Wettability alteration of locally synthesized cationic gemini surfactants on carbonate rock

Carbonate reservoirs are mostly oil-wet and tend to be naturally fractured, leaving a large portion of oil inside the rock matrix after secondary recovery. Cationic surfactants that combine interfacial tension (IFT) reduction, wettability alteration, and low adsorption are potential efficient materials for carbonate reservoirs. Gemini surfactants are known for reducing IFT and altering wettability at a much lower concentration than conventional surfactants. This study focuses on the wettability alteration performance of 14 locally synthesized cationic gemini surfactants and the influencing factors. Contact angle results showed that, due to molecular structural differences, their wettability alteration performance varied widely from weak to strong. Surfactant with one phenyl group in the spacer had the best wettability alteration performance that changes oil-wet rock to water-wet condition. An increase in carbon atom number in the straight-chain spacer had negligible influence. However, when unsaturation was introduced to the spacer, the wettability alteration of surfactant changed from good to poor. Compared to the spacer structure, counter-ion had a less significant influence. Br^- showed better influence on wettability alteration than Cl^-. Wettability alteration performance of surfactants was also influenced differently by different salts. For low permeability carbonate rock, the wettability alteration performance of surfactants is an important parameter. Surfactants that showed better wettability alteration performance obtained a higher oil recovery rate from spontaneous imbibition experiments.