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

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, chang-ing 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 macro -scale, 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 surfac-tant 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.(c) 2022 Elsevier Inc. All rights reserved.