Waterflooding has been regarded as an efficient method for pressure maintenance of oil reservoirs. x Improved techniques such as Smart Water flooding as a new EOR/IOR process has gained more momentum based on the recent research activities in this field and the reduction of oil price. Despite many efforts on achieving the governing mechanisms of Smart Water flooding in many individual fields, most of data are sparse and more possible mechanisms which explains all the interactions yet to be introduced. This experimental study used a systematic laboratory framework which is based on seawater treatments at fixed ionic strength to eliminate the ionic strength effects, NaCl considered as the adjusting salt, as the injecting water. An oil-wet carbonate asphaltenic and fractured reservoir is the subject of this study. In order to investigate the impact of divalent ions in Smart Water and determining the governing mechanisms, both fluid-fluid and rock-fluid interactions are carefully studied through contact angle, IFT and pH measurements. The best Smart Water recipes from these experiments are chosen for Amott cell imbibition tests to combine all of the rock-fluid and fluid-fluid interactions of species during Smart Water injection in fractured rocks. According to the obtained results, sulfate ion has the most impact on IFT reduction for the crude oil and various Smart Water recipes and also causes the most reduction in contact angle tests. The imbibition experiments confirm these results, since the lowest recovery was obtained by removing sulfate in seawater while increasing this ion up to 4 times in seawater causes more than 8% of the ultimate recovery efficiency. The results indicated that sulfate is the most efficient divalent ion in seawater to improve the wettability alteration process for carbonate rocks during Smart Water flooding due to the expansion of electrical double layer mechanism. It is also believed that the acceleration of wettability alteration process would be mostly through rock dissolution mechanism. In addition, in the condition of high concentrations of sulfate ions, increased amount of Ca2+ and Mg 2+ concentrations and the absence of monovalent ions in the injecting water, result in significant enhancements in wettability alteration which lead to 17.5% increase in ultimate oil recovery efficiency.