A Model for Investigating Mechanical Transport in Fracture Networks

Abstract

A technique is presented to determine when anisotropic fracture systems can be modeled as equivalent porous media (continua) for transport. In order to use the continuum approach, one must demonstrate that the fracture system has the same transport behavior as an equivalent porous medium. Hydraulic effective porosity is calculated as the product of specific discharge and mean travel time, divided by linear length of travel. Specific discharge and hydraulic effective porosity are measured in different directions of flow in regions of varying size with constant hydraulic gradients. If the fracture system behaves like an equivalent porous medium, directional flow has the following properties: (1) specific discharge can be predicted from a permeability tensor and (2) hydraulic effective porosity is independent of direction of flow. A numerical model has been developed to simulate mechanical transport under steady flow in a discrete fracture network. The model is used to determine the distribution of travel times from inlet to outlet for fluid traveling in stream tubes. We have examined only systems with parallel fracture sets in which all fractures are long compared to the region under study. These systems satisfy criterion 1 in that flux can be calculated using a porous medium equivalent. However, these systems do not satisfy criterion 2 because hydraulic effective porosity is shown to be directionally dependent. Thus, even though flux can be accurately predicted using porous medium assumptions for some fracture systems, it may not be possible to accurately predict mechanical transport using these same assumptions.

A lattice‐gas and lattice Boltzmann study of mixing at continuous fracture Junctions: Importance of boundary conditions

“…To include contaminant transport in a network model, one must devise an algorithm to redistribute solute at the fracture junctions. Two common methods are: streamline routing, appropriate for higher flow speeds [Endo et al, 1984]; or complete mixing, useful for slow flow [Travis, 1984]. However, it is important to determine the range of Peclet numbers where each algorithm is accurate, and to estimate the extent 6f solute mixingin the transition region between low and high speed flows [Berkowitz et al, 1994;Li, 1995].…”

Tracer Transport in Fractures: Analysis of Field Data Based on a Variable‐Aperture Channel Model

“…A number of approaches for tran-, sport in fractured media have been proposed in the literature (Endo et al, 1984;Robin-... , 1984;Schwartz and Smith, 1988). In this paper we extend and apply a variableaperture channel model (Tsang and Tsang, 1987;Tsang et al, 1988;Tsang and Tsang, 1989) as the basis for the analysis of the Stripa-3D data .…”

Repository seals requirement study

“…Fluid travels much faster along the fractures than through the blocks, and the speed differs along fractures having different apertures (Endo et al 1984).…”