Investigation into fluid-fluid interaction phenomena during low salinity waterflooding using a reservoir-on-a-chip microfluidic model

Injection of modified salinity brines (MSB), i.e. brine with seawater-like salinity (SWS) and low salinity water (LSW) in oil-wet carbonate rocks has proven to be relevant to improved oil recovery operations. Many reports in the literature related the underlying mechanisms to rock-fluid interactions such as ionic exchange and electrical double layer expansions, which caused wettability alterations at the rock surface. Little attention appears to have been paid to fluid-fluid interactions as a potential mechanism in displacement processes. In this work, we investigated the role of fluid-fluid interactions in improved oil recovery using MSBs. Interfacial tension and surface elasticity calculations were correlated to visual observations of displacement processes in order to investigate the role of crude oil snap-off. Microfluidic devices featuring pore throats 50 mu m in diameter were used to observe snap-off as a function of salinity of the displacing fluid. Flow experiments suggested that, in a water-wet constricted pore throat, SWS brines suppressed crude oil snap-off as compared to formation water brine (FWS) brine. This behavior was correlated to the higher surface elasticity of oil-SWS interfaces than that of oil-FWS interfaces. Higher surface elasticity suppressed the expansion of the thin water film coating pore throat walls and hence it increased the capillary number at which snap-off of the crude oil phase was expected to occur. Moreover, water interacted with the polar components to form reverse micelles called microdispersions. These microdispersions were observed in both aged and non-aged microfluidic devices near oil-brine interfaces in the pore-network. Similarly, in a vial test performed by Tetteh and Barati (2019), microdispersion formation was only observed very close to oil-brine interfaces, caused by transport of water molecules into the oil phase due to reduced intermolecular forces at oil-LSW interfaces. This microdispersion formation was visualized using environmental scanning electron microscopy. These microdispersions remobilized, redistributed the oil, and along with a slight change in wettability in the medium, they improved the observed recovery. In the pore-network flow experiments, the use of SWS brines resulted in the formation of relatively larger oil droplets, which is attributed to the suppression of crude oil snap-off and enhanced oil coalescence resulting from changes in oil-brine interfaces. The integrated experimental study presented in this work demonstrates the importance of fluid-fluid interactions in improved oil recovery using MSBs.