Low-salinity/smart waterflooding is a technique, used in oil reservoirs, where the salinity and/or ionic composition of the injection water is tuned to improve oil recovery. Low-salinity (LS) waterflooding can enhance oil recovery by altering the wettability of carbonate rock surfaces from oil-wet to water-wet. Though wettability alteration is generally agreed to be the main mechanism for the improved oil recovery, the contributing parameters and necessary conditions for wettability alteration are not clearly understood. Hence, it is essential to decouple the effects of salinity, ionic composition, and oil composition on the wettability alteration of rock surfaces. This study systematically investigated the effects of brine type and ionic strength on the wettability alteration of naphthenic-acid-adsorbed calcite surfaces.

Firstly, the extent of wettability alteration was assessed by performing contact angle measurements on smooth Iceland Spar calcite surfaces that were aged at 120°C in 5M NaCl brine, model oil, and single-electrolyte-based brine solutions of different salinity and ionic composition. Secondly, oil recovery due to low salinity waterflooding was verified by conducting coreflooding experiments at room temperature using limestone core samples. The feasibility of assessing in-situ wettability alteration within the limestone core was also investigated by performing micro-CT scans. Thirdly, in order to observe temporal evolution of in-situ wettability alteration directly, a procedure to fabricate calcite-coated glass surfaces was developed and the process parameters affecting the coating density and the formation of various CaCO3 polymorphs were investigated. Finally, the effects of brine type and ionic strength on the wettability alteration of naphthenic-acid-adsorbed calcite surfaces were investigated using calcite-coated straight-channel microfluidic chips.

The results showed that, irrespective of the brine type, low-salinity brine altered wettability of the naphthenic acid-adsorbed calcite surfaces from oil-wet state to water-wet state. Wettability alteration was found to increase with the decrease in brine salinity. It was also found that wettability alteration can happen at ambient temperature and mostly within few hours after exposure to low salinity brine. The wettability alteration potential observed in this study was LS Na2SO4 > LS NaCl > LS MgSO4 ? LS MgCl2 > SW NaCl (LS=0.164 M and SW=0.656 M ionic strength). Magnesium ions were found to be unfavorable towards wettability alteration in the presence or absence of sulfate ions. Exposure to high temperature did not result in any further significant wettability alteration in all the brine solutions tested.