Thomas A. Buscheck

Infiltration of a liquid front in an unsaturated, fractured porous medium

We consider liquid infiltrating by gravity flow into a system of parallel, regularly spaced fractures in an unsaturated porous medium. The position of the fracture liquid front as a function of time, under some simplifying assumptions, is shown to obey a nonlinear integrodifferential equation. Approximate analytic solutions are developed, showing that the movement of the liquid front exhibits three major flow periods: (1) at early time, the frontal position is determined by the fracture inlet boundary condition and the gravity-driven flow behavior of the fracture with negligible influence by the matrix; (2) at intermediate time, matrix imbibition retards the frontal advance against the pull of gravity; (3) at late time, the matrix approaches saturation and the frontal velocity approaches a limiting value. A two-dimensional numerical model is used to confirm the approximate solutions. Implications of the model for nuclear waste storage are discussed. The analysis is applicable not only to fractured rock but also to lateral infiltration into coarse-grained sediments lying between layers of fine-grained soil.

Analytical solutions for reactive transport of N-member radionuclide chains in a single fracture

Several numerical codes have been used to simulate radionuclide transport in fractured rock systems. The validation of such numerical codes can be accomplished by comparison of numerical simulations against appropriate analytical solutions. In this paper, we present analytical solutions for the reactive transport of N-member radionuclide chains (i.e., multiple species of radionuclides and their daughter species) through a discrete fracture in a porous rock matrix applying a system decomposition approach. We consider the transport of N-member radionuclide chains in a single-fracture-matrix system as a starting point to simulate more realistic and complex systems. The processes considered are advection along the fracture, lateral diffusion in the matrix, radioactive decay of multiple radionuclides, and adsorption in both the fracture and matrix. Different retardation factors can be specified for the fracture and matrix. However, all species are assumed to share the same retardation factors for the fracture and matrix, respectively. Although a daughter species may penetrate farther along the fracture than its parent species when a constant-concentration boundary condition is applied, our results indicate that all species retain the same transport speed in the fracture if a pulse of the first species is released into the fracture. This solution scheme provides a way to validate numerical computer codes of radionuclide transport in fractured rock, such as those being used to assess the performance of a potential nuclear-waste repository at Yucca Mountain.

Validation of the Multiscale Thermohydrologic Model used for analysis of a proposed repository at Yucca Mountain

Performance assessment and design evaluation of the proposed repository at Yucca Mountain are facilitated by a thermohydrologic modeling tool that simultaneously accounts for processes occurring at a scale of a few tens of centimeters around individual waste packages and emplacement drifts, and accounts for processes at the multi-kilometer scale of the mountain. The most straightforward approach is to account for the 3-D drift- and mountain-scale dimensionality all within a single monolithic thermohydrologic model. This approach is too computationally expensive to be a viable simulation tool capable of addressing all waste-package locations in the repository. The Multiscale Thermohydrologic Model (MSTHM) is a computationally efficient alternative to addressing these modeling issues. In this paper, we describe the principal calculation stages to predict temperature, relative humidity, and liquid-saturation, as well as other thermohydrologic variables, in the drifts and in the host rock. Using a three-drift repository example (which is a scaled-down version of the proposed repository), we demonstrate the validity of the MSTHM approach against a nested monolithic thermohydrologic model.

An Analytical Solution of Tetrachloroethylene Transport and Biodegradation

In this manuscript, we consider a transport system with a dechlorination reaction network, in which tetrachloroethylene (PCE) reacts to produce trichloroethylene (TCE), TCE reacts to form three daughter products, cis-1,2-dichloroethylene (cis-1,2-DCE), trans-1,2-dichloroethylene (trans-1,2-DCE), and 1,1-dichloroethylene (1,1-DCE), three DCEs further react to produce vinyl chloride (VC), finally VC reacts to produce ethylene (ETH). Because the partial differential equation describing the reactive transport of VC, is coupled by three reactant concentrations, currently the problem must be solved numerically. Following Lu et al. (2003), we extend the analytical solution from five species to the entire PCE reaction network. Using the singular value decomposition (SVD) the system of transport equations with convergent reactions is decoupled into seven orthogonal subsystems. Previously published analytical solutions of single species transport become the basic solutions in the transformed domain for each independent subsystem. The solutions in real concentration domain are obtained using the inverse transform. The solution derived in this study can then be used instead of Sun et al. (1999) in BIOCHLOR for simulating more realistic systems of biodegradation and reactive transport.

