How does phase behavior of surfactant/fluid/fluid systems affect fluid-fluid displacement in porous media?

Surfactant flooding is widely used in subsurface applications where high displacement efficiency is needed, such as nonaqueous phase liquid (NAPL) remediation and enhanced oil recovery (EOR). For surfactant/water/oil systems, the partitioning of surfactants in each phase is controlled by external factors, such as salinity. The third phase, microemulsions, which locates between water and oil, can be formed spontaneously at certain salinities. Therefore, the transfer of surfactants from water to other phases, which depends on salinity, can occur during surfactant flooding, resulting in spatially non-homogeneous surfactant concentrations. The critical parameters governing the displacement dynamics, such as contact angle and interfacial tension (IFT), can vary considerably due to spatially non-homogeneous surfactant concentrations. However, the influence of spatially non -homogeneous contact angle and IFT on the displacement in porous media has not been sufficiently explored. In this work, we visualized the surfactant flooding at different salinities in 2D micromodels. Our results suggest that the displacement patterns vary considerably with salinity. One displacement front was observed at low salinity, while displacement front separation occurred at intermediate and high salinity, forming two different displacement fronts. At intermediate and high salinity, we noticed that the downstream displacement was imbibition, while the upstream displacement was drainage. We suggest that the separation of displacement fronts was capillary-driven, arising from spatially non-homogeneous contact angle and IFT. Our study provides new insights into the effect of the phase behavior of surfactant/water/oil systems on displacement patterns.