Water-Based Enhanced Oil Recovery (EOR) by Smart Water: Optimal Ionic Composition for EOR in Carbonates

The composition of the injecting brine has a profound effect on the efficiency of water-based enhanced oil recovery (EOR) methods. Recently, we observed that not only is the concentration of the active ions Ca(2+), Mg(2+), and SO(4)(2-) important for wettability alteration in carbonates but also the amount of non-active salt, NaCl, has an impact on the oil recovery process. Removing NaCl from the synthetic seawater improved the oil recovery by about 10% of original oil in place (OOIP) compared to ordinary seawater. The results were discussed in terms of electrical double-layer effects. In this work, we have modified the seawater depleted in NaCl by adjusting the concentration of active ions, Ca(2+) and SO(4)(2-). Oil displacement studies in outcrop chalk samples by spontaneous imbibition were performed at temperature ranges of 70-120 degrees C using different oils and imbibing fluids. When the concentration of SO(4)(2-) in the seawater depleted in NaCl was increased 4 times, the ultimate oil recovery increased by about 5-18% of OOIP compared to the seawater depleted in NaCl. The amount of Ca(2+) in the seawater depleted in NaCl had no significant effect on the oil recovery at 100 degrees C, but significant improvements were observed at 120 degrees C. Chromatographic wettability analysis confirmed that the water-wet area of the rock surface increased as the oil recovery increased, emphasizing the importance of the ionic composition and the ion concentration of the injecting brine in the water-based EOR methods.