Eco-Friendly Hybrid System Combining Smart Water and Natural Surfactants for Enhanced Oil Recovery in Carbonate Reservoirs

Smart water injection, when combined with surfactants, represents an efficient and environmentally sustainable technique for enhancing oil recovery (EOR) in carbonate reservoirs. This study examines the synergistic effects of smart water (SW) and two surfactants the synthetic cationic cetyltrimethylammonium bromide (CTAB) and the natural nonionic Ziziphus Spina Christi (ZSC) on wettability alteration, interfacial behavior, viscosity modification, and oil recovery performance. Laboratory analyses including contact angle measurement, FTIR-ATR spectroscopy, interfacial tension (IFT) determination, viscosity and rheological tests, micromodel displacement, and spontaneous imbibition experiments were conducted on carbonate samples from the North Azadegan oil field. The results showed that increasing the concentration of potential determining ions (PDIs: Ca2+, Mg2+, SO42–) in smart water significantly shifted rock wettability from oil-wet (≈135°) to moderately (mildly) water-wet (≈55°). The addition of surfactants further enhanced this effect, with ZSC showing superior wettability alteration and CTAB achieving ultralow IFT (0.15 mN/m). Viscosity analysis revealed that low-salinity and Ca2+ enriched waters produced a salting-in effect that reduced crude oil viscosity and promoted oil-in-water (O/W) emulsions, whereas high-salinity brines caused salting out and increased viscosity through water-in-oil (W/O) emulsions. The integration of surfactants, particularly CTAB and ZSC, stabilized O/W emulsions and improved fluid mobility. Micromodel and core-scale tests confirmed enhanced oil recovery, 26% with SW(2Mg2+), 28% with ZSC + SW(2Mg2+), and 31% with CTAB + SW(2Mg2+). Spontaneous imbibition tests showed further improvement from 7–8% (SW) to 69% (CTAB + SW). Overall, optimizing ionic composition and salinity while combining smart water with appropriate surfactants effectively alters wettability, reduces IFT and viscosity, and enhances both capillary and viscous displacement mechanisms. Although CTAB achieved the highest efficiency, ZSC offers a sustainable and environmentally friendly alternative for smart water EOR in carbonate formations.