In this paper, the underlying mechanisms behind the low salinity waterflooding were investigated. It has been ascertained that wettability alteration towards more water-wet is the leading cause to enhance oil recovery and observe the low salinity effect (LSE). The double-layer expansion mechanism has been considered the most probable reason for the wettability alteration due to low salinity waterflooding. Therefore, mechanistic modeling of low salinity waterflooding was approached by developing a diffuse layer surface complexation-based model that accounts for the double-layer expansion phenomena. The model was developed using the geochemical reaction path code PHREEQC, wherein the speciation and surface charge development at the oil-brine and kaolinite-brine interfaces were determined. Consequently, both the electrostatic pair linkages and the electrostatic forces were calculated, which led to predicting the qualitative change in the rock wetness owing to the low salinity effect. Wettability alteration owing to low salinity water flooding was predicted as a function of the oil composition, pH environment, ionic strength, divalent ions of the injected brine, and the concentration of the divalent cations in the formation water. The wettability change was qualitatively estimated using three approaches: bond product sum, zeta potential, and stability number.
Overall, the high levels of pH in the system and the high acid content in the oil composition proved to have a beneficial impact in observing the low salinity effect and thus wettability alteration towards more water-wet. In contrast, increasing the ionic strength and the divalent cation concentrations at both formation water and injected brine suppressed the low salinity effect. Also, the abundance of Ca+2 in the formation water was more pronounced than Mg+2 to suppress the LSE. The results also showed that the bond product sum approach could not predict the wettability alteration owing to low salinity waterflooding when the pH level is higher than five.