Promising synergies to address water, sequestration, legal, and public acceptance issues associated with large-scale implementation of CO 2 sequestration

Stabilization of CO2 atmospheric concentrations requires practical strategies to address the challenges posed by the continued use of coal for baseload-electricity production. Over the next two decades, CO2 capture and sequestration (CCS) demonstration projects would need to increase several orders of magnitude across the globe in both size and scale. This task has several potential barriers which will have to be accounted for. These barriers include those that have been known for a number of years including safety of subsurface sequestration, pore-space competition with emerging activities like shale gas production, legal and regulatory frameworks, and public acceptance and technical communication. In addition water management is a new challenge that should be actively and carefully considered across all CCS operations. A review of the new insights gained on these previously and newly identified challenges, since the IPCC special report on CCS, is presented in this paper. While somewhat daunting in scope, some of these challenges can be addressed more easily by recognizing the potential advantageous synergies that can be exploited when these challenges are dealt with in combination. For example, active management of water resources, including brine in deep subsurface formations, can provide the additional cooling-water required by the CO2 capture retrofitting process while simultaneously reducing sequestration leakage risk and furthering efforts toward public acceptance. This comprehensive assessment indicates that water, sequestration, legal, and public acceptance challenges ought to be researched individually, but must also be examined collectively to exploit the promising synergies identified herein. Exploitation of these synergies provides the best possibilities for successful large-scale implementation of CCS.

Surrogate-based optimization of hydraulic fracturing in pre-existing fracture networks

Hydraulic fracturing has been used widely to stimulate production of oil, natural gas, and geothermal energy in formations with low natural permeability. Numerical optimization of fracture stimulation often requires a large number of evaluations of objective functions and constraints from forward hydraulic fracturing models, which are computationally expensive and even prohibitive in some situations. Moreover, there are a variety of uncertainties associated with the pre-existing fracture distributions and rock mechanical properties, which affect the optimized decisions for hydraulic fracturing. In this study, a surrogate-based approach is developed for efficient optimization of hydraulic fracturing well design in the presence of natural-system uncertainties. The fractal dimension is derived from the simulated fracturing network as the objective for maximizing energy recovery sweep efficiency. The surrogate model, which is constructed using training data from high-fidelity fracturing models for mapping the relationship between uncertain input parameters and the fractal dimension, provides fast approximation of the objective functions and constraints. A suite of surrogate models constructed using different fitting methods is evaluated and validated for fast predictions. Global sensitivity analysis is conducted to gain insights into the impact of the input variables on the output of interest, and further used for parameter screening. The high efficiency of the surrogate-based approach is demonstrated for three optimization scenarios with different and uncertain ambient conditions. Our results suggest the critical importance of considering uncertain pre-existing fracture networks in optimization studies of hydraulic fracturing.

Analysis of thermohydrologic behavior for above-boiling and below-boiling thermal-operating modes for a repository at Yucca Mountain

We report results from a multi-scale thermohydrologic modeling study for two alternative thermal-operating modes for the potential repository system recently analyzed by the Yucca Mountain Project. These include a Higher-Temperature Operating Mode (HTOM), which results in a localized boiling zone around each emplacement drift, and a Lower-Temperature Operating Mode (LTOM), which always maintains sub-boiling temperatures throughout the repository. The HTOM places all waste packages nearly end to end, making the lineal power density greater than in the LTOM. The lower lineal power density in the LTOM was achieved by placing some waste packages farther apart (which results in a larger repository footprint), and through an increased reliance on pre-closure ventilation to remove the waste-package-generated heat. We focus on temperature T and relative humidity RH at the waste-package and drift-wall surfaces, and on in-drift evaporation. In general, HTOM temperatures are greater than corresponding LTOM temperatures, exhibit similar spatial variability and have a stronger dependence on infiltration flux. The duration of RH reduction on waste packages is similar for the LTOM and HTOM. A major difference between the LTOM and HTOM is the lower waste-package temperature at any given value of waste-package RH for the LTOM. Waste-package temperatures in the LTOM, by design, remain below approximately 85 degrees C; the absence of RH reduction arising from host-rock dryout causes waste-package RH to remain above about 40%. The HTOM waste packages experience higher temperatures and correspondingly lower RH conditions as a result of RH reduction arising from host-rock dryout. For most of the repository area, the HTOM delays the potential onset of gravity-driven seepage compared to the LTOM (as indicated by the duration of boiling at the drift wall). Boiling conditions in the HTOM also delays the onset of capillary-driven seepage into the granular invert, causing the HTOM to have less evaporation in the invert during the first 800-1500 years than the LTOM; subsequent evaporation rates are higher in the HTOM, due to the higher power density.

