LAUSR

Abstract Despite many attempts for the development of smart water flooding (SWF), as a new enhanced oil recovery (EOR) approach, mechanisms of EOR by SWF and their dependency on the salt type and concentration in carbonate rocks still remain in a state of ambiguity. In this study, various aspects of smart water such as salt type and concentration, temperature, pressure, and pH on the crude oil-brine-rock (CBR) system are investigated through measuring contact angle (CA) and interfacial tension (IFT), followed by elaborating all governing mechanisms in this regard. For this purpose, five salts (i.e., NaCl, KCl, CaCl2, MgCl2, and Na2SO4) and formation water (FW) are used as brine part of the system. The experiment results show that the presence of an optimum concentration of CaCl2 in the injected fluid leads to a greater IFT reduction (20.3%) in the oil-water system than other salts such as Na2SO4 (18.3%), KCl (17.9%), and MgCl2 (14.4%). Also, it was observed that a slight change in temperature has a stronger effect on the IFT than pressure. Accordingly, it is concluded that increasing reservoir pressure has a small negative effect and reducing reservoir fluid temperature has a significant negative effect on the performance of the SWF process. Moreover, to yield a proper wettability condition, the concentration of NaCl, KCl, CaCl2, and MgCl2 should be decreased as much as possible while maintaining the concentration of Na2SO4. Furthermore, changing the pH of the smart water toward basic values greatly reduces the IFT but acidic pH values do not have a considerable effect on the IFT. Thus, it is suggested adding some additives to make the carrier fluid (water) basic and thus greatly reduce the IFT. Besides, the CA in the basic environments becomes water-wet (60.8°) while no significant change in the acidic aqueous phase was observed. Giving time to the process (soaking) can play an important positive role in the SWF of carbonate reservoirs such that it leads to the rock dissolution (rising pH) as well as stabilization of wettability alteration. The IFT reduction is due to the breakdown of hydrogen bonds in the water. The wettability alteration by the mentioned salts, except Na2SO4, is based on the salting-in/salting-out effect while for Na2SO4, it is due to replacing the rock surface carboxyl by SO42− ion. The mechanism of pH effect is the replacement of rock surface carboxyl with the produced OH−.