Investigating the role of [PF₆]⁻ and [BF₄]⁻ based ionic liquids for enhanced oil recovery in carbonate reservoirs: Experimental and molecular simulation insights

In chemical enhanced oil recovery (cEOR), various chemicals such as surfactants, nanofluids, polymers, and co-solvents are used. These chemicals modify molecular interactions at the rock-oil-water interface. Consequently, they enhance oil displacement by changing wettability, capillary forces, viscosity, and relative permeability. This study is focusing on the role of [PF₆]⁻ and [BF₄]⁻ anions in imidazolium-based ionic liquids (ILs) on enhancing oil displacement in carbonate reservoirs under harsh reservoir conditions. To do so, we ran a series of wettability (static and dynamic), spontaneous imbibition, coreflood oil displacement tests, and molecular simulations. Wettability studies showed that [PF₆]⁻ based ILs perform better than [BF₄]⁻ ILs. Furthermore, the use of ILs improved the oil recovery by about 18–39 % and 18–24 % in the spontaneous imbibition and coreflood oil displacement tests, respectively. However, in both studies, the [PF₆]⁻ based ILs outperformed [BF₄]⁻. The simulations emphasized the significance of molecular charge density in controlling the action mechanism. [PF₆]⁻ exhibited lower charge density than [BF₄]⁻, resulting in attenuated interactions with the metal cations and water. This allowed [PF₆]⁻ to penetrate more effectively at the oil-rock interface, leading to a more efficient change in wettability to water-wet compared to [BF₄]⁻. Another interesting result of our DFT and MD simulation is that the ILs are more salt-tolerant than the canonical surfactants. They exhibit stronger interactions with the monovalent cations than the divalent, therefore, in presence of Cl⁻ that display higher charge density than the [PF₆]⁻ and [BF₄]⁻, the interactions with the metal cations will be minimal. Our results indicate that ionic liquids (ILs) containing larger anions exhibit promising potential for wettability alteration.