Pore-Scale Mechanisms and Enhanced Oil Recovery Performance of Polymer-Assisted Winsor Microemulsion Systems: From Single Systems to Optimized Slug Design.

Polymer flooding is a highly promising enhanced oil recovery (EOR) technique for improving sweep efficiency, particularly in complex reservoirs at advanced stages of water production. While polymer flooding effectively improves sweep efficiency, efficient mobilization of residual oil requires further reduction in interfacial tension. Surfactant systems capable of forming microemulsions have therefore been introduced to enhance oil displacement through improved oil mobilization. The underlying oil displacement mechanisms of microemulsions are strongly dependent on phase behavior, which is governed by Winsor phase conditions. In this study, the pore-scale oil displacement mechanisms of Winsor I, II, and III microemulsion systems were systematically investigated using glass micromodel experiments. Winsor I mainly promoted oil detachment and emulsification, leaving residual oil as corner-bound oil and dispersed droplets. Winsor II showed limited efficiency due to its oil-continuous nature and viscous water-in-oil emulsions, resulting in persistent columnar residual oil. In contrast, Winsor III formed a continuous middle-phase microemulsion, enabling a solubilization-migration mechanism that effectively mobilized and transported oil. Accordingly, Winsor III achieved the highest recovery (81.37%), followed by Winsor I (75.6%) and Winsor II (64.9%). Optimized microemulsion slug injection further improved performance, with Winsor II-III-I reaching 82.2% and Winsor III-I sequence achieved the highest recovery of 85.6%. This study provides a mechanistic framework linking Winsor phase behavior to oil mobilization and demonstrates that both phase optimization and slug design are critical for improving microemulsion flooding performance in complex reservoir conditions.