Low-salinity water-alternating-CO2 EOR

• By combining low-salinity waterflood EOR and gas injection EOR, a new EOR process is proposed. The proposed EOR process is low-salinity water-alternate-gas/CO2 (LS-WACO2 or LS-WAG) injection.
• Coreflood in carbonate reservoir core and sandstone core were performed.
• Contact angle, Interfacial Tension (IFT), and minimum miscibility pressure (MMP) measurements were performed on carbonate, sandstone, and shale reservoirs.
• Thermodynamic modeling indicated that CO2-brine IFT decreases with decreasing brine salinity.
• Improved wettability alteration towards hydrophilic and IFT reduction are among the possible mechanisms by which LS-WACO2 or LS-WAG EOR mobilizes additional residual oil.

Carbon dioxide flooding is currently the most technically and economically viable enhanced oil recovery (EOR) process in carbonate and sandstone reservoirs. Low-salinity waterflood is a relatively new EOR process; and our experiments in carbonate cores show significant oil recovery improvements with low-salinity waterflood. We propose a new EOR process to improve recovery, which involves low-salinity water-alternating-CO2/gas (LS-WACO2 or LS-WAG) injection.To evaluate the proposed idea, three core floods and several contact angle and IFT measurements were performed. The core floods include: seawater flood, followed by low-salinity waterflood, followed by CO2 injection, which yielded fourteen, twenty-five, and thirty-eight percent additional oil recovery by CO2 from two carbonate and one sandstone experiments.We performed contact angle measurements on several low-permeability carbonate, medium-permeability Berea sandstone, and ultra-low permeability Three Forks mudstone core discs using different salinities brine with and without CO2 gas. The contact angle measurements confirmed that favorable wettability alteration is achievable with the proposed EOR process. In addition, visual observations suggested that the proposed EOR process could be effective for cleaning the matrix-fracture interface in conventional and unconventional reservoirs. Interfacial tension (IFT) measurements and correlation relevant to the EOR process is also included in this study.