Geology Reference
In-Depth Information
THE DISCRETE FRACTURE NETWORK APPROACH
Conceptual Model
Discrete fracture network approaches have been developed in the past few
years to handle the question of fluid and mass transport in fractured
heterogeneous systems where discontinuities are likely to exist at many
scales. Various conceptual approaches were proposed to describe the geometry
of network of discontinuities, and capture the uneven nature of flow and
solute transfer within a single fracture. 2D flow as well as channel models
in random or structured network of planar fractures were investigated and
tested against a variety of in situ experiments. Different softwares now exist
with most of these capabilities (i.e. NAPSAC developed by AEA Tech in
UK, FRACMAN package, developed by Goldberg ass.). The FRACAS
software used hereafter is a similar product, gradually developed at the Paris
School of Mines by Cacas et al. (1990) and Bruel et al. (1994), in the
framework of National and European research programmes dedicated to
nuclear waste insulation, to geothermal projects in hard rocks (Bruel, 2002),
and to the estimation of hydrogeological properties in the vicinity of mined
areas. Some of the capabilities of the code were recently tested in an
international benchmark exercise (Rejeb and Bruel, 2001).
Geometry
The FRACAS modelling approach is based on the assumption that fluid
moves through a rock mass within a system of interconnected fractures and
that flow in the rock matrix is negligible by comparison. To alleviate the
problems faced in the interpretation of fractured rock geometry the following
are adapted for a better representation and interpretation of the aquifer
characteristics.
The three-dimensional hydraulically conductive network of planar, disc-
shaped fractures is generated within a rectangular block of rock based on
stochastic descriptions of fracture density (Poisson distribution), fracture
orientation (Fisher von Mises distribution), and fracture diameter (log-normal
distribution) for specific fracture sets. Fractures may arrange into five types
of fracture systems for modelling purposes.
1. Pure random network of disk shaped fractures.
2. Fisher type set of disk shaped fractures, that is a directional set.
3. Sub-vertical, with no preferential strike, set of disk shaped fractures.
4. Planar structures of deterministic location, can be infinite or with a finite
elliptical extension, partly glued/non-persistent.
5. For an array of discs, with a fully deterministic description.
