Properties, preparation, stability of nanoemulsions, their improving oil recovery mechanisms, and challenges for oil field applications—A critical review

With the development of the oil industry, improving oil recovery (IOR) technology is becoming more and more important, and an excellent working fluid is a key to IOR. Among them, nanoemulsions have attracted a lot of attention. However, due to the similarity of nanoemulsions to other emulsions or surfactant solutions, the properties and characteristics of nanoemulsions are confusing, and the IOR mechanisms of nanoemulsions have not been systematically reviewed. Also, how the functions of nanoemulsions affect the IOR process has not been discussed. This comprehensive review stretches from the properties of nanoemulsions all the way to the applications and challenges and attempts to clarify the above problems. Nanoemulsion is a dispersed system (oil-in-water (O/W), water-in-oil (W/O)) with a diameter of 20–500 nm, usually stabilized by surfactants. Nanoemulsions are kinetically stable systems and can keep stable for a long period (a few months to years), but the Ostwald ripening progress is the main instability mechanism. Nanoemulsions can be prepared by high-energy methods (∼1010 W/kg) and low-energy methods (∼103 W/kg), and they can be characterized in terms of particle size, morphology, and stability. In the area of well-bore cleaning, nanoemulsions can improve the cleaning efficiency through small size, dissolving oil, reducing the adhesion work, and wettability alteration. In the area of well stimulation, nanoemulsions can provide suitable interfacial tension and positive capillary forces by wettability alteration, thus removing water blockage and enhancing imbibition. In the nanoemulsion flooding process, nanoemulsions can enhance the oil recovery (EOR) by increasing the capillary number, sweep efficiency, and mobility of crude oil, displacing the residual oil and dissolving oil. However, limited by reservoir environments, large-scale production, and the cost of nanoemulsions, the applications of nanoemulsions are still challenging. This review systematically explains the IOR mechanisms of nanoemulsions and provides insights and directions for the further development of nanoemulsions in the oil and gas industries.