Exploring Surfactant-Polymer Flooding in Challenging Environments: A Study in Carbonate Reservoirs

Enhanced oil recovery (EOR) in carbonate reservoirs faces significant challenges, particularly under high-temperature, high-salinity (HTHS) conditions, where traditional chemical methods may lose effectiveness. Surfactant-polymer (SP) flooding has emerged as a promising method to improve oil recovery by combining the interfacial tension reduction and wettability alteration effects of surfactants with the mobility control provided by polymers. However, achieving stability and optimal performance for SP formulations in harsh reservoir conditions remains challenging, especially for conventional surfactants. This study investigates using zwitterionic surfactants combined with anionic sulfonated polymers to address these challenges. Several experiments were used to highlight the potential of this SP formulation, including stability tests at varying temperatures and salinities, contact angle, interfacial tension (IFT), zeta potential, and static adsorption. Stability tests indicated that surfactants generally retain stability up to 80°C across a range of salinities, including high salinity conditions (214,000 ppm), with no precipitation or cloudiness observed at both 0.25 wt% and 0.5 wt% concentrations over 30 days. At 90°C, stability appears to be influenced by surfactant concentration and brine composition, with selected conditions showing potential for maintaining phase consistency. Contact angle, zeta potential, and static adsorption measurements illustrated how surfactants drive considerable wettability alteration in carbonate rocks. Zeta potential analysis demonstrated that by shifting the rock surface charge to more negative, surfactants can promote a transition from oil-wet to intermediate or water-wet states, enhancing oil recovery. Contact angle studies confirmed this shift, with the effectiveness of wettability alteration impacted by surfactant concentration, salinity, and temperature. It was found that the average contact angle drop achieved by zwitterionic surfactants can reach more than 100° at specific conditions, making the rock more water wet. Interestingly, the wettability alteration effect was slightly reduced at higher surfactant concentrations (0.5 wt%) in diluted brines. The static adsorption tests showed that this is likely due to lower surfactant adsorption on rock surfaces at diluted salinities, highlighting the importance of optimizing surfactant concentration for specific reservoir conditions. IFT measurements indicated that while the zwitterionic surfactant demonstrates some reduction in interfacial tension, its primary mechanism of action is through promoting wettability alteration toward a water-wet state, as opposed to significant IFT reduction. The addition of polymers to the SP solution revealed minimal impact on surfactant stability, with solutions retaining viscosity at 70°C and experiencing only moderate viscosity reduction at 90°C, pointing to polymer resilience in thermal environments. These findings highlight the potential of SP flooding with optimized zwitterionic surfactants as a robust EOR strategy, achieving both stability and enhanced performance in challenging HTHS carbonate reservoirs.