Temporal evolution of pore geometry, fluid flow, and solute transport resulting from colloid deposition

Deposition of colloidal particles is one of many processes that lead to the evolution of the structure of natural porous media in groundwater aquifers, oil reservoirs, and sediment beds. Understanding of the mechanisms and effects of this type of structural evolution has been limited by a lack of direct observations of pore structure. Here, synchrotron X-ray difference microtomography (XDMT) was used to resolve the temporal evolution of pore structure and the distribution of colloidal deposits within a granular porous medium. Column filtration experiments were performed to observe the deposition of relatively high concentrations of colloidal zirconia (200 mg/l of particles having diameter similar to 1 mu m) in a packed bed of glass beads (diameters 210-300 mu m). Noninvasive XDMT imaging of the pore structure was performed three separate times during each column experiment. The structural information observed at each time was used to define internal boundary conditions for three-dimensional lattice Boltzmann (LB) simulations that show how the evolving pore structure affects pore fluid flow and solute transport. While the total deposit mass increased continuously over time, colloid deposition was observed to be highly heterogeneous and local colloid detachment was observed at some locations in a low ionic strength medium. LB simulations indicated that particle accumulation greatly reduced the permeability of the porous medium while increasing the tortuosity. The colloidal deposits also increased the spatial variability in pore water velocities, leading to higher dispersion coefficients. Anomalous dispersion behavior was investigated by simulation at the scale of the experimental system: weak tailing was found in the clean bed case, and the extent of tailing greatly increased following colloid deposition because of the development of extensive no-flow regions. As a result of this coupling between pore fluid flow, colloid accumulation, and the pore geometry, colloid deposition is expected to strongly influence long-term solute dynamics in cases where solute transport is either accompanied by high colloid influx or where the passage of the solute front mobilizes and then redistributes material from the porous matrix.