Experimental Study of Deep Eutectic Solvents for Chemical Enhanced Oil Recovery in Harsh Reservoir Conditions: Effects on Interfacial Tension, Wettability, and Core Flooding Performance

The growing global energy demand and steady decline of the conventionally available reserves have increased reliance on enhanced oil recovery (EOR) to mobilize residual oil, especially in oil-wet carbonates with harsh reservoir conditions. In this study, three deep eutectic solvents (choline chloride–citric acid (ChCl–CA), choline chloride–urea (ChCl–U), and zinc chloride–urea (ZnCl2–U)) were assessed as chemical EOR agents under reservoir-relevant conditions. Interfacial tension (IFT) and contact angle measurements were performed at high temperature and pressure (100 to 5000 psi) and brine chemistry (seawater vs high-salinity formation water), followed by tertiary core flooding. All deep eutectic solvents (DES) reduced oil–water IFT and shifted wettability toward less oil-wet conditions, with stronger responses at higher DES concentrations. Pressure provided an additional, consistent IFT reduction, supporting better performance under realistic reservoir pressures. Brine composition was decisive: seawater generally enhanced DES effectiveness, whereas the ZnCl2–U system showed incompatibility in sulfate-rich seawater due to precipitation. Core floods confirmed that ChCl–CA delivered the highest tertiary recovery, increasing oil recovery by 17.85% of original oil in place in seawater and 15.30% in formation water. These increments corresponded to relative increases of 67.8% and 61.2%, respectively, compared with the corresponding secondary waterflood recovery. These results highlight that the improved performance of acidic DES under the tested conditions is related to the combined contribution of moderate IFT reduction, wettability alteration, and possible carbonate rock alteration/reactivity, offering an effective alternative to conventional chemical agents under harsh reservoir conditions. It is finally concluded that the acidic and multifunctional DESs such as ChCl–CA can be considered as promising EOR agents for industrial applications.