Microemulsion flushing of petroleum hydrocarbons in porous media: Microfluidic insights into residual oil morphologies and removal mechanisms.

The flushing technology for petroleum hydrocarbon-contaminated soil is constrained by technical bottlenecks, including limited solubilization capacity and low remediation efficiency of traditional flushing agents. Microemulsions, with their superior solubilization performance, low interfacial tension, and low surface tension, present a potential solution. However, the flushing mechanisms at the pore scale remain unclear. This study employs microfluidic-based techniques to simulate the flushing process of petroleum hydrocarbon-contaminated soil by observing fluid flow in porous media chips. We compare the removal efficiency of various flushing agents, including microemulsions, and elucidate the microscopic mechanisms underlying microemulsion-mediated petroleum hydrocarbon removal. Results reveal that residual petroleum hydrocarbons exhibit six distinct morphologies within pores: pool pattern, bridge pattern, ganglia pattern, dead-end pattern, film pattern, and disconnected droplets pattern. Their distribution is influenced by fluid dynamics, interfacial properties, and pore structure. Microemulsions enhance contaminant removal through two synergistic mechanisms: (1) wettability alteration to reduce adhesion work and promote contaminant detachment; (2) solubilization and emulsification to enhance oil-phase solubility and mobility. Surfactants and microemulsions can both significantly enhance the displacement efficiency of petroleum hydrocarbons. Compared with water displacement, the displacement time of petroleum hydrocarbons by surfactants and microemulsions is reduced from 260 s to 36 s and 16 s, respectively. Moreover, microemulsions can achieve efficient augmentation of residual oil within the subsequent 360 s. This study discloses the flushing mechanism of microemulsions at the microscopic scale, offering a theoretical basis for the optimization of flushing processes in contaminated sites and holding great application value.