Role of Low Flow and Backward Flow Zones on Colloid Transport in Pore Structures Derived from Real Porous Media

To examine the relevance of low flow zones and flow vortices to colloid transport in real porous media, lattice-Boltzmann (LB) simulations were combined with X-ray microtomography (XMT) to simulate flow fields in glass beads and quartz sand. Backward flow zones were demonstrated to be widely present in both porous media, with a greater volume fraction in the former relative to the latter porous media. Glass beads in the XMT images were approximated as spheres and their coordinates and radii were extracted to allow reconstruction of pore structures. LB simulations were again performed and the simulated flow fields in the reconstructed pore structures were coupled to a three-dimensional particle tracking algorithm. Particle tracking simulations demonstrated that significant amounts of colloids stayed in the simulated domains for long periods (up to 50 pore volumes). The percentages of colloids with long residence time increased as the depth of the secondary energy minimum increased. The majority of the colloids with long residence time were translated to low flow zones while being associated with grain surfaces via secondary minima. A small fraction of colloids entered low flow zones without being associated with the grains surfaces. Backward flow zones were also found to trap a small fraction of colloids for significantly long time (up to 10 pore volumes). In overall, however, backward flow zones trapped fewer colloids for shorter durations than low flow zones. In summary, this work demonstrates the importance of temporary trapping of colloids by the low flow and backward flow zones in real porous media. This trapping process can explain a number of intriguing experimental observations.