(68a) Influence of Surfactant Ion Types on Chelating Agent Performance in Enhanced Oil Recovery

In enhanced oil recovery (EOR), many different types of chemicals, such as surfactants, polymers, and alkalis, are applied to help recover residual oil in the formation. Surfactants are used primarily for interfacial tension (IFT) reduction purposes. With the development in the understanding and design of surfactants, their applications are enlarged to include other purposes such as wettability alteration, viscoelasticity, etc. Nevertheless, the applications of surfactants are largely limited by stability issues that arise in high salinity conditions. A possible solution is to add chelating agents that capture multivalent metal ions to accommodate surfactants in harsh conditions. This work focuses on the EOR implications of the surfactant + chelating agent combination, with a special focus on the impact of surfactant ion type. Surfactants with different ion types, including SDS (anionic), DTAB (cationic), Triton X100 (nonionic), and a zwitterionic surfactant, were tested. EDTA was adopted to represent chelating agents. The oil/water IFT values of oil drop in the mixtures of surfactant and EDTA were measured by the pendant drop method. Indiana limestone substrates were pretreated to be oil-wet by using crude oil. After that, they were soaked in the mixtures of surfactant and EDTA for 4 days. The resulting contact angle on the substrates’ surfaces were measured by the static contact angle measurement. Results showed that the chelating agent EDTA can reduce IFT to a limited extent. When combined with a surfactant, the oil/water IFT value can be reduced to 0.1~1 mN/m level. This reduction is more significant if pH is increased above 9. The oil-wet Indiana limestone substrates treated by EDTA were turned to intermediate-wet condition. Those treated by EDTA + surfactant showed varying results, with most of the surfactants having a synergic effect with EDTA, while almost no wettability alteration was achieved by EDTA + SDS. Besides, SDS was not stable in lifted salinity even with EDTA at a concentration of 3 wt%. In conclusion, between a surfactant and a chelating agent, there is a synergic effect on IFT reduction. In the aspect of wettability alteration, however, it is largely dependent on the ion type of the surfactant. Anionic surfactants could impact negatively, possibly due to competitive adsorption of anionic surfactants and the chelating agent.