Waterflooding has been the most successful method for recovering oil from reservoirs. The salinity and ion composition of the injection water in the past have not been considered as a key parameter in oil recovery from water-flooded reservoirs. In recent years, evidence - from laboratory studies verified by some field tests mainly targeting sandstones - has shown that injecting low salinity water has a significant impact on oil recovery. The potential for carbonates has not been thoroughly investigated where some of the reported studies have excluded carbonates from this effect.

Saudi Aramco, through its upstream research arm (the Advanced Research Center) has initiated a research program tagged "Smart WaterFlood" to explore the potential of increasing oil recovery by tuning the injection water properties. Based on the research work for the last three years, we present in this paper the results of reservoir condition laboratory coreflooding studies, conducted using composite rock samples from a carbonate reservoir, to investigate the impact of salinity and ionic composition on oil recovery. Also, we report a broad range of laboratory studies addressing oil recovery mechanisms.

The experimental results revealed that substantial tertiary oil recovery beyond conventional waterflooding can be achieved by altering the salinity and ionic content of field injection water. The new trend is distinct from what have been addressed in previous reported studies on topics of low salinity waterflooding for sandstones, or seawater injection into high temperature chalk reservoirs. Laboratory studies have also shown that altering the salinity and ionic content of the injection water has a significant impact on the wettability of the rock surface. The results, observations, and interpretations addressed in this study provided compelling evidence to suggest that the key mechanisms for substantial oil recovery by altering salinity of field injection brine is wettability alteration.

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