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

摘要

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 ℃ 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 S-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 ℃, but significant improvements were observed at 120 ℃. 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.