Time-dependent closure and permeability of a small-scale hydraulic fracture created in granite and sandstone were measured under constant normal stress. The time-dependent closure increased to be constant at 50 to 70 hours for both rocks. The ultimate time-dependent closure did not depend on the normal stress when the elastic closure curve obeyed the Goodman's formula as for granite, while it increased with the normal stress when the elastic closure curve did not obey the Goodman's formula as for sand- stone. The hydraulic aperture decreased with time to be constant at about 50 hours for both rocks. The hydraulic aperture produced by the elastic closure was very small (2.2 to 3.5 um), and was finally 1.3 to 1.8um. The larger the initial hydraulic aperture was, the more the hydraulic aperture decreased with time, regardless of the normal stress.The decrease in the hydraulic aperture was small in comparison with the increase in the time-dependent closure for both rocks. This was because the fractures used in this study were all well mated and, accordingly, the contact area was developed well.
There have been many researches on the mechanical properties and fluid permeability of fractures such as joints in a rock mass (Kranz et al. 1979, Walsh 1981, Raven & Gale 1985, Witherspoon et al. 1980, Brown 1987, Kojima et al. 1995), since fracture systems affect the stability of engineered rock structures and excavations and also control the dispersion of chemical contaminants into and through the subsurface. The physical properties of fracture systems may be more complex than a simple linear superposition of the properties of single fractures. Nevertheless, the properties of a single fracture provide the basis for understanding and estimating the properties of fracture systems. A fracture is closed by the compressive normal stress. However, only a part of the fracture surfaces is in contact and void spaces or apertures provide paths for fluid flow. The aperture distributions in a fracture are governed by the rock stresses, the mechanical properties of rock, and the topography of the fracture surfaces (e.g. U. S. National Committee for Rock Mechanics 1996). A natural tensile fracture is a ‘mated’ fracture. The two surfaces are more or less related to each other at long wave- lengths (Brown et al. 1986, Matsuki et al. 1995, Glover et al. 1998). Accordingly, the characteristics of a fracture aperture cannot be estimated from those of the two fracture surfaces because the aperture distribution depends on the degree of correlation between the height distributions of the fracture surfaces. A fracture under constant normal stress may close time dependently since rock is more or less viscoelastic (Jaeger and Cook 1965). Accordingly, fluid flow through a fracture may decrease with time. Prediction of the long-term closure and permeability of a fracture is necessary for designing engineered rock structures, particularly in the fields of underground nuclear waste disposal and geothermal energy extraction. However, there have been very few investigations of the time-dependent closure and permeability of a fracture with rough surfaces.
