Surfactant flooding is one technique of chemical enhanced oil recovery (EOR) aimed at improving the microscopic displacement efficiency of trapped residual oil via reducing the oil–water interfacial tension and wettability alteration. Success of surfactant flooding strongly relies on surfactant loss through its adsorption onto reservoir minerals to ensure maximum transfer to target reservoir. The current study examines the adsorption behavior of saponin natural surfactant onto carbonate rock outcrops. As an environmentally friendly extract from plants, saponins have shown the potential to increase oil recovery, although saponin loss or adsorption on surfaces is yet to be studied. Common synthetic surfactants of various types (i.e., cationic and anionic) and different molecular structures (other nonionic surfactants) have also been studied to provide comparisons to saponin. The surfactant adsorption onto carbonate samples was studied by batch adsorption experiments, with the residual surfactant concentration determined by the surface tension technique. It was found that saponin, a natural nonionic surfactant, adsorbed less than the ionic surfactants, since saponin adsorption is not governed by electrostatic interactions but weaker hydrogen bonding. Such data concludes that saponins are likely to yield less retention than the ionic surfactants, but compared to other nonionic surfactants its retention is greater. This is likely attributed to differing surfactant molecular structures. Due to its branch-like structure with more terminal functional groups, saponin adsorbs more on the rock surface compared to other long-chain nonionic surfactants. The findings of the current study provide a useful guide in surfactant selection for EOR and highlight a potential of natural and environmentally friendly surfactants.

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