| Summary: | Low salinity water is an emerging enhanced oil recovery (EOR) method that causes wettability alteration towards a favorable state to reduce residual oil saturation, while water alternating gas (WAG) is a proven EOR process that enhances oil recovery by controlling mobility of both water and gas. Therefore, combining the two EOR processes as low salinity water-alternating CO2 EOR injection (LSWA CO2) can further improve oil recovery by promoting the synergy of the mechanisms underlying these two methods.Core flooding experiments, contact angle, interfacial tension (IFT), and CO2 solubility measurement in oil and brine were conducted to investigate the viability and performance of LSWA CO2 in sandstone reservoirs. A favorable wettability alteration, along with IFT reduction and mobility control, are the mechanisms that contribute to residual oil mobilization efficiencies during the LSWA CO2 EOR process. In addition, LSWA CO2 core flooding experiments result in a significant incremental oil recovery.Three smart waters were tested in our research, to examine the impact of changing cationic composition on oil recovery. The solutions are designed brines as NaCl (SW1), MgCl2 (SW2), and KCl (SW3). Of the three solutions, SW1 yields the highest incremental oil recovery and highest IFT reduction. In addition, it results in a favorable wettability alteration towards a more water-wet state.In all cases, introducing CO2 to the brine/oil system shows a great advantage in terms of enhancing wettability modification, promoting IFT reduction, and controlling the displacement front of the injected fluid through mobility control.
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