Investigation of the effects of low-salinity waterflooding for improved oil recovery in carbonate reservoir cores

Water injection for both pressure maintenance and oil displacement is the most important secondary recovery method in sandstones. It has also been implemented with success in a few carbonate reservoirs, but because the most carbonate reservoirs worldwide are characterized as neutral to preferential oil-wet, normal waterflooding is usually not successful as an enhanced oil recovery (EOR) technique. It has been proved that seawater can be used as an EOR fluid for hot, fractured carbonate oil reservoirs since it is able to modify the wetting conditions and to enhance the oil recovery. The potential determining ions in seawater such as Ca2+, Mg2+, and SO42- played a crucial role in altering the wettability from oil-wet to more water-wet condition because of their reactivity towards the carbonate surface. In this paper, the potential of low-salinity brine to enhance the oil recovery has been studied. Four flooding tests were conducted on both limestone cores containing anhydrite and chalk core containing no sulfate. It is observed that low-salinity brine had only effect on rocks containing anhydrite. The dissolution of anhydrite, CaSO4, which is the source for SO42-, is depending on salinity/composition of brine and the temperature. The dissolution of anhydrite normally increases as the temperature decreases. Lowering the salinity of injection brine increases the reactivity of the surface-active ions SO42- and Ca2+.