Combined Low Salinity Brine Injection and Surfactant Flooding in Mixed-Wet Sandstone Cores

The concept of surfactant flooding has long been recognized as a promising supplement to water-based enhanced oil recovery methods. In recent years, core displacement experiments performed on elastic rock samples using low salinity brine as injection fluid have proven to give a moderate increase in crude oil recovery. This paper presents a new hybrid EOR process where the effect of low salinity brine injection is combined with surfactant flooding. An anionic surfactant formulation was selected to give low interfacial tension (IFT) in a low salinity environment containing 0.50 wt % NaCl. The surfactant forms Winsor type I microemulsion at this salinity. Improved surfactant solubility and reduced adsorption or retention is among the advantages of using surfactant at low salinity conditions. This paper reports core flood experiments performed on outcrop sandstone cores using crude oil as the oil phase. Results show high oil recovery of more than 90% of original oil in place (OOIP) when surfactant is used in tertiary mode, i.e., after secondary waterflood. Destabilization of oil layers caused by change in brine salinity and simultaneous mobilization of the residual oil at low IFT is discussed as the possible underlying mechanism for the combined process of low salinity water injection with surfactant flooding. Low surfactant retention under these conditions may makes this hybrid EOR process more economically attractive compared to applying low salinity waterflood or surfactant flooding separately.