Functionalization of Imidazolium-Based Ionic Liquids for the Improved Oil Recovery from Carbonate Reservoirs under Harsh Conditions

Chemical enhanced oil recovery (cEOR) is one of the promising technologies for boosting the oil recovery in depleted oil reservoirs by injecting various chemicals, including commercial chemicals such as polymers, surfactants, alkalis, and some specialized or functionalized chemicals like nanoparticles, macromolecules, and biomolecules. These chemicals typically tend to alter the interfacial forces at both oil-water and rock-oil-water interfaces by modifying their physicochemical interactions. Consequently, it shifts the reservoir’s wettability more toward water wet, thereby contributing to the improved oil recovery. However, the harsh reservoir environments often face huge challenges such as chemical instability, adsorptivity of the chemicals in the near-wellbore region, and poor deliverability of chemicals into dead-ends and deep reservoirs. In this study, we investigated the influences of functionalization in the ionic liquids (ILs) on the oil-water interfacial tension (IFT) and rock-oil-water wettability changes and thus for the oil recovery studies in the strongly oil-wet carbonate reservoirs. The experiments were confirmed that the functionalized ILs achieved a more significant IFT reduction and wettability shifts from a strongly oil-wet state (160°) to a water-wet state (52-76°) compared to the unfunctionalized ILs. Furthermore, the spontaneous imbibition tests confirmed the specificity of the IL functionalization by improving the oil recovery in a range of 28-44%. Molecular dynamics simulations were performed to investigate the interfacial behavior of functionalized [C₁₂C₁COOHim]⁺[Br]⁻ and unfunctionalized [C₁₂C₂im]⁺[Br]⁻ ILs at oil-brine and rock-brine interfaces as well as in bulk brine solutions. At the oil-brine interface, simulations revealed that the functionalized ILs had a higher interfacial packing density and a greater reduction in IFT compared to the unfunctionalized IL. This improved performance is due to its ion-pair headgroup forming strong ionic interactions with cations (e.g., Ca²⁺) and anions (e.g., Cl^-) in the aqueous phase. The absence of such interactions in the unfunctionalized IL resulted in a weaker interfacial affinity and lower density and influenced by competing bulk solvation forces. At the rock-oil interface, the functionalized ILs maintained continuous oil displacement through dual interactions: its cationic imidazolium interacted with organic acids in the crude oil, while its carboxylate anions engaged with calcite surfaces. Additionally, it formed complexes with Ca²⁺ ions that could interact with both the organic acids and the carbonate of the calcite surface. Conversely, the unfunctionalized IL prompted a quick initial displacement but reached a plateau soon after. The dual functionality of the functionalized IL disrupts calcite-oil adhesion and facilitates oil displacement. This study highlights the potential of functionalized ILs in enhancing oil recovery from carbonate reservoirs, presenting a promising approach for challenging reservoir environments.