HR: 1340h
AN: H43F-0418    [Abstracts]
TI: Multiple approaches to studying pore connectivity in rock matrices
AU: * Hu, Q
EM: hu7@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550 United States
AU: Ewing, R P
EM: ewing@iastate.edu
AF: Iowa State University, 2101 Agronomy Hall, Ames, IA 50011 United States
AB: The current conceptual model of contaminant transport in unsaturated fractured rock has water moving through fractures, with migration of the entrained contaminants being retarded by diffusion into and within the rock matrix. The diffusion coefficient is implicitly assumed to have a constant, scale-invariant value. However, rocks whose pores are poorly interconnected are known to have anomalous transport properties that strongly impact long-term net diffusion. For example, rocks with low pore connectivity have a cross-over length below which diffusivity decreases with increasing distance, and above which diffusivity appears constant but with a diffusion coefficient less than that measured on samples smaller than the cross-over length. In order to examine the extent of pore connectivity in a variety of rock types from around the world and further investigate the cross-over lengths of several selected rocks, we developed several complementary experimental techniques: (1) imbibition tests wherein the water uptake was monitored over time, with the slope of log imbibed water mass versus log time indicative of the pore connectivity of the rock matrix; (2) tracer saturation tests by vacuum-saturating rock samples with a tracer solution, followed by micro-scale mapping to obtain tracer distribution with depth, which is related to pore connectivity; (3) gas diffusion experiments to measure diffusion coefficients for a rock at several different sample thicknesses to examine scaling effect; and (4) synchrotron microtomography to analyze the iso-concentration surfaces following tracer diffusion into a rock matrix. These experiments were also modeled using random walk methods on a 3-D lattice with different values of pore coordination. The results indicate that porespace in many rocks is close to the percolation threshold. Use of standard diffusion models for rock matrices with low pore connectivity can yield incorrectly calculated diffusivity values from measured data, and predict incorrect matrix diffusion and retardation coefficients for contaminants in fractured rock.
DE: 1719 Hydrology
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting