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Papers by Eric Sonnenthal

Journal of Energy Resources Technology, 2021

Enhanced geothermal systems (EGS) offer the potential for a much larger energy source than conven... more Enhanced geothermal systems (EGS) offer the potential for a much larger energy source than conventional hydrothermal systems. Hot, low-permeability rocks are prevalent at depth around the world, but the challenge of extracting thermal energy depends on the ability to create and sustain open fracture networks. Laboratory experiments were conducted using a suite of selected rock cores (granite, metasediment, rhyolite ash-flow tuff, and silicified rhyolitic tuff) at relevant pressures (uniaxial loading up to 20.7 MPa and fluid pressures up to 10.3 MPa) and temperatures (150–250 °C) to evaluate the potential impacts of circulating fluids through fractured rock by monitoring changes in fracture aperture, mineralogy, permeability, and fluid chemistry. Because a fluid in disequilibrium with the rocks (deionized water) was used for these experiments, there was net dissolution of the rock sample: this increased with increasing temperature and experiment duration. Thermal-hydrological-mechani...

Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2017

A diverse suite of numerical simulators is currently being applied to predict or understand the p... more A diverse suite of numerical simulators is currently being applied to predict or understand the performance of enhanced geothermal systems (EGS). To build confidence and identify critical development needs for these analytical tools, the United States Department of Energy, Geothermal Technologies Office sponsored a Code Comparison Study (GTO-CCS), with participants from universities, industry, and national laboratories. A principal objective for the study was to create a community forum for improvement and verification of numerical simulators for EGS modeling. Teams participating in the study were those representing U.S. national laboratories, universities, and industries, and each team brought unique numerical simulation capabilities to bear on the problems. Two classes of problems were developed during the study, benchmark problems and challenge problems. The benchmark problems were structured to test the ability of the collection of numerical simulators to solve various combinations of coupled thermal, hydrologic, geomechanical, and geochemical processes. This class of problems was strictly defined in terms of properties, driving forces, initial conditions, and boundary conditions. The challenge problems were based on the enhanced geothermal systems research conducted at Fenton Hill, near Los Alamos, New Mexico, between 1974 and 1995. The problems involved two phases of research, stimulation, development, and circulation in two separate reservoirs. The challenge problems had specific questions to be

The DECOVALEX project is an international cooperative project initiated by SKI, the Swedish Nucle... more The DECOVALEX project is an international cooperative project initiated by SKI, the Swedish Nuclear Power Inspectorate, with participation of about 10 international organizations. The general goal of this project is to encourage multidisciplinary interactive and cooperative research on modeling coupled thermo-hydro-mechanical-chemical (THMC) processes in geologic formations in support of the performance assessment for underground storage of radioactive waste. One of the research tasks, initiated in 2004 by the U.S. Department of Energy (DOE), addresses the long-term impact of geomechanical and geochemical processes on the flow conditions near waste emplacement tunnels. Within this task, four international research teams conduct predictive analysis of the coupled processes in two generic repositories, using multiple approaches and different computer codes. Below, we give an overview of the research task and report its current status.

Portions of this document may be illegible in electronic image products. Images are produced from... more Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

Research Objectives The objectives of this study were to evaluate the thermal-hydrological-chemic... more Research Objectives The objectives of this study were to evaluate the thermal-hydrological-chemical (THC) effects on flow and geochemistry in the unsaturated zone (UZ) at Yucca Mountain at a mountain scale. The major THC processes important in the UZ are (1) mineral precipitatiorddissolution affecting flow and transport to and from the potential repository, and (2) changes in the compositions of gas and liquid that may seep into drifts.

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An integrated modeling approach was developed to investigate long-term coupled thermal, hydrologi... more An integrated modeling approach was developed to investigate long-term coupled thermal, hydrological, and chemical (THC) processes that could take place around nuclear waste emplacement tunnels (drifts). The approach involves the development of process models, followed by numerical implementation and validation against field and laboratory experiments before conducting long-term predictive simulations. An outcome of this work was the refinement and validation of an existing reactive transport numerical code for applications specific to the geologic storage of nuclear waste. The model was applied to the case of the proposed high-level nuclear waste repository at Yucca Mountain, Nevada, to evaluate the chemistry of waters potentially seeping into drifts and the effect of water-rock interaction on long-term hydrological behavior

Research Obiectives The emplacement of heat-generating nuclear waste in the potential geologic re... more Research Obiectives The emplacement of heat-generating nuclear waste in the potential geologic repository at Yucca Mountain, Nevada, will result in enhanced water-rock interaction around the emplacement drifts. Water present in the matrix and fractures of the rock around the drift may vaporize and migrate via fractures to cooler regions where condensation would occur. The condensate would react with the surrounding rock, resulting in mineral dissolution. Mineralized water flowing under gravity back towards the heat zone would boil, depositing the dissolved minerals. Such mineral deposition would reduce porosity and permeability above the repository, thus altering the flow paths of percolating water. The objective of this research is to use coupled thermal-hydrological-chemical (THC) models to simulate previously conducted laboratory experiments involving tuff dissolution and mineral precipitation in a boiling, unsaturated fracture.

