In situ formation of microemulsions is studied by co-injection of its individual phases: surfactant solution and decane oil. The experiments were conducted in micro-channels etched on a glass micromodel which facilitates the direct observation of the microemulsion formation. The micromodel was horizontally placed under an inverted fluorescence microscope. The aqueous phase was injected via one micro-channel and the oil phase was injected via another. The two microchannels merged at a T-junction where the microemulsion phase started forming upon mixing of its two individual phases. The oil phase was doped with an oil-soluble fluorescent dye, called Nile Red, which shifts its fluorescent peak based on the polarity of the environment. Under fluorescent light, the microemulsion and oil phase fluoresced bright red and amber brown, respectively.
Microemulsions found their application in chemical enhanced oil recovery (cEOR) due to their ability to eliminate the capillary forces by generating ultra-low interfacial tension (IFT) between the aqueous phase and the oil-in-place. The flow regime shifts from immiscible to a quasi-miscible one, allowing the brine to access the more isolated parts of the rock. Inevitably the microemulsion becomes part of the flowing system and it has to be recovered. Therefore understanding of the microemulsion rheology as well as its IFT is important. The common knowledge on microemulsion formation is limited to the phase behavior analysis performed in static test tubes. Such tests answer some questions on microemulsion properties once they are formed, but they do not provide any insight to how a microemulsion forms and flows.
This study presents a methodology to visualize the microemulsion together with its aqueous and oil phase components at the time scale necessary to study mixing. These visualizations can provide insights into questions regarding in situ microemulsion formation, and consequently, the rheological behavior of multiphase systems with ultra-low IFT. Our experiments indicated that at flowing conditions the fluids may mix spontaneously, and the mixing profile depends on the salinity of the environment as well as the injection rates.