Eco-Friendly Ionic Liquids for Enhancing CO2 Miscibility in Heavy Oil: Applications in EOR and Carbon Storage

Abstract The increasing global demand for energy continues to drive interest in efficient enhanced oil recovery (EOR) methods, particularly for challenging reservoirs containing extra-heavy crude oil. Traditional thermal recovery methods often prove economically and technically unsuitable for high-pressure, high-temperature (HPHT) environments. In this context, CO2-based miscible flooding has emerged as a favorable EOR technique due to its capacity to reduce interfacial tension and improve displacement efficiency. However, the high injection pressures required to achieve miscibility frequently exceed formation fracture gradients, making practical implementation difficult. To address this challenge, the objective of this study is to investigate the use of an environmentally friendly ionic liquid (IL), 1-methyl-3-octylimidazolium chloride ([MOIM]Cl), to reduce the minimum miscibility pressure (MMP) and first contact miscibility pressure (FCMP) in a CO2–extra-heavy oil system under elevated temperatures. Experimental investigations were conducted using the vanishing interfacial tension (VIT) technique, a proven approach for determining critical miscibility conditions. A crude oil sample with a density of 1.067 g/cm³ and viscosity of 9900 cP was selected to represent extra-heavy oil reservoirs. The sample was characterized through SARA analysis and TAN measurements to evaluate its compositional polarity and interaction potential with the IL. Interfacial tension (IFT) measurements were performed under varying pressure and temperature conditions at two targeted temperatures, 50 °C and 90 °C. The effect of incorporating 0.5 wt.% of [MOIM]Cl into the oil sample was evaluated through a comparison of the MMP and FCMP before and after IL addition. The findings showed that the addition of [MOIM]Cl had no beneficial impact on miscibility at 50 °C, with MMP and FCMP values increasing slightly from 1690 psi to 1749.7 psi and from 2705.8 psi to 2869.5 psi, respectively. However, at 90 °C, the inclusion of [MOIM]Cl resulted in a notable reduction in miscibility pressures. The MMP decreased from 2783.9 psi to 2351.7 psi (a 15.5% reduction), and the FCMP dropped from 4183.9 psi to 3756.1 psi (a 10.2% reduction). These results confirmed that the IL performs more effectively under elevated temperature conditions, particularly in systems containing polar and viscous crude oil with high asphaltene and aromatic content. This study highlights the novel application of a biodegradable and thermally stable IL for enhancing CO2 miscibility in extra-heavy oil systems, offering a promising and safer alternative for EOR and carbon storage in HPHT reservoirs.