**5. LSW/nanofluid hybrid EOR technique**

Nanoparticles are used in EOR processes due to their size and high surface area; thus, nanoparticles flow without difficulty through the pore/throat network in porous media. Nanoparticles enhance oil recovery by the following mechanisms: IFT reduction, wettability alteration, improvement of mobility ratio, and in situ emulsification [52, 53]. For example, SiO2 particles are hydrophilic and can be injected into the porous media to alter the rock wettability toward more water wet [54, 55].

Despite the positive effects of LSW on oil recovery, LSW flooding changes the chemical environment (pH, ionic strength, and temperature) in the porous media, which may lead to the detachment of reservoir particles. As the salinity of the injected brine becomes less than the critical salinity concentration, fine migration initiates, which leads to formation damage [56, 57]. Fine migration can enhance oil recovery by mobility control through the blockage of high-permeability layers, but fine migration can also cause severe damage to the near-wellbore zone. Therefore, it is desirable to control fine migration and to take advantage of its positive effects far from the wellbore to minimize its damaging effects near the wellbore. Some researchers have stated that combining LSW and NPs may help to overcome the detrimental effects of formation damage associated with low-salinity flooding [58, 59]. Hence, nanoparticles in nanofluid form can be used as a hybrid approach with LSW flooding to improve the performance of this method.

Nanoparticles enhance the attractive forces between fine particles and grain surfaces, particularly by changing the surface zeta potentials of fine particles [60]. Nanofluid pretreatment prior to the injection of LSW can reduce the side effects of fine migration by decreasing the injection pressure drop [53]. Abhishek et al. [52] studied nanoparticle adsorption at different salinities and observed that during hybrid LSW and nanofluid injection, the adsorption of nanoparticles prevents fine migration.

Another approach is to apply the hybrid nanofluid/LSW method to alter wettability and interfacial tension. Hydrophilic nanoparticles adsorb on the rock surface, and water molecules accumulate around them. This changes the wettability of the rock to be more water wet and improves oil recovery. Sadatshojaei et al. [60] coupled silica nanoparticles with LSW prepared by dilution of Persian Gulf brine. In this study, IFT, wettability, and zeta potential were measured to analyze the carbonate rock-low-salinity fluid interaction in the presence of nanofluid. It was observed that the influence of the nanofluid on the wettability of the porous media was dominant relative to the IFT reduction; thus, wettability alteration can be

considered as the main mechanism for oil recovery. Moreover, Ding et al. [61] introduced nanoparticle-assisted low-salinity hot water (LSHW) injection for heavy oil recovery. Flooding tests were conducted on silica sand packs saturated with heavy oil to compare the effect of LSW flooding, 0.05 wt% SiO2 nanoparticle-dispersed LSW flooding, and 0.05 wt% Al2O3 nanoparticle-dispersed LSW flooding on oil recovery. They observed higher oil recovery by the injection of NP/LSW than LSW alone. Also, they found that Al2O3 NPs were more effective in recovering oil due to the greater reduction in IFT.

Generally, the average nanoparticle size, specific surface area, and stability of nanofluids are important in the performance of this hybrid method. The issue of nanofluid stability at low-salinity conditions is challenging and should be considered in the application of this hybrid method. The range of stability constraints (such as zeta potential) of colloidal systems of nanofluids coupled with ions (i.e., LSW) is typically wider so that in the presence of both elements (nanoparticles and ions), longer-lasting stable solutions can be attained [60].
