5 Chapter II Literature Review Previous field studies have shown that poor oil recovery factors are the biggest challenge in developing oil and gas fields. The reasons can be rapid pressure depletion, complex reservoir characteristics or operational challenges. To overcome this situation, EOR is often presented as the most promising technology. EOR (Enhanced Oil Recovery) refers to various methods used to increase the amount of crude oil that can be extracted from an oil field after the primary and secondary recovery stages have been exhausted. EOR is crucial for extending the life of oil fields and increasing the overall oil recovery rate, sometimes by 30-60% more than conventional methods (Donaldson & Chilingar, 1989). Without the EOR implementation methods, about 65-80% of the OOIP oil remains in the reservoir (Thomas S., 2008). The main types of EOR techniques include thermal, gas injection, chemical injection, and microbial. In chemical EOR, various chemicals can be additives to modify the interaction of oil-brine-rock in the reservoir to obtain more favorable conditions for final oil recovery. The widely used chemicals in CEOR include Low Salinity Water/Smart water, surfactants, nanoparticles, polymers, microorganisms, ionic solutions, and combinations of them, which role as injection fluids. II.1 Low Salinity Water Low Salinity Water (LSW) in the application of Low Salinity Water Injection was first known around 1960 when Reiter and Kaye realized the effect of low salinity on injection fluids on oil recovery from sandstone core plug (Reiter PK, 1961). Low salinity water flooding has attracted much attention due to its various advantageous characteristics, including its relatively low cost (compared to other types of chemical EOR), its abundant availability (especially in offshore operations), its ease of application, and its simple feasibility study procedure (Bartels et al., 2019). Results from the BP Laboratory (Lager et al., 2006) showed an average profit of 14% using low-salinity brine. Other studies show varying results from +4% to 6 +40%. Single-well chemical tracer tests conducted by BP Alaska resulted in a decrease in residual oil saturation of 6-12% from OOIP (McGuire et al., 2005). It is believed that the initial condition of a reservoir is water-wet due to the presence of fluids deposited along with the diagenesis process of sedimentary rocks in geological basins over geological time scales. Then, the presence of oil in the form of an oleic phase during the migration process from the source rock to the reservoir for millions of years can replace the presence of brine in the reservoir and cause alteration of the wettability of the rock from water-wet to oil-wet. (McAuliffe CD, 1979). Various techniques have been developed to modify these wettability properties to their original conditions through various chemicals which able to modify the surface properties of rocks with their specific mechanisms, including the use of low salinity water. Basically, oil, brine, and rock are in equilibrium for a particular system, and the injection of LSW as secondary phase disrupts fluid conditions and their interface properties. Injection of water at various ion concentrations or compositions can result in an imbalance of forces between the phases, thus forming a new equilibrium condition that is different from the initial state (Alshakhs MJ & Kovscek, 2016; Tetteh et al., 2020).