Injection of low salinity water with a correct composition can act as an effective enhanced recovery method. This paper will discuss the recovery efficiency of low salinity water flooding in carbonate and sandstone samples representing Saudi reservoirs and explore the possible mechanisms behind the extra oil recovery obtained by such process. Synthetic formation brine at 20% salinity and Saudi medium crude oil were used as formation fluids. Sea water collected from the Arabian Gulf and different diluted versions of sea water were used as displacing fluids in tertiary and secondary flooding modes.
Results indicate the potential of low salinity water flooding for both carbonate and sandstone samples investigated in both tertiary and secondary flooding modes. Interfacial tension, Zeta potential and contact angle measurements were performed to investigate the possible recovery mechanisms. Interfacial tension (IFT) of oleic and different salinity aqueous solutions resulted in insignificant IFT drop with salinity reduction. Zeta potential and contact angle measurements confirmed that wettability alteration is the main mechanism for improved oil recovery in both carbonate and sandstone samples. Wettability was shifted from intermediate wetting condition in presence of formation brine to preferentially water wet condition in presence of ten times diluted sea water for both rock types. Multi ion exchange and increasing sulfate concentration present in injected water in addition to that generated in-situ due to anhydrite dissolution may explain the substantial incremental recovery of the carbonate sample by low salinity water flooding. The recovery mechanism of the sandstone sample is attributed to fines migration and the release of existing oil and mixed-wet kaolinite clay particles exposing the underlying water wet surface.
