Mechanistic study of wettability alteration of oil-wet calcite: The effect of magnesium ions in the presence and absence of cationic surfactant

• Interactions between oil-wet calcite surface and magnesium ions were studied.
• Magnesium ions are able to displace adsorbed carboxylate group from surface.
• Mg2+ ions change the wettability of calcite surface to more water wet status.
• Mg2+ linked to carboxylate group and remove them from oil-wet surface.
• Presence of cationic surfactant promotes the process of wettability alteration.

Over 60% of the world’s discovered oil reserves are held in carbonate reservoirs, which are mostly naturally fractured. Conventional water flooding results in low oil recovery efficiency in these reservoirs as most of them are oil-wet. On account of negative capillary forces, injected brine cannot penetrate simply into an oil-wet matrix of fractured formations to force the oil out. Wettability alteration of the rock surface to preferentially more water-wet state has been extensively studied using both smart water and surfactants separately. This study aims to study the effects of Mg2+ as one of the most important wettability influencing ions on the wetting properties of oil-wet carbonate surfaces in the presence and absence of cationic surfactant. Different analytical tools were utilized to examine the interactions between calcite surface, Mg2+ and surfactant. Results show that, magnesium ions are able to remove the strongly adsorbed carboxylate group from the surface and change the wettability of calcite surface to more water wet status. However, the presence of surfactant improves this process and enhances the wettability alteration toward more water wet. It has been demonstrated that the interaction of magnesium ions with adsorbed carboxylate on the surface hence forming a complex with carboxylate group as well as ion–pair formation between cationic surfactant and adsorbed carboxylate are the main mechanisms of wettability alteration which has resulted in removing the carboxylate from the calcite surface.

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