Surfactant-enhanced oil recovery from an interfacial perspective: Multiscale mechanisms, adsorption loss, and performance boundaries under realistic reservoir conditions

Surfactant-enhanced oil recovery (SEOR) is widely applied to improve the mobilization of residual oil through interfacial tension reduction, wettability alteration, and emulsification. However, field performance remains highly inconsistent, largely due to adsorption losses, interfacial instability, and strong coupling between surfactant molecular structure and reservoir physicochemical conditions. In this review, we systematically analyze the interfacial behavior of surfactants and surfactant-based composite systems under realistic reservoir environments, with emphasis on molecular adsorption and film formation, pore-scale displacement processes, and pilot-scale performance. By integrating recent advances up to 2025 in laboratory experiments, microfluidic visualization, molecular dynamics simulations, and field studies, we establish quantitative relationships between surfactant class (anionic, nonionic, amphoteric, and hybrid systems), rock mineralogy (sandstone, carbonate, shale), and key reservoir parameters including salinity, divalent-ion concentration, temperature, and pressure. Particular attention is given to adsorption capacity, interfacial tension stability, wettability reversal efficiency, and emulsification behavior, and their tradeoffs in different geological settings. The analysis demonstrates that adsorption loss is not only an economic constraint but also a critical factor governing the dominance and sustainability of interfacial mechanisms, defining operational thresholds beyond which ultra-low interfacial tension or stable microemulsions become ineffective. Furthermore, this review highlights emerging surfactant systems such as bio-based surfactants, ionic liquids, and nanoparticle-assisted formulations, and evaluates their performance windows and limitations in high-salinity and high-temperature reservoirs. A multiscale analytical framework is proposed to link molecular structure, dynamic interfacial processes, and reservoir-scale responses, enabling the transition from qualitative mechanism descriptions to engineering-oriented performance boundaries and formulation guidelines. Overall, this work provides actionable insights for surfactant selection, formulation design, and risk assessment prior to field deployment, and identifies key challenges including divalent-ion tolerance, adsorption control, cost reduction, and dynamic interfacial characterization. The framework established herein aims to accelerate the translation of SEOR technologies from laboratory studies to reliable and economically sustainable field applications.