a. Department of Geoscience and Technology, Tohoku University, Aoba-ku, Sendai 980-8579, Japan

The permeability of a fracture may decrease with time due to time-dependent closure under in-situ normal stress. Estimation of the long-term permeability of a fracture is necessary for designing facilities for nuclear waste disposal and geothermal energy extraction. In this study, by using specimens of a hollow cylinder, which contained a hydraulic fracture perpendicular to the axis, the changes with time in the closure and permeability of the fracture under constant normal stress were measured for normal stresses of up to 25 MPa for granite and up to 15 MPa for sandstone. The power spectral density of the aperture showed that the fractures used in this study had the surfacestrongly correlated to each other. Main results obtained in this study were summarized as follows:
1) The time-dependent closure increased to be constant at 50 to 70 hours for the 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 sandstone. The difference in the elastic closure curve between the two rocks was due to that in the increase rate of the contact area, which was related to the initial part of the cumulative probability distribution function of the aperture.
2) The hydraulic aperture decreased with time to be constant at about 50 hours for the both rocks. The hydraulic aperture was very small, 2.2 to 3.5 mm after the elastic closure and was finally 1.3 to 1.8 mm. The larger the initial hydraulic aperture was, the more the hydraulic aperture decreased with time. The ultimate hydraulic aperture was 40 to 75 % of the initial value.
3) The hydraulic aperture did not decrease according to the Darcy's law with the time-dependent closure for the both rocks. The decrease in the hydraulic aperture was small in comparison with the increase in the time-dependent closure. This was because the fractures used in this study were all well mated and, accordingly, the contact between the two fracture surfaces was developed well.