The main problem during field scale implantation of low salinity water injection (LSWI) is the decline in injectivity versus time. Moreover, the actual mechanisms that result in incremental oil recovery are not completely known. In previous studies, the geomechanical effects have not been considered, and pore volume changed while bulk volume is still constant which in turn can bring uncertainty to the simulation results. In this paper, both geochemical and geomechanical models have been coupled with the flow model. For coupling geomechanical model, three equations have been solved simultaneously in each time step. Then, the geochemical model has been coupled by adding the necessary aqueous and mineral reactions and ion concentration of both formation and injection waters. Increasing the Ca2+ concentration in the injected brine cause a reduction in the ultimate oil recovery. Also, increasing SO42− concentration in the injected brine up to about 70 ppm, resulted in increased oil recovery, while increasing the concentration caused a reduction in oil recovery. Injection above formation parting pressure (FPP) is beneficial but, there is a high uncertainty during injection above the FPP that can affect ultimate oil recovery and net present value. The results of this study show that geomechanical and rock parameters have intensive effects on the simulation results and rough estimating them in the simulation process can result in major errors and uncertainties. Further, it is very important to precisely include the dominant mechanisms of low salinity or smart water process during simulation studies.