Elsevier Geo-Engineering Book Series, 2004

A numerical model was developed to predict the coupled thermal, hydrological, and chemical (THC) ... more A numerical model was developed to predict the coupled thermal, hydrological, and chemical (THC) processes accompanying the Drift Scale Test (DST) at Yucca Mountain, NV. The DST has been closely monitored through the collection of gas, water, and mineral samples as well as thermal, hydrological, and mechanical measurements. A two-dimensional dual permeability model was developed to evaluate multiphase, multicomponent, reaction-transport processes in the fractured tuff. Comparisons between results using the TOUGHREACT code and measured water (e.g., pH, SiO 2 (aq), Na + , K +) and gas (CO 2) compositions show that the model captures the chemical evolution in the DST. Non-reactive aqueous species (e.g., Cl) show strong dilution in fracture waters, indicating little fracture-matrix interaction. Silica concentrations are higher than in the initial pore water and show a trend of increasing reaction with fracturelining silicates at higher temperatures. The narrow precipitation zone of predominantly amorphous silica observed above the heaters was also captured.

6.3. Geochemical Input (chemical.inp) An example of chemical.inp is given in Figure 8.2.2.

$Research paper thumbnail of Modeling coupled thermal–hydrological–chemical processes in the unsaturated fractured rock of Yucca Mountain, Nevada: Heterogeneity and seepage$

Physics and Chemistry of the Earth, Parts A/B/C, 2006

An understanding of processes affecting seepage into emplacement tunnels is needed for correctly ... more An understanding of processes affecting seepage into emplacement tunnels is needed for correctly predicting the performance of underground radioactive waste repositories. It has been previously estimated that the capillary and vaporization barriers in the unsaturated fractured rock of Yucca Mountain are enough to prevent seepage under present day infiltration conditions. It has also been thought that a substantially elevated infiltration flux will be required to cause seepage after the thermal period is over. While coupled thermal-hydrological-chemical (THC) changes in Yucca Mountain host rock due to repository heating has been previously investigated, those THC models did not incorporate elements of the seepage model. In this paper, we combine the THC processes in unsaturated fractured rock with the processes affecting seepage. We observe that the THC processes alter the hydrological properties of the fractured rock through mineral precipitation and dissolution. We show that such alteration in the hydrological properties of the rock often leads to local flow channeling. We conclude that such local flow channeling may result in seepage under certain conditions, even with nonelevated infiltration fluxes.

Journal of Porous Media, 2009

When hot radioactive waste is placed in subsurface tunnels, a series of complex changes occurs in... more When hot radioactive waste is placed in subsurface tunnels, a series of complex changes occurs in the surrounding medium. The water in the pore space of the medium undergoes vaporization and boiling. Subsequently, vapor migrates out of the matrix pore space, moving away from the tunnel through the permeable fracture network. This migration is propelled by buoyancy, by the increased vapor pressure caused by heating and boiling, and through local convection. In cooler regions, the vapor condenses on fracture walls, where it drains through the fracture network. Slow imbibition of water thereafter leads to gradual rewetting of the rock matrix. These thermal and hydrological processes also bring about chemical changes in the medium. Amorphous silica precipitates from boiling and evaporation, and calcite from heating and CO 2 volatilization. The precipitation of amorphous silica, and to a much lesser extent calcite, results in long-term permeability reduction. Evaporative concentration also results in the precipitation of gypsum (or anhydrite), halite, fluorite and other salts. These evaporative minerals eventually redissolve after the boiling period is over, however, their precipitation results in a significant temporary decrease in permeability. Reduction of permeability is also associated with changes in fracture capillary characteristics. In short, the coupled thermal-hydrological-chemical (THC) processes dynamically alter the hydrological properties of the rock. A model based on the TOUGHREACT reactive transport software is presented here to investigate the impact of THC processes on flow near an emplacement tunnel at Yucca Mountain, Nevada. We show how transient changes in hydrological properties caused by THC processes often lead to local flow channeling and saturation increases above the tunnel. For models that include only permeability changes to fractures, such local flow channeling may lead to seepage relative to models where THC effects are ignored. However, coupled THC seepage models that include both permeability and capillary changes to fractures may not show this additional seepage.

