Dynamic adsorption kinetics of the functionalized ionic liquids and their impact on enhanced oil recovery in harsh carbonate reservoirs

The enhanced oil recovery (EOR) in harsh carbonate reservoirs faces enormous challenges due to their extreme complexity and strong oil-wet tendency. Additionally, the long-term chemical instability, high adsorption and retention of chemicals in the near-wellbore region, and poor diffusivity and the lack of deliverability further increase the challenges in the oil recovery process. In the current study, we investigated the dynamic adsorption–desorption kinetics of two imidazolium-based ionic liquids (ILs: unfunctionalized ILs, [C₁₂C₂im]⁺[Br]⁻, and the carboxyl-functionalized IL, [C₁₂C₁COOHim]⁺[Br]⁻) in harsh carbonate reservoirs and thus evaluated their influence on incremental oil recovery using a high-temperature and high-pressure (HTHP) coreflood setup. Initially, we demonstrated the compatibility of these ILs under high-temperature (100 °C) and high-salinity (240 kppm TDS) conditions. After that, we assessed these ILs for their static adsorption in the carbonate rocks at various concentrations. Subsequently, we studied their dynamic adsorption–desorption kinetics in the carbonate cores under reservoir conditions of 100 °C and 3200 psi pore-pressure. In this, we demonstrated both slug and continuous injections of ILs, recorded their adsorption–desorption kinetics, and thus quantified their ultimate adsorption or chemical loss in reservoirs. The dynamic adsorption tests clearly demonstrated that the functionalized ILs, [C₁₂C₁COOHim]⁺[Br]⁻, exhibited a slower breakthrough, with the relatively lower effluent concentrations at the beginning, and overall higher retention in the carbonate reservoirs. This indicates the stronger electrostatic and/or hydrogen bonding interactions of this ILs with the minerals present on the pore surfaces. In contrast, the unfunctionalized ILs, [C₁₂C₂im]⁺[Br]⁻, demonstrated faster breakthrough and lower retention. However, the overall measured adsorption of both ILs fell well within the field requirements, with 0.1 mg of [C₁₂C₂im]⁺[Br]⁻ per gram of rock and 0.3 mg/g for [C₁₂C₁COOHim]⁺[Br]⁻. Later, by integrating the density functional theory (DFT) with the molecular dynamics (MD), we demonstrated that the carboxyl-functionalized ILs, [C₁₂C₁COOHim]⁺[Br]⁻ adsorbs significantly more strongly to calcite surfaces than its non-functionalized analogue, [C₁₂C₂im]⁺[Br]⁻. The DFT calculations revealed that this enhanced adsorption is driven by the direct coordination of the carboxylate group to surface Ca^{2 +} sites. Furthermore, MD simulations in SW brine confirm that these specific interactions allow the functionalized ILs to accumulate densely within the Stern layer, whereas the non-functionalized ILs is restricted to the diffuse layer. In parallel, we also conducted coreflood oil displacement tests using these ILs (1–10 mM) to evaluate their contribution to increased oil recovery over neat seawater flooding. It was noted that these ILs yielded about 12–26% of incremental oil recovery, with the maximum total recovery reaching 64.4%. The results showed that the functionalized ILs consistently outperformed than the unfunctionalized one, primarily due to the better surface interactions followed by the wettability alteration effects. This study confirms that molecular functionalization enhances ILs–rock interactions, and that enables for improved wettability shifts, and better oil displacement efficiency even in the harsh environments of carbonate reservoirs.