Ionic Liquids for CO₂-EOR in Carbonates: Reduced Miscibility Pressure, Altered Interfacial Properties, and Improved Oil Recovery

CO₂ flooding is widely applied for enhanced oil recovery and can provide additional storage capacity for carbon management; however, its effectiveness in carbonate systems is often constrained by the high minimum miscibility pressure (MMP) required to achieve efficient displacement. In high-pressure/high-temperature environments, operating above the MMP can necessitate elevated injection pressures that increase the likelihood of formation fracturing and injectivity impairment, while operating below the MMP reduces displacement efficiency and leaves substantial residual oil. A strategy was therefore evaluated in which a low-dosage ionic liquid (IL) is used to improve CO₂–oil miscibility while simultaneously reducing capillary trapping. The objective of this work was to quantify the impact of an imidazolium-based IL, 1-methyl-3-octylimidazolium chloride ([C8mim]Cl), on miscibility pressure, interfacial properties, and ultimate recovery during tertiary CO₂ flooding in carbonate cores. A staged experimental workflow was implemented in which miscibility behavior was screened across multiple IL concentrations, followed by confirmatory interfacial tension and wettability measurements at reservoir-relevant temperature. Two composite-core floods were then performed under identical flow rate and thermal conditions to provide a base case (seawater followed by CO₂) and an IL-assisted case (seawater followed by an IL slug and soak, then CO₂). CT scanning was performed before and after flooding to characterize axial heterogeneity and support the interpretation of fluid redistribution and remaining oil. A compositional history-matching workflow was subsequently applied in a CT-informed numerical model to reconcile relative-permeability behavior and displacement trends with the measured production response. Meaningful reductions in miscibility pressure were obtained with [C8mim]Cl. At the optimum concentration (0.5 wt.%), the MMP decreased from 11.32 MPa to 10.33 MPa (8.7% reduction), and the first-contact miscibility pressure decreased by 16.27%. Interfacial properties were also improved, with the oil–seawater IFT decreasing from 26.9 to 20.9 mN/m (≈22% reduction) and the carbonate surface condition shifting toward less oil-wet behavior as the contact angle decreased from 134.4° to 124.5°. In coreflooding, the base case produced 52.2% OOIP during seawater injection and an additional 29.7% OOIP during CO₂ injection, yielding a total recovery factor of 82.0%. With the IL pre-slug, 55.5% OOIP was recovered by the end of the seawater/IL stage, and a further 39.7% OOIP was recovered during CO₂ injection, increasing total recovery to 95.2% OOIP. The CO₂-stage incremental recovery factor was therefore increased by 10.0 percentage points (reflect 33.7 % improvement) relative to the base case, while the total recovery was increased by 13.2 percentage points. This integrated evaluation demonstrates that a low-dosage IL pre-slug can be used to lower miscibility pressure while improving displacement efficiency during tertiary CO₂ injection in carbonates. The novelty lies in coupling miscibility-pressure reduction with interfacial-property modification and validating the combined mechanism using coreflood volumetrics, CT-supported interpretation, and compositional history matching, enabling a practical pathway to higher CO₂-EOR recovery with reduced risk of high-pressure operational constraints.