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The modification of the surface wetting characteristics in fractured oil-wet carbonate reservoirs, by reversing wettability from oil-wet to water-wet, leads to improved oil recovery. However, in order to obtain a successful oil recovery process, it is crucial to understand the active mechanisms of wettability alteration. This study looks at the effect of sulfate ions as one of the most promising wettability influencing ions on the wetting properties of oil-wet calcite; the effect is studied both with and without the presence of cationic surfactant and possible mechanisms of wettability alteration are explored. A number of analytical techniques were utilized to analyze the mineral surface before and after treatment. The study presents a thorough discussion of the influence of sulfate ions in displacing adsorbed carboxylate from the oil-wet surface, both with and without the presence of cationic surfactant are discussed thoroughly. The interaction between sulfate ions and the calcium ions attached to carboxylate groups on the surface is believed to be the main active mechanism of wettability alteration at high concentration of sulfate ions. Ion-exchange between the hydroxide group and the adsorbed stearate ion on the calcite surface is shown to act as a supplementary mechanism that desorbs stearate ions from the surface. In the treatment of an aged calcite surface with sulfate ions, a combination of these two mechanisms resulted in a more water-wet surface. The copresence of sulfate ions and cationic surfactant resulted in a further reduction in the amount of adsorbed carboxylate on the surface. This can be attributed to the release of adsorbed carboxylate groups on the surface through ion pair formation with the cationic surfactant. The desorption of negatively charged carboxylate groups from the surface facilitates the approach of negatively charged sulfate ions to the aged calcite surface. It can therefore be concluded that sulfate ions accompanied by cationic surfactant molecules can alter the wetting properties toward water-wet state more effectively than sulfate ions alone.

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Wettability alteration toward a more water-wet state was found to be a promising approach for oil recovery improvement in oil-wet and naturally fractured carbonate reservoirs. This approach has been extensively studied in the literature using low-salinity/smart water and surfactant injection separately. However, application of surfactants in enhanced oil recovery is limited by their compatibility with the conditions of the reservoirs. In this study, polyethoxylated non-ionic surfactants with different ethylene oxide units were combined with low-salinity brine for a more efficient and cost-effective process. The compatibility of the surfactant solutions highly improved by reducing the salinity in the range of 200–2 g/L. Interfacial tension (IFT) measurements revealed that IFT decreased with increasing salinity. Contact angle measurements of calcite surfaces showed that wettability can be altered from a strong oil-wet state to a water-wet state after treatment with solutions of non-ionic surfactants over a wide range of salinities (∼2–110 g/L). ζ potential, Fourier transform infrared spectroscopy, and thermogravimetric analysis revealed that the non-ionic surfactant could partially displace carboxylate compounds from the surface and adsorb by forming a hydrogen bond with the hydroxyl group on the calcite surface. The formation of hydrogen bonds between ethoxy groups of the surfactant and hydroxyl or carboxylic groups on the solid surface can result in the replacement of organic compounds on the calcite surface. The organic compounds could form a new layer on the layer of adsorbed surfactant molecules via hydrophobic interactions. In addition, adsorption of the hydrophobic part of the non-ionic surfactant on the hydrophobic calcite surface and the formation of a surfactant double layer could partially contribute to the wettability alteration process.