Journal of Contaminant Hydrology, 2003

The evolution of fluid chemistry and mineral alteration around a potential waste emplacement tunn... more The evolution of fluid chemistry and mineral alteration around a potential waste emplacement tunnel (drift) is evaluated using numerical modeling. The model considers the flow of water, gas, and heat, plus reactions between minerals, CO 2 gas, and aqueous species, and porositypermeability-capillary pressure coupling for a dual permeability (fractures and matrix) medium. Two possible operating temperature modes are investigated: a "high-temperature" case with temperatures exceeding the boiling point of water for several hundred years, and a "low-temperature" case with temperatures remaining below boiling for the entire life of the repository. In both cases, possible seepage waters are characterized by dilute to moderate salinities and mildly alkaline pH values. These trends in fluid composition and mineral alteration are controlled by various coupled mechanisms. For example, upon heating and boiling, CO 2 exsolution from pore waters raises pH and causes calcite precipitation. In condensation zones, this CO 2 redissolves, resulting in a decrease in pH that causes calcite dissolution and enhances feldspar alteration to clays. Heat also enhances dissolution of wallrock minerals leading to elevated silica concentrations. Amorphous silica precipitates through evaporative concentration caused by boiling in the high-temperature case, but does not precipitate in the low-temperature case. Some alteration of feldspars to clays and zeolites is predicted in the high-temperature case. In both cases, calcite precipitates when percolating waters are heated near the drift. The predicted porosity decrease 2 around drifts in the high-temperature case (several percent of the fracture volume) is larger by at least one order of magnitude than in the low temperature case. Although there are important differences between the two investigated temperature modes in the predicted evolution of fluid compositions and mineral alteration around drifts, these differences are small relative to the model uncertainty and the variability of water compositions at Yucca Mountain.

Geochemical Transactions, 2009

Background: This study investigates the generation of HCl and other acid gases from boiling calci... more Background: This study investigates the generation of HCl and other acid gases from boiling calcium chloride dominated waters at atmospheric pressure, primarily using numerical modeling. The main focus of this investigation relates to the long-term geologic disposal of nuclear waste at Yucca Mountain, Nevada, where pore waters around waste-emplacement tunnels are expected to undergo boiling and evaporative concentration as a result of the heat released by spent nuclear fuel. Processes that are modeled include boiling of highly concentrated solutions, gas transport, and gas condensation accompanied by the dissociation of acid gases, causing low-pH condensate. Results: Simple calculations are first carried out to evaluate condensate pH as a function of HCl gas fugacity and condensed water fraction for a vapor equilibrated with saturated calcium chloride brine at 50-150°C and 1 bar. The distillation of a calcium-chloride-dominated brine is then simulated with a reactive transport model using a brine composition representative of partially evaporated calcium-rich pore waters at Yucca Mountain. Results show a significant increase in boiling temperature from evaporative concentration, as well as low pH in condensates, particularly for dynamic systems where partial condensation takes place, which result in enrichment of HCl in condensates. These results are in qualitative agreement with experimental data from other studies. Conclusion: The combination of reactive transport with multicomponent brine chemistry to study evaporation, boiling, and the potential for acid gas generation at the proposed Yucca Mountain repository is seen as an improvement relative to previously applied simpler batch evaporation models. This approach allows the evaluation of thermal, hydrological, and chemical (THC) processes in a coupled manner, and modeling of settings much more relevant to actual field conditions than the distillation experiment considered. The actual and modeled distillation experiments do not represent expected conditions in an emplacement drift, but nevertheless illustrate the potential for acid-gas generation at moderate temperatures (<150°C).

