A comprehensive insight on the impact of individual ions on Engineered Waterflood: With already strongly water-wet sandstone

Abstract

Engineered Waterflood (EWF) has drawn increasing attention as an emerging economic-friendly technique. However, the underlying mechanisms still remain ambiguous. Wettability alteration has been claimed as the main reason to the additional oil recovery by EWF. In contrast, limited work was devoted to investigate the contribution of fluid-fluid interaction. To fill this gap, in the study, strongly water-wet rock samples (artificial clay-free sandstones) were used to minimize wettability alteration, thereby highlighting the fluid-fluid interaction. A high-salinity single-component NaCl solution (100 kppm) was used as a base line; three low salinity (1 kppm) brines (i.e., NaCl, CaCl2 and Na2SO4) were adopted in our experiments to explore the effect of brine salinity and ion types. Moreover, an offshore heavy oil was chosen in our study to emphasize fluid-fluid interfacial response. First, secondary EWFs were conducted to demonstrate the brine chemistry effect on core scale. Subsequently, oil/brine interfacial properties including IFT and IVE were analyzed at the interfacial film scale. Thereafter, visible micromodel experiments were performed to manifest the fluid-fluid interaction on pore-scale displacement events. On the other hand, zeta potential measurements along with the disjoining pressure isotherm calculation by the extended-DLVO theory were used to study the rock-fluid interaction. Our results show that both low salinity and the sulfate enrichment increase the interfacial visco-elasticity, which hinders the microscopic displacement events of both snap-off and coalescence. In addition, as engineered water is injected at secondary mode, the overall oil continuity is primarily improved by suppressing snap-off. Additionally, disjoining pressure calculation reveal that a lower brine salinity and an enrichment of sulfate can decrease the attraction force between oil and rock, imparting a lubricating effect on the flow of crude oil. Therefore, this work has shed light on EWF mechanisms by considering both fluid-fluid and fluid-rock interactions from multi-scales. It also clarifies the effect of sulfate on EWF in sandstone, unlike previous studies that mainly evaluated the performance of sulfate in carbonate.