George A. Zyvoloski

The site-scale saturated zone flow model for Yucca Mountain: calibration of different conceptual models and their impact on flow paths

This paper presents several different conceptual models of the Large Hydraulic Gradient (LHG) region north of Yucca Mountain and describes the impact of those models on groundwater flow near the potential high-level repository site. The results are based on a numerical model of site-scale saturated zone beneath Yucca Mountain. This model is used for performance assessment predictions of radionuclide transport and to guide future data collection and modeling activities. The numerical model is calibrated by matching available water level measurements using parameter estimation techniques, along with more informal comparisons of the model to hydrologic and geochemical information. The model software (hydrologic simulation code FEHM and parameter estimation software PEST) and model setup allows for efficient calibration of multiple conceptual models. Until now, the Large Hydraulic Gradient has been simulated using a low-permeability, east-west oriented feature, even though direct evidence for this feature is lacking. In addition to this model, we investigate and calibrate three additional conceptual models of the Large Hydraulic Gradient, all of which are based on a presumed zone of hydrothermal chemical alteration north of Yucca Mountain. After examining the heads and permeabilities obtained from the calibrated models, we present particle pathways from the potential repository that record differences in the predicted groundwater flow regime. The results show that Large Hydraulic Gradient can be represented with the alternate conceptual models that include the hydrothermally altered zone. The predicted pathways are mildly sensitive to the choice of the conceptual model and more sensitive to the quality of calibration in the vicinity on the repository. These differences are most likely due to different degrees of fit of model to data, and do not represent important differences in hydrologic conditions for the different conceptual models.

CO2/Brine transport into shallow aquifers along fault zones.

Unintended release of CO(2) from carbon sequestration reservoirs poses a well-recognized risk to groundwater quality. Research has largely focused on in situ CO(2)-induced pH depression and subsequent trace metal mobilization. In this paper we focus on a second mechanism: upward intrusion of displaced brine or brackish-water into a shallow aquifer as a result of CO(2) injection. Studies of two natural analog sites provide insights into physical and chemical mechanisms controlling both brackish water and CO(2) intrusion into shallow aquifers along fault zones. At the Chimayó, New Mexico site, shallow groundwater near the fault is enriched in CO(2) and, in some places, salinity is significantly elevated. In contrast, at the Springerville, Arizona site CO(2) is leaking upward through brine aquifers but does not appear to be increasing salinity in the shallow aquifer. Using multiphase transport simulations we show conditions under which significant CO(2) can be transported through deep brine aquifers into shallow layers. Only a subset of these conditions favor entrainment of salinity into the shallow aquifer: high aspect-ratio leakage pathways and viscous coupling between the fluid phases. Recognition of the conditions under which salinity is favored to be cotransported with CO(2) into shallow aquifers will be important in environmental risk assessments.

FEHMN 1.0: Finite element heat and mass transfer code

A computer code is described which can simulate non-isothermal multiphase multicomponent flow in porous media. It is applicable to natural-state studies of geothermal systems and ground-water flow. The equations of heat and mass transfer for multiphase flow in porous and permeable media are solved using the finite element method. The permeability and porosity of the medium are allowed to depend on pressure and temperature. The code also has provisions for movable air and water phases and noncoupled tracers; that is, tracer solutions that do not affect the heat and mass transfer solutions. The tracers can be passive or reactive. The code can simulate two-dimensional, two-dimensional radial, or three-dimensional geometries. A summary of the equations in the model and the numerical solution procedure are provided in this report. A user`s guide and sample problems are also included. The main use of FEHMN will be to assist in the understanding of flow fields in the saturated zone below the proposed Yucca Mountain Repository. 33 refs., 27 figs., 12 tabs.

