Pore-Scale Insights into the Nonmonotonic Effect of Injection Water Salinity on Wettability and Oil Recovery in Low-Salinity Waterflooding

Detailed investigation of the time-scale and the combined effect of fluid-rock and fluid-fluid phenomena in low-salinity enhanced oil recovery (EOR) is essential to gain fundamental understanding of the efficiency of oil recovery by this method. By focusing on a brine-oil-quartz system and developing fit-for-purpose methods and apparatus, we performed microfluidic flooding experiments to study this as a function of salinity. Brines made of single monovalent or divalent salts were considered to identify the most potent brines for micromodel flooding experiments. The results of the microfluidic experiments reveal that the oil recovery factor by injection of brine containing sodium chloride follows a nonmonotonic trend with salinity. The nonmonotonic oil recovery behavior at the macro-scale is caused by the existence of several concomitant micro-scale phenomena which behave nonmonotonically with salinity and have their own optimum salinity: the wettability alteration, the coalescence time of oil droplets and interface elasticity, and the interfacial tension (IFT). The optimum conditions for these phenomena are reached at a lower ionic strength with the brines containing divalent cations than those with monovalent cations. Since different working phenomena in low-salinity EOR have their own time-scale of action, it is critically important to allow sufficient time, about a few days to a few weeks, to account for low-salinity kinetics and obtain the full effect of low salinity in the system, particularly for fluid-rock interactions such as wettability alteration which is a dominant slow process. Fluid-fluid interactions such as IFT change and coalescence were found to be much faster. Not accounting for the time effect can result in an underestimation of oil recovery by as much as 10%. The novel results of this study highlight that the optimal salinity for maximal oil recovery can be different from that for each underlying phenomena; however, it matters most in the design of the process.