Improved Oil Recovery in Carbonates by Modified Seawater – Optimal Ionic Composition and Salinity

Not only the concentration of the active ions Ca2+, Mg2+, and SO42 is important for wettability alteration in a water-based EOR process, but also the amount of non-active salt, Na+ and Cl-, has an impact on the wettability alteration. Removing NaCl from the composition of the seawater improved the oil recovery about 10% of OOIP compared to seawater. In this work, we have modified the seawater depleted in NaCl by adjusting the concentration of the potential determining ions Ca2+ and SO42-. Outcrop chalk cores were used as the porous medium with an initial formation brine of 10%, saturated and aged in a crude oil with AN=2.0 mgKOH/g. The oil displacement tests were performed at 100 and 120oC by spontaneous imbibition of the optimized seawater. The concentration of SO42- in the imbibing brine had the largest effect on the oil recovery. When the concentration of SO42- in the seawater depleted in NaCl was increased four times, the ultimate oil recovery increased by about 20% of OOIP compared to the seawater depleted in NaCl. The amount of Ca2+ in the seawater depleted in NaCl had no significant effect on the oil recovery at 100 oC, but small improvements were observed at 120oC.