ABSTRACT:

Conceptual models for fluid flow in fractured geologic formations include, in order of increasing complexity, the equivalent porous media, the dual-permeability and the discrete fracture approach. Currently, no simple criteria exist to determine the suitability of one model for a particular geological system. The objective of this paper is to link in situ block size distribution and the hydraulic behaviour of a rock mass, by integrating the information from in situ measurements into a three-dimensional numerical flow model that can be based on any of the above mentioned conceptual models.

RESUME:

Les differents modèles conceptuels developpes pour I'ecoulement dans des massifs fractures sont, en ordre de complexite croissante, Ie milieu poreux equivalent, Ie modèle de double-porosite et l'approche a fracturation discrète. Cependant, il n'existe presentement pas de critère permettant d'evaluer I'applicabilite de chaque modèle. L'objectif de cet article est d'etablir un lien entre la blocometrie et Ie comportement hydraulique de massifs rocheux, en integrant les mesures in situ à un modèle tridimensionel d'ecoulement en milieu fracture base sur l'un ou I'autre des modèles conceptuels enumeres,

ZUSAMMENFASSUNG:

Die Strömungsmodellierung in klueftigen Gesteinen erfordert die Kenntniss der hydraulischen Eigenschaften des Felsmaterials. Die konzeptionellen Modelle zur Beschreibung klueftiger Medien reichen (in der Reihenfolge zunehmender Komplexitat) von Ansatzen mit aquivalenten porösen Medien ueber Zweibereichs-Durchlassigkeitsmodelle bis zu diskreten Kluft-Modellen. Zur Zeit gibt es noch kein einfaches kriterium zur Bestimmung der Eignung eines Modells fuer ein bestimmtes System. Ziel dieser Veröffentlichung ist es, eine Verbindung zwischen der in situ Grössenverteilung einzelner Blöcke und den hydraulischen felseigenschaften herzustellen. Dazu wird die Information aus den in situ Messungen in ein numerisches Modell eingearbeitet. Dieses numerische Modell kann auf einem der zuvor genannten konzeptionellen Modellen basieren.

1 INTRODUCTION

Modelling of fluid flow in fractured rock requires the determination of the hydraulic properties of jointed rack masses. Conceptual models of varying degrees of complexity have been developed to represent the fractured material. The simplest representation is the continuum approach, where the fractured rock is represented by an equivalent anisotropic porous medium. Average properties representing the combined fracture/rock matrix system are derived for a REV or representative elementary volume (Bear, 1972). This approach has been used by, among others, Long et al. (1982) and Schwartz and Smith (1988) to study groundwater flow in fractured geologic formations. Previous work has shown that it is not always possible to define a REV for all fractured rock masses. Furthermore, the scale over which the REV is defined might be too large to measure with any precision the hydraulic properties of the system (Neuman, 1987). However, when a suitable REV can be defined, the continuum approach is useful as a first step in the investigation of the general hydraulic behaviour of fractured rock. It can help in developing a conceptual understanding of the system and orient more detailed local studies. A practical advantage of continuum analysis is that it can be undertaken with existing numerical models for porous media flow. A potentially more realistic representation of flow can be obtained by employing the dual-permeability (or dual-porosity) approach, where two continua are defined, Barenblatt et al. (1960). All flow is assumed to occur in the more permeable fractures and the rock matrix represents a storage reservoir for fluid. Fluid exchange terms between the two systems are derived from fracture geometry. Finally, the most complex conceptual model of fluid flow is based on a discrete fracture approach where each fracture is modelled individually. This requires explicit definition of the precise location and hydraulic properties of all fractures. A common assumption consists in treating fractures as parallel plates with hydraulic conductivity given by the cubic law. In reality, however, fractures may exhibit flow along preferential channels resulting from surface roughness. Applications of models using a discrete fracture representation can be found in Schwartz et al. (1983), Cacas et al. (1990) and Therrien and Sudicky (1996).

This content is only available via PDF.
You can access this article if you purchase or spend a download.