Combining Simulation and Emulation for Calibrating Sequentially Reactive Transport Systems

Reaction rates are usually identified at laboratory scale, by comparing measured concentrations with those of the corresponding mathematical models. However, laboratory-scale reaction rates may not necessarily reflect the reactive transport scenarios at the field scale. Thus, a major challenge for field-scale modeling is the determination of reaction kinetics and rates. The conventional inversion of reaction rates relies on optimization approaches that require expensive computation to obtain the gradient of objective functions. In this manuscript, we present a combined simulation–emulation approach for calibrating the first-order reaction rates at the field scale. A number of sample points are adaptively selected to represent the high-dimensional parametric space including dimensions of reaction rates. Correspondingly, reactive transport models are generated and executed for constructing response surfaces of objective functions. Taking the advantage of smooth response surfaces, optimization of reaction rates is efficiently performed. For several benchmark cases, the advantage of using global sensitivity analysis and uncertainty quantification of the objective functions in terms of uncertain reaction rates is demonstrated.

Managing geologic CO2 storage with pre-injection brine production: a strategy evaluated with a model of CO2 injection at Snøhvit

CO2 capture and storage (CCS) in saline reservoirs can play a key role in curbing CO2 emissions. Buildup of pressure due to CO2 injection, however, can create hazards (wellbore leakage, caprock fracturing, and induced seismicity) to safe storage that must be carefully addressed. Reservoir pressure management by producing brine to minimize pressure buildup is a potential tool to manage these risks. To date, research studies on the effectiveness of brine production have largely focused on generic, hypothetical systems. In this paper, we use data from the Snohvit CCS project to perform a data-constrained analysis of its effectiveness under realistic geologic conditions. During the first phase of the Snohvit project, CO2 was injected into the compartmentalized Tubaen Fm. with lower-than-expected injectivity and capacity, which resulted in pressure buildup sooner than was expected. Using a reservoir model calibrated to this observed behavior, we analyze an alternative scenario in which brine is produced from the storage unit prior to injection. The results suggest that pre-injection brine production in this particular formation would be 94% efficient on a volume-per-volume basis – i.e. for each cubic meter of brine removed, an additional 0.94 cubic meters of CO2 could have been injected while maintaining the same peak reservoir pressure. Further, pressure drawdown observed during brine production is a mirror image of pressure buildup during CO2 injection, providing necessary data to estimate reservoir capacity before CO2 is injected. These observations suggest that this approach can be valuable for site selection and characterization, risk management, and increasing public acceptance.

Localized dryout: An approach for managing the thermal hydrologi-cal effects of decay heat at Yucca Mountain

For a nuclear waste repository in the unsaturated zone at Yucca Mountain, there are two thermal loading approaches to using decay heat constructively -- that is, to substantially reduce relative humidity and liquid flow near waste packages for a considerable time, and thereby limit waste package degradation and radionuclide dissolution and release. ``Extended dryout`` achieves these effects with a thermal load high enough to generate large-scale (coalesced) rock dryout. ``Localized dryout``(which uses wide drift spacing and a thermal load too low for coalesced dryout) achieves them by maintaining a large temperature difference between the waste package and drift wall; this is done with close waste package spacing (generating a high line-heat load) and/or low-thermal-conductivity backfill in the drift. Backfill can greatly reduce relative humidity on the waste package in both the localized and extended dryout approaches. Besides using decay heat constructively, localized dryout reduces the possibility that far-field temperature rise and condensate buildup above the drifts might adversely affect waste isolation.