Microemulsion flooding based on surfactant system: Recent advances and current challenges in enhanced oil recovery

Driven by the continued depletion of conventional oil resources and the rapid growth in global energy demand, advanced enhanced oil recovery (EOR) technologies have attracted widespread attention. By integrating core attributes of surfactant, emulsion, and micellar solution flooding, microemulsion flooding demonstrates considerable application advantages in EOR owing to its thermodynamic stability, nanoscale droplet size, ultra-low interfacial tension, and strong crude-oil solubilization capacity. In response to the developmental demands of low-permeability reservoirs and the imperative of sustainable energy progress, this review explores pathways for advancing microemulsion flooding through targeted innovation in surfactant system. First, the colloidal theoretical foundations and recent progress of three representative surfactant-based displacement technologies are summarized. For surfactant flooding, optimization strategies have focused on improving tolerance to temperature, salinity, and low-permeability conditions, reducing reservoir adsorption, and developing environmentally friendly formulations. For emulsion flooding, major advances include functional modification, performance regulation and stimuli-responsive behavior. For micellar solution flooding, recent breakthroughs center on wormlike micelles and viscoelastic surfactants. Collectively, these advances furnish critical theoretical and technical guidance for the innovative design and evolutionary development of microemulsion flooding systems. Subsequently, the review systematically elucidates the oil displacement mechanisms of microemulsions by integrating phase behavior evolution, interfacial properties and crude oil mobilization dynamics to clarify the comprehensive EOR efficacy of microemulsion flooding. It further bridges the gap between laboratory-scale performance evaluations and field-scale applications, while critically assessing engineering adaptability and the major developmental bottlenecks that still constrain the technology. On the basis of the above systematic elaboration, corresponding targeted optimization pathways are proposed to address these technical challenges and to support further improvement of microemulsion systems and their practical application in EOR.