When injecting low salinity (LS) water, it is believed that destabilization of oil layers adhering to mineral surfaces could be a contributing mechanism to enhanced oil recovery (EOR). Surfactant flooding is a proven EOR technique by increasing the capillary number. The combination of LS water at reduced capillarity can avoid retrapping of destabilized oil and exceed recoveries of either of the techniques applied alone. In this study, we have used an alcohol propoxy sulfate mixed with an internal olefin sulfonate to compare the oil recovery in a low salinity surfactant (LSS) flooding process at moderately low IFTs to that of an optimal salinity surfactant (OSS) injection process at ultralow IFT. The surfactant formulation was selected on the basis of an initial screening phase using a North Sea crude oil and diluted seawater. Its effect on oil recovery efficiency in different injection scenarios was investigated using crude oil aged Berea sandstone cores. The results showed comparable recoveries for the LSS flooding at a capillary number 2 orders of magnitude lower than that for the surfactant flooding at ultralow IFT. In addition, retention values in the latter case were around 60% higher than for the LS case. On the basis of this, it appears that the LSS process may be more economically efficient than an OSS injection process at ultralow IFT.

The thesis includes research on the hybrid EOR process of low salinity surfactant injection. The main objective was to investigate whether combining the two processes of low salinity injection with surfactant injection would be more efficient than either of the processes alone. I big thank you to all fellow students and colleagues at CIPR that I have met over the years, many of which I still consider as close friends. A big thank you goes to a very special person, Bartek Vik, for his support and motivation on both a personal and professional level. I admire your always optimistic way of being which have helped me to keep things in perspective. Last but not least I would like to thank the rest of my family and friends for all their support, motivation and for always having belief in me.

It is well-established that injecting water with significantly lower salinity than the formation water salinity may give increased oil recovery. Although less well studied, the observed low-salinity effect has drawn attention to possible benefits from combining low-salinity water with traditional enhanced oil recovery techniques, such as surfactant, polymer, alkali, etc., to make the overall recovery process more efficient. Surfactant injection, for example, reduces the interfacial tension (IFT) between the injected surfactant solution and the oil, thus mobilizing capillary trapped oil and/or reducing the tendency for capillary trapping. The majority of literature on the topic of low salinity and surfactant flooding primarily addresses one or the other. This study, however, compares the combined effect of reduction in IFT and low-salinity conditions to the effect of a sole reduction in IFT on oil recovery in intermediate-wet Berea sandstone cores. We find that reductions in residual oil saturation, at similar capillary numbers and phase behavior conditions, are higher for the low-salinity surfactant injection experiments compared to regular surfactant injection experiments. This strongly indicates that there is a combined effect of IFT reduction and low salinity on recovery compared to that from a reduction in IFT alone.