Newberry Volcano in Central Oregon is the site of a Department of Energy funded Enhanced Geotherm... more Newberry Volcano in Central Oregon is the site of a Department of Energy funded Enhanced Geothermal System (EGS) Demonstration Project. Stimulation and production of an EGS is a strong perturbation to the physical and chemical environment, giving rise to coupled Thermal-Hydrological-Mechanical-Chemical (THMC) processes leading to permeability changes as a result of mineral dissolution and precipitation, rock deformation, and fracture reactivation. To evaluate these processes, and to help guide EGS stimulation and reservoir development strategies, a combined native-state and reservoir model of the west flank of Newberry Volcano was created that encompasses the planned stimulation zone and a several km region of the west flank from the surface down to the supercritical region, likely close to a postulated cooling intrusive body. Temperature and pressure distributions were first modeled using TOUGHREACT with boundary conditions estimated from nearby drill holes, and compared to measurements made in the over 3 km deep NWG 55-29 drill hole. With estimates of the porosity and heat capacities for the major hydrogeologic units, thermal conductivities were calibrated by matching to the measured temperature profile. To simulate the development of the observed hydrothermal mineralogy, a reaction-transport model (THC) was developed using the pre-alteration mineralogy and shallow groundwater chemistry as the initial geochemical conditions, assuming that modeled temperature and pressure distributions were relatively constant over several thousand years. Close correspondence of modeled and observed epidote distributions support the observation that past hydrothermal activity took place under thermal gradients similar to current values, whereas calcite and sulfide abundances at depth likely require a magmatic gas component. Multicomponent geothermometry was used to estimate potential temperatures of equilibration of waters, and to evaluate the effects of kinetics on calculated mineral equilibration temperatures. The ultimate goal will be to capture both the local chemical and mechanical changes in the rock owing to stimulation as well as the potential long-term response and sustainability of the larger-scale geothermal reservoir.

The interaction between hydrothermal fluids and the rocks through which they migrate alters the e... more The interaction between hydrothermal fluids and the rocks through which they migrate alters the earlier formed primary minerals and leads to the formation of secondary minerals, resulting in changes in the physical and chemical properties of the system. We have developed a comprehensive numerical simulator, TOUGHREACT, which considers nonisothermal multi-component chemical transport in both liquid and gas phases. A variety of subsurface thermo-physical-chemical processes is considered under a wide range of conditions of pressure, temperature, water saturation, and ionic strength. The code can be applied to problems in fundamental analysis of the hydrothermal systems and in the exploration of geothermal reservoirs including chemical evolution, mineral alteration, mineral scaling, changes of porosity and permeability, and mineral recovery from geothermal fluids.

Coupled modeling of subsurface multiphase fluid and heat flow, solute transport and chemical reac... more Coupled modeling of subsurface multiphase fluid and heat flow, solute transport and chemical reactions can be used for the assessment of acid mine drainage remediation, waste disposal sites, hydrothermal convection, contaminant transport, and groundwater quality. We have developed a comprehensive numerical simulator, TOUGHREACT, which considers non-isothermal multi-component chemical transport in both liquid and gas phases. A wide range of subsurface thermo-physical-chemical processes is considered under various thermohydrological and geochemical conditions of pressure, temperature, water saturation, and ionic strength. The code can be applied to one-, two-or three-dimensional porous and fractured media with physical and chemical heterogeneity.

Transport in Porous Media, 2014

In this study, we use the TOUGH-FLAC simulator for coupled thermo-hydro-mechanical (THM) modeling... more In this study, we use the TOUGH-FLAC simulator for coupled thermo-hydro-mechanical (THM) modeling of well stimulation for an Enhanced Geothermal System (EGS) project. We analyze the potential for injection-induced fracturing and reactivation of natural fractures in a porous medium with associated permeability enhancement. Our analysis aims to understand how far the EGS reservoir may grow and how the hydroshearing effect depends upon system conditions. We analyze the enhanced reservoir, or hydrosheared zone, by studying the extent of the failure zone using an elasto-plastic model, and accounting for permeability changes as a function of the induced stresses. Results, for both fully saturated and unsaturated medium cases, demonstrate how the EGS reservoir growth depends on the initial fluid phase, and how the reservoir extent changes as a function of two critical parameters: the coefficient of friction and the permeabilityenhancing factor. Moreover, whereas the stimulation is driven by the pressure exceeding a hydroshearing threshold, the modeling also demonstrates how the injection-induced cooling boosts the stimulation a bit farther.

Modeling coupled water-gas-rock interactions in unsaturated fractured rock requires conceptual an... more Modeling coupled water-gas-rock interactions in unsaturated fractured rock requires conceptual and numerical model considerations beyond those developed for saturated porous media. This paper focuses on the integration of the geological and hydrological parameters into the calculation of reactive-transport parameters and the feedback of mineral precipitation/dissolution to flow and transport. These basic relations have been implemented in the reactive transport code TOUGHREACT (Xu et al., 2003) that couples equilibrium and kinetic water-gas-rock interactions with multiphase flow and aqueous and gaseous species transport. Simulation results are presented illustrating the effects of water-rock interaction accompanying the heating of unsaturated heterogeneous fractured tuff.