Ionic Liquid and Surfactant Mixture for Wettability Alteration and Imbibition in Carbonate Reservoirs

Summary

Surfactant-mediated spontaneous imbibition (SI) is an effective way to enhance oil production for carbonates. However, conventional surfactants alone may not satisfy the need for high oil production rates in challenging reservoir conditions. This study investigated the use of ionic liquid/surfactant mixtures as a cofunctional system to improve wettability alteration and imbibition efficiency. In detail, three surfactants (ANS-1, betaine-1, and ANS-2) and four ionic liquids, 1-ethyl-3-methylimidazolium acetate (EMIM Ac), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM BF4), 1-ethyl-3-methylimidazolium ethyl sulfate (EMIM SO4), and 1-decyl-3-methylimidazolium chloride (DMIM Cl), with different interfacial tension (IFT) and wettability alteration ability were systematically evaluated through contact angle, surface tension (SFT), IFT, zeta potential, and imbibition efficiency in oil-wet carbonate core plugs. After analyzing the effects of ionic liquids on wettability alteration of rock surface, IFT changes of the chemical solutions with crude oil, and ultimate imbibition efficiency, it turns out that the short-carbon chain ionic liquid EMIM Ac exhibits negligible surface activity and does not significantly reduce IFT but effectively enhances wettability alteration, reducing contact angle from ~145 to ~56°. The ionic liquids can form ion pairs with polar components that exist in crude oil and assist the detachment of crude oil from carbonate rock surfaces. The optimized formulation (0.15 wt% ANS-1 + 0.05 wt% EMIM Ac) improved SI efficiency by 6% compared with surfactant alone. Analysis using the inverse Bond number indicates that imbibition occurs in a gravity-dominated regime, where low IFT provided by surfactants is the primary driving force, while ionic-liquid-enhanced wettability contributes to incremental oil production. This work provides new insight into the design of hybrid chemical systems for imbibition in carbonate by demonstrating ionic liquid function as an effective comodifier to complement surfactant performance.