An Integrated Modeling Approach for Characterizing Multiphase Flow, Chemical Transport, and Heat Transfer in Fractured Reservoirs

Even with the significant progress made in modeling flow and transport in fractured rock in the last few decades, characterizing fractured reservoirs remains a great challenge. This is because of the inherent complexity within actual fractured reservoirs, from multiphase flow and transport to the spatial variability of the fracture network. To investigate coupled processes of fluid and heat flow and chemical isotopic transport in the highly heterogeneous, unsaturated fractured tuff of Yucca Mountain (a proposed underground repository site for storing U.S. high-level radioactive waste), we present an integrated modeling methodology. The proposed modeling approach integrates a wide variety of moisture, pneumatic, thermal, and geochemical isotopic field data into a comprehensive three-dimensional numerical model for modeling analyses. In particular, the results of this evaluation indicate that moisture data, such as water potential and liquid saturation in the rock matrix, are not sufficient to determine in situ percolation flux, whereas temperature and geochemical isotopic data provide better constraints to net infiltration rates and flow patterns. In addition, pneumatic data are found to be extremely valuable in estimating large-scale fracture permeability. The integration of hydrologic, pneumatic, temperature, and geochemical data into modeling analyses is thereby demonstrated to provide a practical modeling approach for characterizing flow and transport processes in complex fractured reservoirs. Introduction The unsaturated zone (UZ) of the highly heterogeneous, fractured tuff at Yucca Mountain, Nevada, U.S.A, has been investigated as a possible repository site for storing high-level radioactive waste since the 1980s. Characterization of flow and transport processes in the fractured rock of the Yucca Mountain UZ has received significant attention and has made significant progress over the last two decades. During the same period of extensive studies, many types of data have been collected from the Yucca Mountain UZ, and these data have helped in developing a conceptual understanding of various physical processes within the UZ system. Even with the progress made so far in data analysis and flow and transport modeling in fractured rock, characterizing fractured reservoirs remains a great challenge, because of the inherent complexity within heterogeneous fractured reservoirs, from multiphase flow and transport to the spatial variability of the fracture network. In this work, we present an integrated modeling methodology of calibrating a UZ flow model for characterizing percolation flux in unsaturated fractured rock. Such integration of multiple processes provides a better understanding of percolation patterns and flow behavior within the Yucca Mountain UZ under different climates and hydrogeological conceptualizations of UZ flow. First, we discuss how to integrate different field-observed data, such as water potential, liquid saturation, perched water, gas pressure, chloride, and temperature logs, into a single 3-D UZ flow and transport model. The combined model calibration will provide a consistent cross-check or verification of modeled percolation fluxes, as well as better insight into UZ flow patterns. Second, we show that this integrated modeling effort provides consistent model predictions for different but interrelated hydrological, pneumatic, geochemical, and geothermal processes in the UZ. As a result, such an integrated approach will generally improve the capability and credibility of numerical models in characterizing flow and transport processes in unsaturated fractured formations. This paper represents our continuing effort in developing and applying the UZ flow and transport models to characterizing the UZ system of Yucca Mountain. Specifically, this modeling study consists of (1) a brief UZ model description and (2) model calibration. Hydrogeology and Conceptual Model The domain of the UZ model encompasses approximately 40 km of the Yucca Mountain area, as shown in Figure 1. The UZ is between 500 and 700 m thick and overlies a relatively flat water table. The repository would be located in the highly fractured Topopah Spring welded tuff unit, more than 200 m above the water table. Geologically, Yucca Mountain is a structurally complex system of Tertiary volcanic rock. SPE 106996 An Integrated Modeling Approach for Characterizing Multiphase Flow, Chemical Transport, and Heat Transfer in Fractured Reservoirs Yu-Shu Wu, SPE, Guoping Lu, Keni Zhang, and G.S. Bodvarsson, Lawrence Berkeley Natl. Laboratory