Visualization of colloid transport through heterogeneous porous media using magnetic resonance imaging

The effects of heterogeneous grain packing on colloid transport were evaluated in flow-through columns using magnetic resonance imaging (MRI). Two columns were packed, each with a core of fine-grained silica gel surrounded by a shell of coarse-grained silica gel. In column 1, 600-850 mu m silica gel was surrounded by 850-1000 mu m silica gel. In column 2, 250-600 mu m silica gel was surrounded by 850-1000 mu m silica gel. Both columns were continuously purged with water and colloids were introduced as pulses. MRI images of column I showed that colloid transport in the core and shell was not distinguishable. However, colloid transport was slightly faster along the bottom of the column. T-1 -weighted images showed that small variations in the packing density of silica gel caused this effect. MRI images of column 2 showed that colloid transport in the core was much slower than colloid transport in the shell. Colloid exchange between the shell and the core was also observed. Colloid transport velocities and collision efficiencies were calculated from the images. In agreement with the visualization, velocities for column I increased from the top to bottom of the column and velocities for column 2 were greater in the shell than in the core. Collision efficiencies were calculated, but trends were not apparent because of the difficulty of applying filtration theory to heterogeneous media. Velocities from images were compared to those from conventional experiments where colloid concentrations were measured at the column effluent. While often comparable, results from the latter mask many of the complexities that control the overall rate of colloid transport. Since these complexities can give rise to very different transport behavior, it is critical to understand their effects in real systems. Hence, MRI is a technique that has the power to elucidate many of the small-scale processes that affect the behavior of colloids in the field. (c) 2005 Elsevier B.V. All rights reserved.