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.

表面活性剂驱油广泛用于需要高驱油效率的地下应用，例如非水相液相 (NAPL) 修复和提高采收率 (EOR)。对于表面活性剂/水/油体系，各相中表面活性剂的分配受盐度等外部因素控制。第三相，微乳液，位于水和油之间，在一定的盐度下可以自发形成。因此，表面活性剂从水转移到其他相，这取决于盐度，在表面活性剂驱期间可能发生，导致空间上不均匀的表面活性剂浓度。由于空间上不均匀的表面活性剂浓度，控制位移动力学的关键参数，例如接触角和界面张力 (IFT)，可能会有很大差异。然而，空间非均匀接触角和IFT对多孔介质位移的影响尚未得到充分探索。在这项工作中，我们在二维微模型中可视化了不同盐度下的表面活性剂驱油。我们的结果表明，位移模式随盐度变化很大。在低盐度下观察到一个置换前沿，在中高盐度出现置换前沿分离，形成两个不同的置换前沿。在中高盐度下，我们注意到下游的驱替是吸吸，而上游的驱替是排水。我们认为位移前沿的分离是毛细管驱动的，由空间非均匀接触角和 IFT 引起。