The saturated zone at Yucca Mountain: An overview of the characterization and assessment of the saturated zone as a barrier to potential radionuclide migration

The US Department of Energy is pursuing Yucca Mountain, Nevada, for the development of a geologic repository for the disposal of spent nuclear fuel and high-level radioactive waste, if the repository is able to meet applicable radiation protection standards established by the US Nuclear Regulatory Commission and the US Environmental Protection Agency (EPA). Effective performance of such a repository would rely on a number of natural and engineered barriers to isolate radioactive waste from the accessible environment. Groundwater beneath Yucca Mountain is the primary medium through which most radionuclides might move away from the potential repository. The saturated zone (SZ) system is expected to act as a natural barrier to this possible movement of radionuclides both by delaying their transport and by reducing their concentration before they reach the accessible environment. Information obtained from Yucca Mountain Site Characterization Project activities is used to estimate groundwater flow rates through the site-scale SZ flow and transport model area and to constrain general conceptual models of groundwater flow in the site-scale area. The site-scale conceptual model is a synthesis of what is known about flow and transport processes at the scale required for total system performance assessment of the site. This knowledge builds on and is consistent with knowledge that has accumulated at the regional scale but is more detailed because more data are available at the site-scale level. The mathematical basis of the site-scale model and the associated numerical approaches are designed to assist in quantifying the uncertainty in the permeability of rocks in the geologic framework model and to represent accurately the flow and transport processes included in the site-scale conceptual model. Confidence in the results of the mathematical model was obtained by comparing calculated to observed hydraulic heads, estimated to measured permeabilities, and lateral flow rates calculated by the site-scale model to those calculated by the regional-scale flow model. In addition, it was confirmed that the flow paths leaving the region of the potential repository are consistent with those inferred from gradients of measured head and those independently inferred from water-chemistry data. The general approach of the site-scale SZ flow and transport model analysis is to calculate unit breakthrough curves for radionuclides at the interface between the SZ and the biosphere using the three-dimensional site-scale SZ flow and transport model. Uncertainties are explicitly incorporated into the site-scale SZ flow and transport abstractions through key parameters and conceptual models.

Model study of the thermal storage system by FEHM code

The use of low-temperature geothermal resources is important from the viewpoint of global warming. In order to evaluate various underground projects that use low-temperature geothermal resources, we have estimated the parameters of a typical underground system using the two-well model. By changing the parameters of the system, six different heat extraction scenarios have been studied. One of these six scenarios is recommended because of its small energy loss.

Laboratory-Scale Experiments of the Methane Hydrate Dissociation Process in a Porous Media and Numerical Study for the Estimation of Permeability in Methane Hydrate Reservoir

An experimental study of the dissociation of methane hydrate (MH) by hot-water injection and depressurization was carried out at the National Institute of Advanced Industrial Science and Technology (AIST). These experiments helped us understand some important aspects of MH behavior such as how temperature, pressure, and permeability change during dissociation and gas production. In order to understand the experimental results, a model of MH dissociation in a porous media was designed and implemented in a numerical simulator. In the model, we treated the MH phase as a two-component system by representing the pore space occupied by MH as a separate component. Absolute permeability and relative permeability were formulated as a function of MH saturation, porosity, and sand grain diameter and introduced into the numerical model. Using the developed numerical simulator, we attempted history matching of laboratory-scale experiments of the MH dissociation process. It was found that numerical simulator was able to reproduce temperature change, permeability characteristics, and gas production behavior associated with both MH formation and dissociation.

Virtual watersheds: simulating the water balance of the Rio Grande Basin

Managers of water resources in arid and semi-arid regions must allocate increasingly variable surface water supplies and limited groundwater resources. This challenge is leading to a new generation of detailed computational models that can link multiple sources to a wide range of demands. Detailed computational models of complex natural-human systems can help decision makers allocate scarce natural resources such as water. This article describes a virtual watershed model, the Los Alamos Distributed Hydrologic System (LADHS), which contains the essential physics of all elements of a regional hydrosphere and allows feedback between them. Unlike real watersheds, researchers can perform experiments on virtual watersheds, produce them relatively cheaply (once a modeling framework is established), and run them faster than real time. Furthermore, physics-based virtual watersheds do not require extensive tuning and are flexible enough to accommodate novel boundary conditions such as land-use change or increased climate variability. Essentially, virtual watersheds help resource managers evaluate the risks of alternatives once uncertainties have been quantified.

