ABSTRACT:

Changes in applied stress affect the hydraulic and mechanical properties of fractures. This paper discusses the determination of fracture aperture geometry and the performance of hydromechanical experiments in which the cores containing three different fractures were subjected to confining pressure cycles while simultaneously measuring fracture flow. The results show the expected dependence of conductivity and transmissivity on confining pressure. In addition, interesting differences depending on the fracture type are observed and permanent changes in hydraulic characteristics can be associated with changes in aperture geometry. The validity of the parallel plate flow assumption is also discussed.

RESUME:

Les contraintes appliquees influencent les proprietes hydrauliques et mecaniques des fissures. Dans cet article on discute d'abord Ia determination de Ia geometrie de I'ouverture des fissures. Apres ceci des essais hydromecaniques sont decrits dans lesquelles des echantillons cylindriques contenants les trois fissures differentes sont soumis à des pressions uniformes accroissantes et decroissantes avec mesure simultanee de l' ecoulement dans Ia fissure. Les resultats demontrent I'influence expectee de la pression sur Ia permeabilite et Ia transmissivite des fissures. En plus on peut observer des differences interessantes qui dependent du type de Ia fissure, et les changements permanents des proprietes hydrauliques des fissures peuvent être associes avec les changements de Ia geometrie des ouvertures des fissures. La validite de Ia relation de Poiseuille est aussi discutee.

ZUSAMMENFASSUNG:

Die hydraulischen und mechanischen Eigenschaften von Klueften werden durch aufgebrachte Spannungen verandert. Dieser Artikel beschreibt die Bestimmung der Klueftöffnungsgeometrie und hydromechanische Versuche, in denen zylindrischen Kerne mit drei verschiedenenen Klueften einem zu- und abnehmenden allseitigen Druck unterworfen werden, wahrend der Kluftdurchfluss gemessen wird. Die Versuche ergeben den erwarteten Einfluss des Aussendruckes auf die Kluftdurchlassigkeit- und Transmissivitat. Zusatzlich können interessante Unterschiede, die von den Klufitypen abhangen, beobachtet werden, und die permanenten Veranderungen der hydraulischen Klufteigenschaften können mit entsprechenden Änderungen in der Kluftàffnungsgeometrie in Zusammenhang gebracht werden. Die Gueltigkeit der Fliessgesetzes fuer Parallelströmung wird auch diskutiert.

1 INTRODUCTION

In most problems involving flow in fractured rocks, rock fractures are subject to some level of stress. When a fracture is stressed, the fracture void space deforms and changes in contact area occur. These changes significantly affect the hydraulic and mechanical properties of a fracture. Consequently, many researchers have investigated the stress-deformation-flow characteristics of fractures subject to different states of stress (Jouanna,1972; Sharp and Maini,I972; Iones,I97S; Pratt et al.,1977). Iwai (1976) studied flow through a fracture as a function of deformation, contact area and stresses up to 20 MPa with the specific intention to check the validity of the "cubic law" (poiseuille relation) for parallel plate flow in rock fractures. The results of his laboratory investigation on tension fractures that were artificially induced in homogeneous specimens of granite, basalt, and marble showed that the cubic law was applicable whether the fractures were open or closed and whether the flow path geometry was straight or radial. This may, however, not be correct as suggested in other studies (Kranz et al, 1979 and Engelder and Scholz 1981) particularly for rough-walled fractures. Gale(1982) showed that the degree of deviation from the cubic law or parallel plate model depends on the choice of a reference point when determining fracture apertures. Raven and Gale (1985) found that their experimental results did not follow the cubic law and that the deviation increased with the number of loading cycles and specimen size. They suggested that these deviations are caused by the stress induced change of contact area in the fracture. Existing experimental work does thus not provide a consistent answer regarding the applicability of the cubic law and the effect of contact area on flow in a fracture.

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