Modified seawater as a smart EOR fluid in chalk

• Modified seawater increased oil recovery by 8% OOIP compared to that by seawater.
• >90% NaCl should be removed from seawater for significant increased oil recovery.
• Additional 10% OOIP to be obtained by adding sulfate to modified seawater.
• 3 times sulfate content of seawater added for significant increased oil recovery.

Water based EOR by wettability alteration is of interest among the oil companies because of economic and environmental reasons. It is well known that seawater acts as a wettability modifier in carbonate reservoirs, and the seawater can even be improved as a smart EOR fluid by removing Na+ and Cl− ions, and spiking this NaCl-depleted seawater with SO42- ions. In this paper we have addressed the degree of seawater modification needed to achieve reasonable improvements in oil recovery in spontaneous imbibition processes at 90 °C. Compared to ordinary seawater as imbibing fluid, more than 90% of the NaCl content must be removed in order to increase the oil recovery by 7–8% OOIP. Furthermore, compared to seawater depleted in NaCl, the oil recovery could be increased by 10% OOIP by spiking the NaCl depleted seawater with sulfate, 3–4 times the sulfate concentration present in seawater. The increased efficiency of modified seawater as a smart EOR fluid in carbonate is discussed in terms of a decrease in the ionic density of the double layer close to the positively charged calcite surface.