The role of electrochemical interaction in the success of engineered water flooding

Abstract

Engineered water flooding (EWF) is widely suggested as a successful EOR method that is economically feasible and environmentally friendly. The current study performed two sets of core-flooding tests on Indiana limestone by two different oil samples in a wide salinity range to understand the role of oil in the success of EWF. According to the results, oil A demonstrated an ascending oil recovery with brine dilution from seawater (SW) to 100-time diluted seawater (SW100D). However, oil B indicated a maximum oil recovery at SW10D. The experimental oil/brine interfacial tension (IFT) could not explain the oil recovery results for any oil sample. However, the counterpart contact angles showed that the rock-fluid mechanisms dominated the oil recovery behavior. For oil A, the rock tended to be more water-wet with brine dilution, which justifies the late breakthrough and more oil recovery for SW100D than other brines. For oil B, the optimum contact angle was observed at SW10D. This explains why the injection of SW10D increases the breakthrough time and the oil recovery rather than SW and SW100D. Rock/brine zeta potential decreased from −0.7 to −13.7 mV with salinity reduction of SW to SW100. The zeta potential of oil B/brine decreased (−7.9 to −61.1 mV) significantly more than oil A/brine (from −2.9 to −15.1 mV) with the same dilution. One can conclude that as salinity decreases, oil is repulsed by rock, and it creates a more water-wet surface, as occurred for oil A. This general view cannot explain the reason behind the optimum CA of oil B. Using the DLVO theory, the current study demonstrated that asymmetry (in absolute value, not only in sign) between the rock surface potential and oil decreases repulsion. Although oil B became more negative by diluting from SW10D to SW100D, the asymmetry increased the contact angle of SW100D rather than SW10D.