Understanding the Interactions at Rock-Water and Oil-Water Interfaces during Controlled-Salinity Water Flooding

Abstract

Over the years, laboratory studies and a limited number of field trials have demonstrated the potential of enhancing oil recovery using controlled-salinity water flooding. The injected brine composition is one of the promising techniques that could alter the wettability of carbonate rocks by changing the concentration of the potential determining ions (PDIs), specifically Ca2+, Mg2+, and SO42− ions. In this study, a comprehensive experimental study was conducted to investigate the rock-fluid and fluid-fluid interactions at rock-water and oil-water interfaces. The first step of the study was to measure the interfacial tension (IFT) using the spinning-drop tensiometer and study the dynamic behavior of the oil-water interactions. The zeta potential of carbonate rock samples was then measured using a specially-designed zeta potentiometer capable of utilizing the whole core plug, rather than the pulverized samples. The streaming potential technique was used for the zeta potential measurements and the experiments were conducted under different modified brine composition and rock saturation conditions. Subsequently, wettability alteration experiments were conducted using a specially designed high-pressure high-temperature (HP/HT) cell. The IFT measurements showed an increasing trend as salinity decreases, clarifying that rock-water interactions are more dominant over oil-water interactions. Results of the zeta potential experiments showed a clear trend of yielding more negative values as the seawater gradually diluted down to 1%dSW, due to the expansion of the electrical double layer. On the other hand, when the brine composition was modified, the increase of the PDIs (Ca2+ and Mg2+) did not have as much impact on zeta potential as the SO42− ions. In the wettability alteration experiments, both diluted and composition-modified brine generated a higher imbibition rate, resulting in a higher total oil production when compared with the experiments using the seawater. Furthermore, the wettability alteration of the rock surface trended more towards water-wetness conditions, as inferred from the contact angle measurements. The measurement of zeta potential before and after wettability alteration tests showed that the zeta potential value became less negative after the experiment, which suggested the expulsion of oil from the rock. This was further verified by the measurements of zeta potential for the unsaturated rock and saturated rock with brine and oil. The findings from this study would provide a better understanding of the rock-fluid and fluid-fluid interactions during controlled-salinity water flooding, which will benefit future studies in this area.