Surface-active compounds induce time-dependent and non-monotonic fluid-fluid displacement during low-salinity water flooding

[Display omitted] Hypothesis: In a porous medium saturated with oil (containing oleic surfactant) and saline water, salinity reduction alters the thermodynamic equilibrium and induces spatial redistribution of surfactants, changing the local fluid configuration. During fluid–fluid displacement, this local change reshapes global fluid flows, and thus results in improved oil displacement. Experiments: We performed microfluidic experiments in a centimeter-long pore-network model with a fracture and a dead-end model to observe both the macroscale flows and microscopic fluid configuration evolution. Water with different salinities and model oils with different surfactant concentrations are used. Findings: When oil contacts low salinity water, we observe (1) the solid surface becomes more water-wet, and (2) water-in-oil emulsion spontaneously emerges near the oil–water interface. At the macroscale, the fluid distribution remains unchanged in short term but dramatically changes after tens of hours, which appears as improved oil recovery. Two modes are identified during fluid redistribution: gradual imbibition and sudden collapse. The displacement efficiency is a non-monotonic function of surfactant concentration. This is attributed to the interplay between two opposing effects by adding surfactant: (1) enhancing initial hydrophobicity which negatively affects the displacement, and (2) allowing stronger oil swelling which is beneficial for displacement.