several studies have been carried out on the effect of cationic surfactant and smart water on the oil recovery in carbonate rocks. However, limited studies have been conducted on the combination of these two solutions. In the present study, the effect of combining smart water and cetyltrimethylammonium bromide (CTAB), a cationic surfactant, was analyzed. The synergic effect of the smart water and the CTAB was studied using the results of contact angle, interfacial tension (IFT), zeta potential, and spontaneous imbibition tests. The seawater was modified by increasing the concentration of SO42− and was used as the smart water. The results showed that, in the presence of seawater with 2 or 4 time SO42− concentration, cationic surfactants are more active. In addition, CTAB monomers approach the surface easier, and, by forming pairs of carboxylic groups/CTAB monomers, the carboxylic groups will be removed from the surface, leading to a more water-wet surface. In addition, the results of zeta potential tests showed that even a small increase in the CTAB critical micelle concentration (CMC) from 0 to 1/4 results in an increase in the zeta potential from −42.7 to −18.8. The IFT results within the water-oil interface indicated that due to the presence of SO42− in the vicinity of the interface, the repulsion of polar heads of the cationic surfactant monomers will decrease. Therefore, a greater number of these monomers are oriented toward the interface, which causes a greater reduction of the IFT from 15.5 to 2.3 mN/m. Ultimately, the effects of smart water and combinations of smart water and CTAB on spontaneous imbibition were studied. The diffusion indices based on the surface areas for the seawater with 2 times SO42− concentration, seawater with 2 times SO42− concentration and 1 CMC CTAB, as well as the seawater with 4 times SO42− concentration and 2CMC CTAB were 0.35, 0.55, and 0.66, respectively, which were in full agreement with the results of contact angle, IFT, and zeta potential tests. Overall, it was found that, in the presence of CTAB, due to less IFT and wettability alteration, the smart water will imbibe into the rock more easily and more quickly.