Using colloidal deposition to mobilize immiscible fluids from porous media

Colloidal particles hold promise for mobilizing and removing trapped immiscible fluids from porous media, with implications for key energy and water applications. Most studies focus on accomplishing this goal using particles that can localize at the immiscible fluid interface. Therefore, researchers typically seek to optimize the surface activity of particles, as well as their ability to freely move through a pore space with minimal deposition onto the surrounding solid matrix. Here, we demonstrate that deposition can, surprisingly, promote mobilization of a trapped fluid from a porous medium without requiring any surface activity. Using confocal microscopy, we directly visualize both colloidal particles and trapped immiscible fluid within a transparent, three-dimensional porous medium. We find that as nonsurface active particles deposit on the solid matrix, increasing amounts of trapped fluid become mobilized. We unravel the underlying physics by analyzing the extent of deposition, as well as the geometry of trapped fluid droplets, at the pore scale: deposition increases the viscous stresses on trapped droplets, overcoming the influence of capillarity that keeps them trapped. Given an initial distribution of trapped fluid, this analysis enables us to predict the extent of fluid mobilized through colloidal deposition. Taken together, our work reveals a new way by which colloids can be harnessed to mobilize trapped fluid from a porous medium.

Figure 1

Colloidal deposition promotes oil mobilization. (a) Schematic of the porous medium composed of glass beads packed in a capillary tube. Fluid flow is imposed along $+x$ and micrographs are taken at a fixed depth $z$ within the medium. (b) Magnified micrograph showing trapped oil before particles are injected. Fluorescent signal from the dyed wetting fluid appears as white; hence, white indicates pore space, black circles show the beads, and additional black shows a trapped oil ganglion. Scale bar represents 150 $\mu \mathrm{m}$. (c) Black points show the variation of the residual oil saturation $S_{OR}$, normalized by its maximal value 0.75, over the course of the experiment. Injecting particle-free wetting fluid results in an initial slight decrease in $S_{OR}$. We then inject the particle suspension at the time indicted by the arrow, which results in a further decrease in $S_{OR}$. Gray points show the increase in the solid fraction $1-\varphi \left(t\right)$ over time due to deposition measured from the micrographs. Error bars show differences in the calculated solid area fraction for $\pm 20\%$ of the image binarization threshold. Dashed line shows our theoretical prediction of $S_{OR}$, and the red region shows uncertainty in the prediction arising from heterogeneities in deposition as described in Fig. 4. (d) and (e) Images of the porous medium before and after colloidal injection, respectively, as indicated in (c), showing that particle deposition—indicated by red—mobilizes trapped oil. Scale bars represent 750 $\mu \mathrm{m}$. Imposed flow direction is from left to right.

Figure 2

Oil mobilization does not occur due to colloidal surface activity. Magnified views of confocal micrographs show an oil ganglion (a) before, (b) during, and (c) after mobilization. White regions show pore space, black circles show the bead matrix, additional black regions show oil, with the additional red showing colloidal particles. Some particles appear inside the ganglion due to the non-zero optical slice thickness. Particles are deposited on the bead surfaces, but do not localize at the ganglion surface, indicating that they are not surface active. Blue regions in (b) and (c) show portions of the ganglion that are mobilized between (a)–(b) and (b)–(c). Scale bars represent 250 $\mu \mathrm{m}$. Imposed flow direction is from left to right.

Figure 3

Oil mobilization does not occur due to selective clogging of pores driving flow redirection. Magnified views of confocal micrographs show an oil ganglion (a) before, (b) during, and (c) after mobilization. White regions show pore space, black circles show the bead matrix, additional black regions show oil, with the additional red showing colloidal particles. Some particles appear inside the ganglion due to the nonzero optical slice thickness. The appearance of glass beads that are not completely black and have a shaded outline is a result of imperfect index matching between the wetting fluid and the beads. Particles are uniformly deposited on the bead surfaces both along and transverse to the ganglion; we do not observe selective clogging of pores. Blue regions in (b) and (c) show portions of the ganglion that are mobilized between (a)–(b) and (b)–(c). Scale bars represent 250 $\mu \mathrm{m}$. Imposed flow direction is from left to right. (d) Data showing the fraction of the pore space immediately transverse to the ganglion that is occupied by deposited particles, measured using the confocal micrographs at three different times. Gray points correspond to (a), maroon points correspond to (b), and red points correspond to the end of the experiment, shown in Fig. 1. In all cases, the deposition is uniform across pores.

Figure 4

Permeability reduction due to colloidal deposition promotes oil mobilization. (a) Magnified view of a confocal micrograph of colloidal deposition. White regions show pore space, black circles show the bead matrix, additional black regions show trapped oil ganglia, with the additional red showing deposited colloidal particles. Small black circles in the beads show the particle-free contacts between adjacent beads. Scale bar represents 250 $\mu \mathrm{m}$. Imposed flow direction is from left to right. Deposition is nearly uniform over each bead surface. (b) Schematic of a pore throat of radius $a_t$, showing a uniform layer of deposited particles of thickness $\varepsilon$. (c) Schematic of a trapped oil ganglion. The grains that comprise the porous medium are shown black with red outlines to indicate deposition. The pore body and pore throat diameters, $2a_b$ and $2a_t$, are indicated at the upstream and downstream ends of the trapped ganglion at the positions $x_0$ and $x_0+L$ along the flow direction, respectively. We also indicate the wetting fluid flow rate, $Q_w$, with directionality from left to right and a constant viscosity, ${\mu }_w$. (d) Confocal measurements of the maximal length along the flow direction of trapped ganglia, $L_{\max }$, over the course of colloidal injection. The initial point corresponds to Fig. 1. As deposition progresses, increasingly smaller ganglia are mobilized from the medium, indicated by the decrease in the measured $L_{\max }$. Dashed line shows our theoretical prediction of $L_{\max }$ from Eqs. (6) and (7). The red region shows uncertainty in the prediction arising from heterogeneities in deposition, calculated from $\pm$ the standard deviation in the measured $\varphi$ over the medium.

Figure 5

Nonuniform deposition promotes mobilization under constant pressure drop $\mathrm{\Delta }{P}_t$. (top) Fluid pressure normalized by the pressure drop $\mathrm{\Delta }{P}_t$. A ganglion of length $L$ has $\mathrm{\Delta }{P}_v<\mathrm{\Delta }{P}_c$ when particle deposition is spatially uniform at a low fixed $\mathrm{\Delta }{P}_t$ (black). However, a nonuniform deposition profile obtained at the same constant $\mathrm{\Delta }{P}_t$ can result in an increase of $\mathrm{\Delta }{P}_v>\mathrm{\Delta }{P}_c$ (red). (bottom) The corresponding profiles of normalized permeability, $k/k_0$, are shown over the length of the porous medium, $x/\ell$.