Impact of Salinity on Waterflood Displacement Efficiency

Abstract

Abstract Fluid properties are among the main parameters that influence waterflood performance. This paper aims to investigate the impact of salinity on displacement efficiency by understanding the effects on physical properties. Besides salinity, dissolved CO2 and temperature effects are also evaluated. Based on these, the paper assesses when the different effects are significant, in which brine tracking and temperature modelling are required. In addition, it provides insights about reservoir applications involving CO2. The majority of the studies in the literature about the effects of salinity, mainly the injection of low salinity water, on oil recovery involve geochemical reactions, and there is a lack of studies about the effects of physical properties in that regard. In this work, a sensitivity analysis by compositional reservoir simulation is conducted, where variable salinity cases are simulated in isothermal and non-isothermal systems, with oil composition considering both with and without dissolved CO2. Both 2D and 3D grids are used. The analysis shows that salinity did not strongly impact the oil recovery; its effect on density and viscosity is insufficient to impact displacement efficiency. Moreover, CO2 increased the oil recovery significantly for both 2D and 3D systems, which is attributed to improving oil viscosity and oil swelling phenomena. The reduction in temperature improved the oil recovery slightly for 2D systems and significantly for 3D systems. This suggests that injecting cool brines into hot reservoirs leads to better oil recovery for 3D systems when there is no CO2 due to gravitational effects, while 2D systems, where gravity is less pronounced, have more recovery in the presence of CO2. Additionally, gravity effects are minimized due to the very low mobility ratio. Based on that, brine tracking is generally not required, while temperature modelling in compositional simulations is advised.