Wettability Alteration on Carbonate Rock by the Mixture of the Gemini Surfactant and Chelating Agent

Gemini surfactants are novel surfactants that show unique properties, including low critical micelle concentrations, good water solubility, and outstanding performance in interfacial tension reduction. A previous study focusing on a group of locally synthesized cationic gemini surfactants found promising wettability alteration performance, which indicates the possible application in carbonate reservoirs that are naturally fractured and oil-wet. However, the tested surfactants seemed not to be efficient in high-salinity conditions. Chelating agents are used to reduce salinity by capturing the metal ions, helping maintain the performance of the selected enhanced oil recovery materials. In this study, wettability alteration by the locally synthesized ethoxylated quaternary ammonium gemini surfactant (GS8, with eight carbon atoms in the spacer group) alone and by the mixture between GS8 and a commonly used chelating agent [diethylenetriaminepentaacetic acid (DTPA)] was assessed by contact angle measurements. Results show that GS8 can modify strongly oil-wet Indiana limestone to intermediate-wet when dissolved in deionized water. Adding salts impairs the ability of the surfactant in altering wettability. The negative effect decreases in the sequence: CaCl2 > Na2SO4 > NaCl ≈ NaHCO3 ≈ MgCl2. The addition of DTPA improved the ability of the surfactant to alter the wettability in the presence of salts. A mixture of DTPA and GS8 showed better performance compared to GS8 and DTPA alone. These results suggested that, for high-salinity reservoirs, adding a chelating agent in combination with a cationic gemini surfactant could improve the wettability alteration.