Microfluidic-Based Analysis on the In Situ Microemulsion Formation and Transport in Fractured Porous Media

In situ microemulsion, a promising tertiary oil recovery technique, involves injecting a surfactant solution capable of forming underground microemulsions (MS) to enhance oil mobilization. Typically, previous scholars have extensively conducted macroscopic salinity or oil-water ratio scanning experiments to examine the static phase behavior of in situ microemulsion. However, there is still limited understanding regarding the dynamic formation and transport mechanisms of three types of in situ microemulsions in dead-end porous media. In this study, salinity and oil-water ratio scanning experiments were first conducted to identify three types of MS capable of forming Winsor I, II, or III type microemulsions with oil and to determine the state of microemulsion phase at varying oil-water ratios. Next, in conjunction with fluorescent technology, a fractured porous micromodel was employed to study the formation and transport mechanisms of the three types of microemulsions under both undisturbed and continuously disturbed flow field. The experiments demonstrated that under undisturbed flow fields, only the formation of Winsor III type microemulsions resulted in the appearance of water droplets within the pores. However, when subjected to continuously disturbed flow conditions, the presence of water droplets within the pores was observed upon the formation of any type of microemulsion. Moreover, distinct microemulsions exhibited distinctive miscible and dilution properties. The Winsor I type microemulsion was the most easily diluted, followed by the Winsor III type microemulsion, while the Winsor II type microemulsion was the most challenging to dilute. Therefore, MS with optimal salinity might not always be the ideal choice for oil extraction in dead-end porous media. Lastly, the formation and transport of microemulsions were quantitatively analyzed using Fick's law. Higher salinity in the MS corresponded to smaller diffusion coefficients, resulting in a slower formation and transport of microemulsions. Conversely, the presence of a disturbed flow field increased the diffusion coefficient and promoted the formation and transport of microemulsion.