Experimental Investigation of Seawater Injection for Enhanced Oil Recovery

Water flooding is the most widely used intervention applied in the field. It started decades ago as a means to maintain the reservoir pressure by voidage replacement. Recently, it has been shown that the composition and the type of the water injected in the reservoir can have a significant impact on the interactions between water, oil, and reservoir rock. These interactions can impact the amount of oil recovery by water injection. Two widely used sources of water are high salinity water (HSW) and seawater (SW), especially for offshore reservoirs. High salinity water and seawater differ usually in terms of total salinity as well as composition of the dissolved ions and salts. In this paper, we present the results of a set of coreflood experiments performed under reservoir conditions using two different crude oil samples in which we have compared the performance of conventional Seawater injection with that of HSW injection. A carbonate outcrop rock was used in the experiments. The coreflood data were also supplemented by advanced analysis of the produced oil and water samples in each experiment including interfacial tension measurements and brine composition analysis. The results reveal that Seawater flooding improved oil recovery by 5% in secondary injection mode and by 3% in tertiary injection mode (enhancing oil recovery from 50% to 53%). Further investigation showed that this improvement was attributed to wettability alteration induced by SW injection. This result is significant because it highlights the potential difference between injecting high salinity HSW and SW injection in terms of oil recovery factor. For the conditions of our experiments, seawater injection recovered considerably higher oil compared to HSW injection. The analysis of end-point relative permeability values obtained for seawater injection and for high salinity formation water indicate that the rock became more water-wet when seawater was injected compared to when HSW was injected. The results demonstrate, for the first time, that seawater injection can be advantageous compared to high salinity water injection and highlights another factor to consider when designing a waterflood project. Seawater has been studied before for enhancing oil recovery but at much higher temperatures than our experiments.