Abstract
Optical microscopy was used to measure depth-averaged oil distribution in a quasi-monolayer of crushed marble packed in a microfluidic channel as it was displaced by water. By calibrating the transmitted light intensity to oil thickness, we account for depth variation in the fluid distribution. Experiments reveal that oil saturation at water breakthrough decreases with increasing Darcy velocity, U_{text {w}}, between capillary numbers text {Ca} = mu _{text {w}} U_{text {w}}/sigma = 9times 10^{-7} and 9times 10^{-6}, where mu _{text {w}} is the dynamic viscosity of water and sigma is the oil/water interfacial tension, under the conditions considered presently. In contrast, end-point (long-time) remaining oil saturation depends only weakly on U_{text {w}}. This transient dependence on velocity is attributed to the competition between precursor film flow, which controls early time invasion dynamics but is inefficient at displacing oil, and piston-like displacement, which controls ultimate oil recovery. These results demonstrate that microfluidic experiments using translucent grains and fluids are a convenient tool for quantitative investigation of sub-resolution liquid/liquid displacement in porous media.
Highlights
Water invasion into oil-saturated porous media is relevant to many natural and industrial processes including water infiltration and contaminant transport in soils and groundwater aquifers contaminated by non-aqueous phase liquids (NAPLs), oil recovery, and geological CO2 storage
Micromodels have been used for decades to study porous media flow under conditions relevant to these applications
Many enhanced oil recovery and NAPL-remediation approaches involve the addition of chemical additives such as surfactants and polymers to the water that is injected into the system to displace the oil
Summary
Water invasion into oil-saturated porous media is relevant to many natural and industrial processes including water infiltration and contaminant transport in soils and groundwater aquifers contaminated by non-aqueous phase liquids (NAPLs), oil recovery, and geological CO2 storage. Microfluidic studies of two-phase, oil/water displacement processes typically involve the addition of dye to the aqueous phase to facilitate visualization (e.g., Geistlinger et al 2016; Frette et al 1997; Chevalier et al 2015; Xu et al 2017; Yun et al 2017; Kumar Gunda et al 2011; Xu et al 2014; Cottin et al 2010; Yeganeh et al 2016; Levaché and Bartolo 2014; Datta et al 2014) unless opaque crude oil is used (e.g., Song and Kovscek 2015; Zhu and Papadopoulos 2012; Bowden et al 2016). Where the aim of a microfluidic experiment is to evaluate the performance of such additives, the addition of dye to the aqueous phase is not desirable as dyes can alter the interfacial properties of the test fluid; in such cases, the oil phase must be dyed instead (e.g., Conn et al 2014; Zhang et al 2011; Beaumont et al 2013; Gauteplass et al 2013; Nilsson et al 2013)
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