Low salinity water flooding (LSWF) is a relatively new enhanced oil recovery (EOR) technology which has been reported to improve oil recovery in several laboratory experiments and some field trials. LSWF is a very attractive EOR technique in terms of the lowness of operating costs and the smallness of the environmental influences around the reservoirs. Although several mechanisms involved in oil recovery by LSWF have been proposed, the validated mechanisms and the optimum conditions are still uncertain.
We conducted the laboratory core flooding experiments in order to evaluate the potential of LSWF and to investigate the possible mechanisms that improve the oil recovery. Berea sandstone cores were flooded with high salinity formation water (FW) followed by low salinity water (LSW). To examine the effects of the compositions of LSW and the effects of the polar oil, LSW and crude oil that have different compositions were used. To investigate the reactions that had possibly happened in a core during water flooding, effluent fluid samples were analyzed for ionic concentrations and pH by using ICP-OES and pH meter, respectively.
The results showed that the injection of LSW after FW increased the oil recovery by 7.5 %. Furthermore, in the experiment using the crude oil sample that contained double resin, LSWF improved the oil recovery by up to 15.7 %. However, by the injection of the adjusted LSW (ALSW) that does not contain the divalent cations, the remarkable oil recovery could not be observed. In these experiments, although the pH value and ionic concentration of the effluent fluids were higher than those of the injected LSW and ALSW, suggesting the exchange of ions, LSWF did not improve the oil recovery.
In parallel with the experimental work, the numerical simulator that can reproduce the core flooding performances is being developed. The mechanism of LSWF is also examined by the numerical approach using this simulator, which suggested the role of the interaction among divalent cations, polar oil and clay minerals in improving oil recovery by LSWF.
The above results reveal that the LSWF should improve the oil recovery when the ionic exchange and the adsorption and desorption between polar oil and clay minerals occurred. Furthermore, the content of polar oil in crude oil and the concentration and compositions of LSW may play a predominant role on the LSW effects.