Pore-scale investigation of colloid deposition, changing pore geometry, fluid flow, and solute transport in porous media.

摘要

The overall objective of this thesis is to develop a package of high-resolution, non-invasive experimental and numerical approaches to observe in situ pore-scale geometry modification due to colloid deposition, and quantitatively predict its effects on the large-scale transport properties of the porous medium. High-energy synchrotron-based X-ray difference micro-tomography (XDMT) was used to obtain the data of pore structure and deposited colloidal ZrO2 particles simultaneously in a single measurement. Tomographic reconstructions of the porous medium and colloidal ZrO2 deposits show that the local pore geometry controls particle deposition and that deposits tend to form in a rather heterogeneous manner at the pore scale. By using a different absorption edge, the distribution of Arsenic (As) in natural porous matrix was observed.;XDMT was combined with lattice Boltzmann (LB) simulations to assess changes in pore-scale flow fields, solute transport, and dispersion behavior resulting from colloidal deposition in a granular porous medium. The detailed structural information obtained from XDMT was used to define internal boundary conditions for simulations of pore water flow and solute transport both with and without colloidal deposits. As colloids accumulated in the pore space, the permeability decreased, the mean tortuosity increased, and the tortuosity distribution became multi-modal. The colloidal deposits also increased the spatial variation in pore velocities, leading to higher dispersion coefficients.;XDMT and LB simulations were applied to study sediment mixing and colloid deposition in streambeds. It was found out that sediment mixing started near the streambed surface, and gradually developed downward. Most ZrO2 particles entered the streambed with stream-subsurface exchange, and deposited on the top layers of the bed. Pore-scale visualization showed that over the upstream side of the bed form, most particles deposited on the top sides of beads. LB simulations demonstrated that the distribution of local permeability was highly heterogeneous in the bed. These related the pore-scale research to the larger scale, and are valuable in investigating the complicated interplay of sedimentation, colloid deposition, pore water flow, solute transport, and stream-subsurface interaction.