Research Papers

Dual-Permeability Modeling of Capillary Diversion and Drift Shadow Effects in Unsaturated Fractured Rock

[+] Author and Article Information
Clifford K. Ho

 Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-1127ckho@sandia.gov

Bill W. Arnold, Susan J. Altman

 Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-1127

J. Heat Transfer 131(10), 101012 (Jul 31, 2009) (6 pages) doi:10.1115/1.3180700 History: Received September 29, 2008; Revised April 01, 2009; Published July 31, 2009

The drift-shadow effect describes capillary diversion of water flow around a drift or cavity in porous or fractured rock, resulting in lower water flux directly beneath the cavity. This paper presents computational simulations of drift-shadow experiments using dual-permeability models, similar to the models used for performance assessment analyses of flow and seepage in unsaturated fractured tuff at Yucca Mountain. Comparisons were made between the simulations and experimental data from small-scale drift-shadow tests. Results showed that the dual-permeability models captured the salient trends and behavior observed in the experiments, but constitutive relations (e.g., fracture capillary-pressure curves) can significantly affect the simulated results. Lower water flux beneath the drift was observed in both the simulations and tests, and fingerlike flow patterns were seen to exist with lower simulated capillary pressures. The dual-permeability models used in this analysis were capable of simulating these processes. However, features such as irregularities along the top of the drift (e.g., from roof collapse) and heterogeneities in the fracture network may reduce the impact of capillary diversion and drift shadow. An evaluation of different meshes showed that at the grid refinement used, a comparison between orthogonal and unstructured meshes did not result in large differences.

Copyright © 2009 by American Institute of Physics
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Figure 5

Distribution of normalized outflow below the drift for a fracture aperture of 500 μm, using a linear fracture capillary-pressure curve with a maximum capillary pressure of 200 Pa (DKM=dual-permeability model)

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Figure 4

Simulated fracture saturation using TOUGH2 with a 500 μm aperture and 0.23 ml/min flow rate using van Genuchten capillary-pressure curves: (a) unbounded maximum pressure (1010 Pa), (b) linear capillary-pressure curve with maximum capillary pressure of 200 Pa, and (c) linear capillary-pressure curve with maximum capillary pressure of 30 Pa

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Figure 3

Computational meshes used in the simulations. (a) Orthogonal mesh used in TOUGH2 simulations and (b) unstructured Voronoi mesh used in FEHM simulations.

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Figure 2

X-ray absorption images of the 500 μm aperture test cell taken (a) before and (b) 5 h after the start of experiment at 0.01 ml/min, and (c) 1 h, (d) 2 h, (e) 3 h, and (f) 5 h after start of experiment with 0.23 ml/min flow rate. Image of cell without tracer (a) shows porous pumice fragments as darker areas. From Ref. 1 (with permission from Elsevier).

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Figure 1

Schematic of (a) tuff slabs and (b) test cell used in Ref. 1 (with permission from Elsevier)



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