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We describe a joint inversion approach that combines geophysical and thermal-hydrological data for the estimation of (1) thermal-hydrological parameters (such as permeability, porosity, thermal conductivity, and parameters of the capillary pressure and relative permeability functions) that are necessary for predicting the flow of fluids and heat in fractured porous media, and (2) parameters of the petrophysical function that relates water saturation, porosity and temperature to the dielectric constant. The approach incorporates the coupled simulation of nonisothermal multiphase fluid flow and ground-penetrating radar (GPR) travel times within an optimization framework. We discuss application of the approach to a large-scale in situ heater test which was conducted at Yucca Mountain, Nevada, to better understand the coupled thermal, hydrological, mechanical, and chemical processes that may occur in the fractured rock mass around a geologic repository for high-level radioactive waste. We provide a description of the time-lapse geophysical data (i.e., cross-borehole ground-penetrating radar) and thermal-hydrological data (i.e., temperature and water content data) collected before and during the four-year heating phase of the test, and analyze the sensitivity of the most relevant thermal-hydrological and petrophysical parameters to the available data. To demonstrate feasibility of the approach, and as a first step toward comprehensive inversion of the heater test data, we apply the approach to estimate one parameter, the permeability of the rock matrix.

The vadose zone can be characterized as a complex subsurface system in which intricate physical and biogeochemical processes occur in response to a variety of natural forcings and human activities. This makes it difficult to describe, understand, and predict the behavior of this specific subsurface system. The TOUGH nonisothermal multiphase flow simulators are well-suited to perform advanced vadose zone studies. The conceptual models underlying the TOUGH simulators are capable of representing features specific to the vadose zone, and of addressing a variety of coupled phenomena. Moreover, the simulators are integrated into software tools that enable advanced data analysis, optimization, and system-level modeling. We discuss fundamental and computational challenges in simulating vadose zone processes, review recent advances in modeling such systems, and demonstrate some capabilities of the TOUGH suite of codes using illustrative examples.

Understanding microclimate dynamics in tunnels is important for designing and maintaining underground facilities. For example, in the geological disposal of radioactive materials, condensation of vapor should be minimized as it can accelerate waste package corrosion and radionuclide release. While microclimate dynamics are known to be dominated by the advection of heat and moisture, additional factors may also be important, such as the presence of fractures or faults. We present a relatively inexpensive method to assess microclimatic perturbations within a tunnel. By combining standard temperature and relative humidity sensors with low-cost sensors designed to detect changes in condensation, we monitored microclimate dynamics along a tunnel at the proposed geological repository at Yucca Mountain, Nevada. We observed significant differences in the pattern of condensation in a faulted zone relative to that of a nonfaulted zone, suggesting that the microclimate dynamics of excavated cavities in fractured, partially saturated rocks can be highly complex.

In this study, we evaluate the response of oceanic subsurface systems to thermal stresses caused by the flow of warm fluids through noninsulated well systems crossing hydrate-bearing sediments. Heat transport from warm fluids, originating from deeper reservoirs under production, into the geologic media can cause dissociation of the gas hydrates. The objective of this study is to determine whether gas evolution from hydrate dissociation can lead to excessive pressure buildup, and possibly to fracturing of hydrate-bearing formations and their confining layers, with potentially adverse consequences on the stability of the suboceanic subsurface. This study also aims to determine whether the loss of the hydrate--known to have a strong cementing effect on the porous media--in the vicinity of the well, coupled with the significant pressure increases, can undermine the structural stability of the well assembly.Scoping 1D simulations indicated that the formation intrinsic permeability, the pore compressibility, the temperature of the produced fluids and the initial hydrate saturation are the most important factors affecting the system response, while the thermal conductivity and porosity (above a certain level) appear to have a secondary effect. Large-scale simulations of realistic systems were also conducted, involving complex well designs and multilayered geologic media with nonuniform distribution of properties and initial hydrate saturations that are typical of those expected in natural oceanic systems. The results of the 2D study indicate that although the dissociation radius remains rather limited even after long-term production, low intrinsic permeability and/or high hydrate saturation can lead to the evolution of high pressures that can threaten the formation and its boundaries with fracturing. Although lower maximum pressures are observed in the absence of bottom confining layers and in deeper (and thus warmer and more pressurized) systems, the reduction is limited. Wellbore designs with gravel packs that allow gas venting and pressure relief result in substantially lower pressures.