Numerical simulation of geothermal reservoirs is useful and necessary in understanding and evaluating reservoir structure and behavior, designing field development, and predicting performance. Models vary in complexity depending on processes considered, heterogeneity, data availability, and study objectives. They are evaluated using computer codes written and tested to study single and multiphase flow and transport under nonisothermal conditions. Many flow and heat transfer processes modeled in geothermal reservoirs are expected to occur in anthropogenic thermal (AT) systems created by geologic disposal of heat-generating nuclear waste. We examine and compare geothermal systems and the AT system expected at Yucca Mountain, Nevada, and their modeling. Time frames and spatial scales are similar in both systems, but increased precision is necessary for modeling the AT system, because flow through specific repository locations will affect long-term ability radionuclide retention. Geothermal modeling experience has generated a methodology, used in the AT modeling for Yucca Mountain, yielding good predictive results if sufficient reliable data are available and an experienced modeler is involved. Codes used in geothermal and AT modeling have been tested extensively and successfully on a variety of analytical and laboratory problems.
Key WordsNuclear waste disposal, simulation, geothermal, Yucca Mountain, post-audit Kneafsey, Pruess, O'Sullivan, and Bodvarsson 2
IntroductionSubsurface disposal of heat-generating, high-level nuclear waste, such as at the potential repository at Yucca Mountain, Nevada will induce changes in the thermal, hydrologic, and chemical processes that will occur for some distance from the waste for many thousands of years.These changes include water boiling within the rock, flow of the water vapor, condensation of the water vapor, flow of the condensate, mineral dissolution in the condensate, mineral precipitation upon boiling, and hydrothermal flows in the saturated zone. Understanding flow through the mountain in its natural state, during the heating phase, and as the mountain cools is necessary to adequately assess the protection provided by the repository, and to predict the behavior of the repository for a time scale of 10,000 years or more. To this end, investigators have constructed numerical models to study present moisture flow, analyze thermal test data, evaluate the impact of the imposed heat source on the local environment, and assess how both natural and anthropogenic thermal (AT) systems may affect the potential repository Buscheck, 1998;Buscheck and Nitao, 1993;Buscheck and Nitao, 1994;Haukwa et al., 1999;Nitao, 1988;Nitao et al., 1992;Pruess and Tsang, 1994;Pruess et al., 1984b;Pruess et al., 1990a;Pruess et al., 1990b;Tsang and Birkholzer, 1999;Tsang and Pruess, 1987;Tsang and Pruess, 1989;Tsang and Pruess, 1990;Wu et al., 1999).In this paper, we critically examine fluid-and heat-flow modeling in geothermal systems and the potential nuclear waste disposal in the unsaturat...