HR: 1340h
AN: H33A-1378    [Abstracts]
TI: Colloid Traps:Microscale Modeling of Solid Transport Through a Single Rock Fracture
AU: * Grasselli, G
EM: ggrasselli@mirarco.org
AF: SNL, p.o box 5800, Albuquerque, nm 87185 United States
AU: Boutt, D F
EM: dboutt@geo.umass.edu
AF: SNL, p.o box 5800, Albuquerque, nm 87185 United States
AU: Fredrich, J T
EM: fredrich@sandia.gov
AF: SNL, p.o box 5800, Albuquerque, nm 87185 United States
AU: Cook, B K
EM: bkcook@sandia.gov
AF: SNL, p.o box 5800, Albuquerque, nm 87185 United States
AU: Williams, J R
EM: jrw@mit.edu
AF: MIT, 77 Mass.Ave, Cambridge, MA 02139 United States
AB: In fractured rock formations with low matrix permeability, fluid flow and transport are often dominated by discontinuities. Understanding the mechanisms that control flow in a single fracture is necessary for evaluating complex phenomena such as transport, precipitation, and dissolution processes in fractured media. However, the effect of micron-scale geometrical characteristics of fractures, e.g. local aperture and roughness, on fluid flow and transport is not yet well understood. In this paper we present results of an investigation designed to provide insight into the effect of micro-scale roughness on fluid flow and colloid transport in a single fracture. We created a fracture aperture using one side of a natural fracture in tuff, with the opposite wall of the fracture being created by a transparent glass plate. Laser scanning confocal microscopy was used to measure directly the aperture of the fracture. In situ flow experiments were then performed using a miniaturized flow cell mounted on the microscope stage, with fluorescent microspheres to simulate the behavior of colloids, with particle-image velocimetry technique used to track the particles. The experimental data reveal clearly local velocity perturbations induced by the roughness of the opposing fracture wall. The experimental aperture data were used as input to a 2D coupled lattice-Boltzmann discrete element method (LBDEM) simulator. Since the numerical code is only 2D, a slice parallel to fluid pressure gradient in the system was used to explore the effects of the fracture roughness on transport of the microspheres. Transport of microspheres in the numerical model is strongly influenced by the macroscopic parabolic velocity profiles in the fracture, with the bulk of microspheres transported according to the parabolic velocity profile of the fracture. However, undulations in the fracture increase the areal extent of low fluid velocity zones that generally retard the movement of microspheres resulting in scattered trapping zones along the fracture. On a macroscopic scale this results in significant tailing of the microsphere breakthrough curve. We also find that anisotropy in the surface roughness results in corresponding anisotropy in the breakthrough behaviour for the two opposite flow directions.
DE: 1831 Groundwater quality
DE: 1847 Modeling
DE: 1894 Instruments and techniques: modeling
DE: 5112 Microstructure
SC: Hydrology [H]
MN: Fall Meeting 2005