A critical review on selection of microemulsions or nanoemulsions for enhanced oil recovery

• Surprisingly droplet size of microemulsions is found to be smaller than nanoemulsions. • Microemulsions shows higher efficiency in oil recovery compared to nanoemulsions. • Microemulsions can reduce the interfacial tension up to 10 -4 mN/m while nanoemulsions cannot. • Field application of microemulsions is easier than nanoemulsions. • Microemulsions have better future prospects rather than nanoemulsions. In this review, an attempt has been taken to show the preferential selection criteria of microemulsions and nanoemulsions for applications in enhanced oil recovery. This paper primarily explains the preparation, characterization, and interaction between microemulsions/nanoemulsions and reservoir rock, applications as well as the similarities and dissimilarities of both the emulsions. The detailed description of the emulsions gives a clear-cut insight into these emulsions in enhanced oil recovery. A well-defined opinion has been provided about the choice of either microemulsions or nanoemulsions for additional oil recovery. The evidence explains that the introduction of nanoemulsions could enhance the oil recovery by 70% of total oil in place. At the same time, the microemulsion flooding reduces the residual oil saturation by about 30% in opposition to water flooding with a total oil recovery of 80%. Microemulsions are more effective than nanoemulsions in the reduction of interfacial tension, alteration of wettability, and trapped oil recovery. Microemulsions help to achieve ultralow interfacial tension up to 10 -4 -10 -3 mN/m whereas nanoemulsions can reduce up to 0.1 mN/m. The particle size of microemulsions remains within 100 nm while up to 500 nm particle-sized nanoemulsions can be formed. From these facts, it is evident that both microemulsions and nanoemulsions have the extraordinary capability which makes them more efficient in oil recovery. But a few qualities make the microemulsions more convenient than nanoemulsions. Therefore, this review gives an understanding regarding the exceptional attributes of microemulsions rather than nanoemulsions. However, it has been found from the literature that microemulsions are more efficient than nanoemulsions in enhanced oil recovery for mitigating the energy crisis by enhanced recovery.