Change of injection brine composition from high to low salinity has been reported to improve the oil recovery in some field pilots and core flooding studies. In other studies low salinity water flooding (LSWF) has not been successful. It has also been reported that LSWF can be successful in secondary floods but not in tertiary floods.
The main objective for the reported study has been to investigate the oil recovery and cation concentrations in effluent samples during secondary and tertiary LSWF experiments using brines of different compositions. These results have been combined with evaluation of cation exchange during these floods by simulations and history matching to estimate the saturation functions.
When diluted formation water (LSW2) was injected in secondary floods, the oil production was only slightly higher than the total oil production during secondary flooding with formation water (FW) followed by tertiary flooding with LSW2. In the secondary flood with LSW2 the oil was produced during a long period, and wettability alteration appeared to occur during the flood. LSW2 was also found to give less water-wet conditions than FW. When LSW2 was injected to a core aged with LSW2, the oil production was slow and rather low remaining oil saturation was obtained. In a secondary flood with optimized low salinity water composition selected based on evaluation of cation exchange on clay surfaces, the oil recovery was found to be faster than in the secondary LSWF with LSW2 (diluted FW). By modeling of the cation exchange in the flooding experiments, it was found that the change in oil recovery profile can be explained with the change in the concentrations of divalent cations on the clay surfaces.
It is concluded that the potential for secondary and tertiary LSWF, will strongly depend on the compositions of rock, FW and LSW. The potential for tertiary LSWF will also strongly depend on the composition of the high salinity brine injected in the secondary flood.
Modeling of cation exchange during the reservoir history can be used to evaluate the potential for LSWF of the oil reservoirs and to optimize the LSW compositions.