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.

As one of the solutions to tackle climate change caused by excess carbon dioxide (CO2) emission, CO2 geological storage has been increasingly implemented globally to store CO2 securely and permanently in the subsurface. In the current study, structural trapping, which shows the potential of initial CO2 containment and integrity of the subsurface structure, is experimentally investigated with CO2 leakage assessed. CO2 containment is quantified by CO2 column height, which describes the amount of CO2 accumulated in the formation underneath seal rock and is controlled by a balance between capillary and gravitational forces acting on formation brine and invading CO2. While previous studies considered only contributions from seal rock (i.e., “nonrelative”), the current study examines a concurrent contribution from reservoir rock as a seal–reservoir “relative” column height since CO2 storage as an analogy to petroleum reservoir is a structural trap consisting of the reservoir and impermeable seal covered. A distinctive discrepancy was found between the resultant relative and nonrelative column heights. The nonrelative column heights were positive (∼3000 m), implying a high potential for CO2 storage. On the contrary, with reservoir rock contribution considered, the relative column heights were negative (∼−1800 m), suggesting CO2 leakage through the structural trap. This was attributed to a relatively larger reservoir pore size (5.72 nm) than that of seal rock (4.04 nm). Hence, the contribution from reservoir rock characteristics is non-negligible when analyzing CO2 storage potential. Owing to CO2 dissolution in formation brine, CO2-induced effects including a geochemical reaction between acidic carbonated brine and rocks were also investigated. Rock dissolutions in both seal (claystone) and reservoir (limestone) rocks were observed with changes in the pore size, leading to lower storage potential. Further attempts to improve the column height were made by hydrophobizing seal rock via surfactant adsorption, although the changes were slight and could only facilitate a possible leakage (less negative height column).

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