Estimating Liquid Fluxes in Thermally Perturbed Fractured Rock Using Measured Temperature Profiles

Abstract

04865686, jtbirkholzer@lbl.gov Abstract: A new temperature-profile method was recently developed for analyzing perturbed flow conditions in superheated porous media. The method uses high-resolution temperature data to estimate the magnitude of the heat-driven liquid and gas fluxes that form as a result of boiling, condensation, and recirculation of pore water. In this paper, we evaluate the applicability of this new method to the more complex flow behavior in fractured formations with porous rock matrix. In such formations, with their intrinsic heterogeneity, the porous but low-permeable matrix provides most of the mass and heat storage capacity, and dominates conductive heat transfer, Fractures, on the other hand, offer highly effective conduits for gas and liquid flow, thereby generating significant convective heat transfer. After establishing the accuracy of the temperature-profile method for fractured porous formations, we apply the method in analyzing the perturbed flow conditions in a large-scale underground heater test conducted in unsaturated fractured porous tuff. The flux estimates for this test indicate a signifcant reflux of water near the heat source, on the order of a few hundred millimeter per year-much larger than the ambient percolation flux of only a few millimeter per year.Key Words: heat, flow, fractured rock, temperature profile, thermal perturbation -1 -REV00 IntroductionEvaluating the magnitude of flux perturbation in superheated subsurface systems can be a challenging task, in part because the direct in situ measurement of such quantities is virtually impossible. Flux perturbations are particularly strong in geologic heat pipes, where vapor is transported away from the heat source while condensate water flows back towards the heat source, thereby creating a continuous recirculation of vapor and water at significant rates (e.g., Udell, 1985;Doughty and Pruess, 1990, 1992). Examples of geologic heat pipes can be found in geothermal systems, near emplacement tunnels for the disposal of heat-producing nuclear wastes, in the vicinity of buried pipelines and electrical cables, in post-accident sites with boiling of fluids from nuclear reactor debris, and in oil fields as a result of thermally enhanced recovery (Udell, 1985).Of specific concern in t h s paper is the expected flux perturbation in the vicinity of the geologic repository for nuclear waste at Yucca Mountain, Nevada (Pruess et al., 1990a(Pruess et al., , 1990b, where the decaying radioactive material produces significant amounts of heat.Determining the two-phase flow conditions in the fractured porous rock at YuccaMountain is relevant to the performance of this repository, because these conditions affect the temperature and relative humidity close to the waste packages-important parameters for their corrosion. Large-scale heater tests have been conducted in underground research tunnels at Yucca Mountain to assess the future repository's response to the decay heat and to determine the impact of thermal perturbation on liquid and g...