Wettability Alteration and Spontaneous Imbibition Mechanisms for Enhanced Oil Recovery Using a Surfactant–Polymer System

Chemical enhanced oil recovery (EOR) methods using surfactant–polymer (SP) systems have the potential to mobilize residual oil remaining after conventional water flooding, particularly in mature sandstone reservoirs. This study experimentally investigates the wettability alteration, interfacial tension (IFT) reduction, and spontaneous imbibition mechanisms associated with a representative SP system comprising alkyl glycine surfactant (AGS) and partially hydrolyzed polyacrylamide (PHPA) in sandstone, under reservoir temperature conditions. Spontaneous imbibition experiments demonstrated an incremental oil recovery of 33.12% by the SP system compared to brine imbibition, indicating enhanced capillary-driven oil displacement. IFT measurements showed a reduction to 0.57 mN/m at 75 °C, facilitating oil mobilization under elevated temperature conditions. Contact angle measurements revealed a shift toward water-wet behavior, confirming wettability alteration induced by the SP system. Rheological measurements indicated suitable viscosity characteristics for mobility control, while phase behavior observations showed stable emulsion formation that supports pore-scale displacement in heterogeneous media. Microscopic image analysis further characterized emulsion morphology, providing insight into displacement efficiency at the pore scale. Fourier transform infrared (FTIR) spectroscopy analysis confirmed the functional groups associated with AGS, supporting its role in interfacial modification. Thermal stability tests demonstrated chemical stability of the SP system under reservoir-relevant temperatures. The results highlight the synergistic physicochemical mechanisms governing wettability alteration, IFT reduction, and spontaneous imbibition in SP flooding, offering mechanistic insight into surfactant–polymer interactions relevant to EOR applications in sandstone reservoirs.