Understanding the impact of upscaling THM processes on performance assessment

The major objective of Benchmark Test 2 (BMT2) is to quantitatively examine the reliability of estimates of repository host rock performance, using large-scale performance assessment (PA) models that are developed by upscaling small-scale parameters and processes. These small-scale properties and processes can be investigated based on either discrete-fracture-network (DFN) models or heterogeneous-porous-medium (HPM) models. While most research teams use DFN, we employ fractal-based HPM for upscaling purposes. Comparison of results based on fundamentally different approaches is useful for evaluating and bounding the uncertainties in estimating repository host rock performance. HPM has both advantages and limitations when compared with DFN. DFM is conceptually more appealing because it explicitly describes fractures and the flow and transport processes that occur within them. However, HPM is more consistent with approaches used to derive field measurements of hydraulic properties (such as permeability). These properties are generally determined based on assumptions related to the continuum approach. HPM is also more straightforward in describing spatial-correlation structures of measured hydraulic properties. For example, potential flow features in the Borrowdale Volcanic Group (BVG) were found to show marked spatial clustering (Nirex, 1997), which is expected to result in a long range correlation in measured permeability distributions. This important behavior may not be captured with conventional DFNs, in which random distribution (or similar distributions) of individual fractures is assumed. The usefulness of HPM will be partially demonstrated in this report by a satisfactory description of the short interval testing data using Levy-stable fractals. (Recently, Jackson et al. (2000) also showed that equivalent HPMs could approximately describe flow processes within subgrid fracture networks.) We use Monte Carlo simulations to determine flow and transport parameters at different scales. Since we have used a fractal-based approach supported by field measurements, effective properties will be scale-dependent. Effects of mechanical processes on flow and transport properties will be also considered in the upscaling procedure. Then, large-scale thermal-hydrologic-mechanical (THM) and transport processes will be modeled.