Influence of halide anions on the performance of imidazolium ionic liquids for enhanced oil recovery from carbonate formations: Experimental and molecular simulation insights

Chemical enhanced oil recovery (cEOR) strategies often involves the use of various chemicals, including surfactants, nanofluids, polymers, alkalis, and co-solvents, to improve oil recovery from unconventional reservoirs. These chemicals primarily function by modifying the physicochemical interactions that exist at the rock-oil–water interface at the molecular level, thereby altering wettability, capillary forces, viscosity, and relative permeability of the reservoir, leading to improved recovery rates. Among these chemicals, imidazolium-based ionic liquids have proven to be highly efficient in enhanced oil recovery by significantly improving oil displacement and reducing the environmental impact compared to traditional chemical methods. This study is primarily aims to investigate the influences of [Cl]⁻, [Br]⁻ and [I]⁻ anions within the imidazolium-based ionic liquids (ILs) on the wettability and thus for the oil displacement investigations, particularly in the harsh environment of Saudi Arabian reservoir. Our findings reveal an important effect of the anion′s size and shift the wettability more towards the water-wet state, as it extensively shifted the contact angle from 158° (strongly oil-wet) to 79° (intermediate-wet). This observation is further supported by subsequent experiments on spontaneous imbibition and coreflood oil displacement. Overall, ILs with [I]⁻ as the anion was outperformed than the ILs that contains the [Cl]⁻ and [Br]⁻ as the anion. The application of these ILs as tertiary flooding agents significantly improves oil recovery by 20–24%, with additional gains of 2–4% associated with the increase in anion sizes, such as [Br]⁻ and [I]⁻. Molecular dynamics (MD) simulations were conducted to investigate the interfacial properties of these ionic liquids at the oil-brine and rock-brine interfaces. Previous studies have suggested that halide-based ionic liquids primarily remain in the bulk solution at the oil-brine interface. However, our MD simulations reveal that the observed reduction in interfacial tension (IFT) is mainly attributed to the synergistic interaction between the ionic liquid cations and the organic acids present in the oil phase. Due to their electrostatic complementarity, they are accumulated at the interface, leading to a reduction in IFT. Simulations of ILs containing the three halides in a calcite nanopore indicate that chloride anions form a protective layer on the rock surface, screening ILs interactions and minimizing their loss into the rock formations. The IL ion pairs flow as hydrated complexes, which separate at the rock-oil interface. At the oil-rock interface, the cation interacts with the organic acids, while the halide anions interact with the positively charged rock surface, resulting in a wettability alteration. These results highlight the potential of enhancing the performance of ILs by controlling the ionic size of their anions, making them effective agents for improving oil recovery in carbonate reservoirs, particularly in challenging environments such as those encountered in Saudi Arabia.