This paper investigates the effect of low salinity water injection (LSWI) on both oil displacement and volumetric sweep efficiencies. A new definition of mobility ratio is proposed and shown to correlate with volumetric sweep efficiency better than other definitions in the literature. A quarter 5-spot field model was created by upscaling recently published coreflood experiments. Simulations were performed using the empirical low salinity injection model implemented in the UTCHEM simulator. The volumetric sweep efficiency for various seawater dilutions was estimated by adapting a fractional flow method and a tracer method for this purpose. The fractional flow method and the tracer method were consistent in estimating the volumetric sweep efficiency. Moreover, an improved estimation of volumetric sweep efficiency was possible with the proposed mobility ratio definition, which applies in the tertiary mode and physically makes more sense compared to other definitions in the literature. In addition, the maximum incremental oil recovery was achieved at the 10 times diluted seawater injection cycle with is consistent with other studies in the literature. Next, the injected number of pore volumes for each of the seawater dilutions was optimized. The optimum was found to be two pore volumes of injected seawater dilutions.