Variation of Permeability with Temperature in Fractured Topopah Spring Tuff Samplesby Wunan Lin, Lawrence Livermore Natl Lab, Livermore, United States,
Abstract: Fractures will play an important role in the near-field hydrology of a nuclear-waste package in a mined repository. Our previous studies showed that the water permeability of fractured Topopah Springs tuff samples decreased by more than three orders of magnitude when the sample's temperature (in the case of a sample under a thermal gradient, the temperature in the hottest zone) increased to 150°C at a constant confining pressure of 5 MPa. 6, 7 When the fractured tuff samples were returned to room temperature the water permeability did not recover. We attributed the permeability decrease to smoothing of the asperities on the fracture surfaces, which was caused by the dissolution and redeposition of silica minerals as water flowed through the sample. Water permeability of an intact tuff sample did not change significantly under similar experimental conditions. In this study, a fractured Topopah Springs tuff sample was used to determine the variation of nitrogen (N2) permeability with increasing temperature or water permeability at room temperature at a constant confining pressure of 5 MPa under the following conditions: dry sample, sample saturated with standing water, and steam flowing through the sample. The N2 permeability of a dry fractured tuff sample was independent of temperature. The water permeability measured at room temperature before and after the sample was heated to 150°C with standing water did not change either. On the other hand, flowing steam through the sample at 127°C for about one week decreased the N2 permeability by more than one order of magnitude. Apparently flowing steam and flowing water have a similar effect on smoothing the asperities of the fracture surfaces.
Subject Headings: Water sampling | Temperature effects | Hydraulic fracturing | Water pressure | Water temperature | Radioactive wastes | Case studies | Permeability (soil)
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