A reduced degree of freedom method for simulating non-isothermal multi-phase flow in a porous medium

Abstract Two solution algorithms, with guaranteed memory savings over fully implicit methods, are presented for solving coupled processes in subsurface hydrology. They are applied to problems of non-isothermal multi-phase flow in porous media using both an equivalent continuum approximation and a dual-permeability approach. One algorithm uses an approximation of the Jacobian matrix during the Newton–Raphson iteration such that the coupled system of equations can be partitioned into a solution of some selected primary variables plus a back substitution procedure for the solution of the other variables. The second algorithm uses a similar procedure, except the operations are performed during the preconditioning phase of the solution of linear equations. Numerical forms were derived and the solution procedures were illustrated to reduce the problem from three unknowns per node to one for the single continuum formulation, and from six unknowns per node to two for the dual-permeability approximation. Simulation examples showed that the proposed method produced nearly identical results compared to the traditional fully implicit method while enjoying large memory savings and competitive CPU times.

Deep permeable fault-controlled helium transport and limited mantle flux in two extensional geothermal systems in the Great Basin, United States

This study assesses the relative importance of deeply circulating meteoric water and direct mantle fluid inputs on near-surface 3 He/ 4 He anomalies reported at the Coso and Beowawe geothermal fields of the western United States. The depth of meteoric fluid circulation is a critical factor that controls the temperature, extent of fluid-rock isotope exchange, and mixing with deeply sourced fluids containing mantle volatiles. The influence of mantle fluid flux on the reported helium anomalies appears to be negligible in both systems. This study illustrates the importance of deeply penetrating permeable fault zones (10 −12 to 10 −15 m 2 ) in focusing groundwater and mantle volatiles with high 3 He/ 4 He ratios to shallow crustal levels. These continental geothermal systems are driven by free convection.

Hydrogeochemistry and gas compositions of the Uinta Basin: A regional-scale overview

The geochemistry of formation fluids (water and hydrocarbon gases) in the Uinta Basin, Utah, is evaluated at the regional scale based on fluid sampling and compilation of past records. The deep formation water is dominated by Na-Cl type where halite dissolution has the greatest effects on water chemistry. Its distribution and composition is controlled by both the lithology of geological formations and regional hydrodynamics. The origin of the saline waters in the southeastern basin is interpreted to be a mix of ancient evaporatively concentrated seawater with meteoric water recharged in the geological past, which has experienced water-rock interactions. At the basin scale, three-dimensional mapping of the dissolved solid contents further reveals that (1) in the northern Uinta Basin bordering the Uinta Mountains, significant flushing of the deep basinal brines up to 6-km (3.7-mi) depth by meteoric water has occurred, and (2) in the central basin groundwater discharge areas along the Green River Valley, regional upwelling of saline waters from 2- to 3-km (1.2- to 1.8-mi) depth is occurring. Moreover, gas composition and water-gas stable isotope characteristics in the central to southeastern basin indicate the presence of a deep, thermogenic, and regionally continuous gas deposit. In particular, gases sampled in this region from the Wasatch Formation and Mesaverde Group indicate a similar source rock (type III kerogen of the deeply buried, thermally mature Mesaverde Group in the central to northern basin) as well as migration from the Natural Buttes gas field toward the southeastern basin. Evidence for biogenic methane formation is observed only in the upper Green River Formation in the central to northern Uinta Basin. Here, the organic-rich, immature Green River shales experience meteoric water invasions and formation fluid chemistry, and stable isotope compositions are diagnostic of microbial methanogenesis.