Low salinity water–Surfactant–CO₂ EOR

Coreflood, interfacial tension (IFT), contact angle, and phase behavior measurements were performed to investigate the viability of a hybrid of low-salinity water, surfactant, and CO₂ flood enhanced oil recovery (EOR) process. Low-permeability carbonate reservoir cores were aged for eight weeks at reservoir temperature and pressure. Coreflood and contact angle between oil droplets and core surface measurements were performed. Additional contact angle measurements on sandstone and shale cores were also performed. The coreflood sequences were seawater flood, followed by low-salinity water flood, followed by surfactant floods until residual oil saturations to each flooding sequences and finally CO₂ injection. Coreflood in low-permeability carbonate cores show that the hybrid 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 decrease are the main oil-mobilizing mechanisms in the hybrid EOR process. The hybrid EOR 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.