Contribution of water-in-oil emulsion formation and pressure fluctuations to low salinity waterflooding of asphaltic oils: A pore-scale perspective

During the low salinity waterflooding (LSWF) of a viscous asphaltic oil reservoir, fluid-fluid interactions have a large influence on the fluid flow, pore-scale events, and thus oil recovery efficiency and behavior. In-situ water-in-oil (W/O) emulsion formation is a consequence of crude oil and brine interfacial activities. Despite the published studies, the pore-scale mechanisms of W/O emulsion formation and the role of injected brine salinity, injection rate, and pore-scale heterogeneity on emulsion formation and stability requires a deeper understanding. To address these, a series of static and dynamic micro-scale experiments were performed. The salinity dependent oil-brine interactions were characterized using interfacial tension (IFT) measurements, brine pH measurement, Fourier-transform infrared spectroscopy (FTIR), and W/O emulsion analysis. The results of static experiments showed that regardless of brine salinity, W/O emulsion does not form spontaneously and shear force is required. However, the microfluidic LSWF experiments revealed considerable formation of W/O emulsions and water-breakup. This was concomitant with fluctuations in the differential pressure. The pressure fluctuation was intensified as salinity was reduced, but was highest at the lowest IFT. Moreover, the severity of emulsion formation was intensified at lower rates and by increasing the pore-scale heterogeneity. Two principal mechanisms of W/O emulsion and water-breakup were identified: water snap-off, and water shortcut. The high viscosity of the asphaltic oil intensifies the former mechanism. While the latter is formed in water-wet regions and via movement of the waterfront through the thin layer of connate water. Emulsion division and rupture are two additional/axillary mechanisms leading to further break-up of the water phase. Emulsion formation leads to pore-scale flow diversion which in addition to other mechanisms contributes to improving oil recovery by LSWF. The novel insights gained in this study contribute to a better understanding of LSWF as well as other water-based EOR methods in viscous asphaltic oil reservoirs.