LAUSR

Abstract Surfactant injection into a depleted hydrocarbon reservoir is one of the ways to reduce residual oil saturation, thereby enhancing crude oil production. However, surfactants emanating from petroleum sources are toxic, costly, and not biodegradable. Hence, several ongoing studies target to replace them with green and cheaper alternatives. Recently, natural surfactants have gained much attention due to their excellent surface activities, readily available sources, and environmentally friendliness. However, an extensive review to understand the performance and mechanisms of natural surfactants as enhanced oil recovery (EOR) agent is missing in the literature. This review article critically examines current published research on the potential of natural surfactants for EOR application. We focused on providing a catalogue of natural surfactants other than those from microbial sources in EOR, discussing the influence of natural surfactants on interfacial tension (IFT) reduction, wettability alteration, and adsorption on the rock surface. Also, an examination on the adsorption levels of natural surfactants to ascertain whether they are within the required range is presented. A summary of the recovery potential, the benefits, challenges of field applications, and future outlook of natural surfactants are provided. Reported works on natural oil-based sodium methyl or ethyl ester sulfonates and polymeric methyl ester sulfonate (PMES) suggest that they can effectively reduce the IFT and alter the wettability towards water-wet. They are stable in hard brine compared to the conventional sodium petroleum sulfonates. Most natural-based zwitterionic surfactants can give ultra-low IFT even in the presence of hard brines, they are temperature tolerant coupled with excellent emulsifying, wetting and foaming properties. Furthermore, soap nut saponin can effectively perform at high temperature. Generally, natural surfactants have excellent thermal stability, salt and hardness tolerance, good wetting and interfacial property, excellent foaming and emulsion properties. They are a cheaper alternative to conventional surfactants and are biodegradable with a recovery potential ranging from 2 to 36% OOIP of additional oil after secondary recovery. This paper is useful as it provides insights into the mechanism of natural surfactants in the EOR application. It also provides recommendations and guidelines for future developments in using natural surfactants in EOR.