Low Salinity Water-surfactant-CO2 Enhanced Oil Recovery: Theory and Experiments

A low salinity water-surfactant-CO2 EOR process is proposed. Coreflood, IFT, contact angle, and phase behavior measurements were performed to investigate the viability of the proposed EOR process. Significant oil recovery in the laboratory was achieved with the proposed EOR process. The process mobilizes part of the residual oil because (i) low-salinity brine improves wettability towards hydrophilic condition favorable for surfactant flooding; (ii) surfactant in low-salinity water solubilizes some of the remaining oil as Winsor type II- microemulsion and lowers IFT between oil and water; and (iii) CO2 will follow surfactant to mobilize more of the remaining oil in the wettability-improved condition. We aged the cores for eight weeks at reservoir temperature and pressure and measured contact angle between oil droplets and core surface for several brine-oil-rock environments to mimic the reservoir conditions. Coreflood in low-permeability carbonate cores show that the proposed EOR process produces incremental oil up to twenty-five percent beyond seawater flooding. Contact angle measurements on carbonate, sandstone and shale cores indicate that wettability alteration and IFT reduction are the main oil-mobilizing mechanisms in the proposed EOR process.