A numerical simulation of coupled multiphase fluid flow, heat transfer, and mechanical deformation was carried out to study coupled thermal-hydrological-mechanical (THM) processes at the Yucca Mountain Drift Scale Test (DST) and for validation of a coupled THM numerical simulator. The ability of the numerical simulator to model relevant coupled THM processes at the DST was evaluated by comparison of numerical results to in situ measurements of temperature, water saturation, displacement, and fracture permeability. Of particular relevance for coupled THM processes are thermally induced rock-mass stress and deformations, with associated changes in fracture aperture and fractured rock permeability. Thermally induced rockmass deformation and accompanying changes in fracture permeability were reasonably well predicted using a continuum elastic model, although some individual measurements of displacement and permeability indicate inelastic mechanical responses. It is concluded that fracture closure/opening caused by a change in thermally induced normal stress across fractures is an important mechanism for changes in intrinsic fracture permeability at the DST, whereas fracture shear dilation appears to be less significant. Observed and predicted maximum permeability changes at the DST are within one order of magnitude. These data are important for bounding model predictions of potential changes in rock-mass permeability at a future repository in Yucca Mountain.
The Yucca Mountain Drift Scale Test (DST) is a multiyear, large-scale, underground heating test conducted by the U.S. Department of Energy at Yucca Mountain, Nevada. The DST is designed to study coupled thermal-hydrological-mechanicalchemical (THMC) processes in unsaturated fractured and welded tuff. The DST evolution has the same processes operating over a similar range of thermal conditions as that of a future Yucca Mountain repository. Therefore, DST data are used to validate models of those processes such that the models can be shown to be useful for modeling the post-closure behavior of the system.
Pre-test predictions of coupled thermal-hydrological (TH) and thermal-mechanical (TM) processes at the DST and validation of TH and TM models have previously been conducted as part of the Yucca Mountain site characterization project. These predictions included three-dimensional simulations of TH processes conducted by the Lawrence Berkeley National Laboratory using the TOUGH2 code [1, 2, 3] and the Lawrence Livermore National Laboratory using the NUFT code [4]. Coupled TM processes have been simulated by the Sandia National Laboratories using the JAS-3D code [5] and by the Lawrence Livermore National Laboratory using the 3-DEC code [6]. However, no fully coupled THM analysis of the DST was performed until recently, when Rutqvist et al. [7, 8] applied a model for the analysis of coupled THM processes under multiphase flow conditions. This paper presents the current results of such a coupled THM analysis of the DST.
Experience from the earlier modeling studies of coupled TH and TM processes at Yucca Mountain has been very valuable for development of the coupled THM model applied in this paper. In the pre-test prediction of coupled TH processes at the DST, Birkholzer and Tsang [1, 2] developed a three-dimensional numerical model based on previous experience in simulating the Yucca Mountain Single Heater Test (SHT) in the same formation. The modeling of the SHT had shown that an overlapping continuum model, is appropriate for modeling fracture and matrix interactions with multiphase, multicomponent fluid flow and heat transfer.
