Effects of emulsification factors on the characteristics of crude oil emulsions stabilized by chemical and Biosurfactants: A review

Crude oil emulsions are commonly encountered in oilfield-related industries. Some of these emulsions are formed in-situ during the tertiary enhanced oil recovery process. Additionally, some recent studies reported that the formulation of emulsions ex-situ and injecting them as emulsion flooding could recover more crude oil than the traditional chemical flooding using surfactant/polymer aqueous solutions. The successful application of emulsion flooding would require the formulation of stable emulsions with specific characteristics. Thus, this article aims to review the effects of various emulsification factors (emulsification method, emulsifier type and concentration, oil type and oil/water ratio, salinity, pH, and temperature) on the key emulsion characteristics (droplet size and distribution, zeta potential, interfacial tension (IFT), flowability, and stability). Furthermore, with the growing interest in sustainability and environment protection, the replacement of chemical surfactants with biosurfactants is expected. The latter class, in addition to being environmentally-friendly and can be produced sustainably, might provide superior performance and a better tolerance to the reservoir harsh conditions relative to chemical surfactants. Thus, the available studies on the emulsion formulation using bioemulsifiers and the characteristics of the produced emulsions have also been reviewed. Moreover, some research gaps have been identified, and future research to address them has been proposed.