Smart water spontaneous imbibition into oil-wet carbonate reservoir cores: Symbiotic and individual behavior of potential determining ions

Abstract

Recently, application of the smart water injection, as a low-cost enhanced oil recovery (EOR) method, has gained increasing attention by researchers. The presence of potential determining ions (PDI) plays a crucial role in the smart water injection performance, especially in carbonate reservoirs. However, some controversial theories have been proposed in the literature concerning the exact influence of PDIs on the carbonate rock and fluid properties. The purpose of this study is to analyze the symbiotic and individual behavior of PDIs in the seawater (SW) and different modified SWs (MSW) at a constant salinity of 40,572 ppm, all as smart waters, justifying the effective mechanism of smart waters in carbonates and then selecting for the spontaneous imbibition at different temperatures. The dynamic/static contact angle and interfacial tension (IFT) experiments were conducted to determine the interactions in the oil/brine/rock system. Then, seven types of smart waters were nominated for analyzing the oil recovery during the spontaneous imbibition at 40 and 60 °C after the SW secondary recovery at 60 °C. According to the contact angle experiments, in the case of symbiotic PDIs, there exists an optimum concentration at which a maximum wettability alteration is occurred, attributed to the impact of co-ions and counter-ions in the smart waters and rock surface. At this optimum concentration, the symbiotic effect of all PDIs is better than their individual effects. The SO42− ion compared to Ca2+ exhibits a good wettability alteration in the presence and absence of other PDIs and the low efficiency of Mg2+ was attributing to its less activity at low temperatures. The IFT experiments showed no direct relation for the IFT reduction of the oil/MSW system at different concentrations of PDIs, indicating a non-governing mechanism of the smart waters at the freezed salinity. No additional oil recovery was observed by the spontaneous imbibition of smart waters at a low temperature of 40 °C, while a significant increase occurred at 60 °C. The smart waters with a better performance of wettability alteration lead to more oil recovery, reconfirming the wettability alteration as the effective mechanism of smart waters in carbonates. The findings of this study can help for better understanding of the symbiotic and individual behavior of PDIs of smart waters through the EOR purpose in carbonate reservoirs.