Low-salinity water-alternating-CO₂ flooding enhanced oil recovery: Theory and experiments

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. We propose a new EOR process to improve economics, which involves low-salinity water-alternating-CO₂ (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, mediumpermeability Berea sandstone, and ultra-low permeability Three Forks mudstone core discs using different salinities. 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. IFT measurments and modeling relevant to the EOR process is also included in this study.