Discrete-Fracture Modeling of Thermal-Hydrological Processes at Yucca Mountain and the Llnl G-Tunnel Heater Test

An in situ heater test was performed at G-Tunnel, Nevada Nuclear Test Site, to investigate the thermal-hydrological response of unsaturated, fractured volcanic tuff under conditions similar to those at Yucca Mountain. The NUFT flow and transport code was used to model the test using discrete-fracture and equivalent-continuum approaches. Nonequilibrium fracture flow and thermal buoyant gas-phase convection were found to be the likely causes for observed lack of condensate imbibition into the matrix. The potential repository at Yucca Mountain was also modeled. Disequilibrium fracture flow is predicted to occur for less than a hundred years after emplacement followed by a period of fracture-matrix equilibrium, during which the equivalent-continuum and discrete-fracture models give almost identical results

Localized Dryout: an Approach for Managing the Thermal-Hydrological 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

Sensitivity analysis of the dynamic CO2 storage capacity estimate for the Bunter Sandstone of the UK Southern North Sea

Carbon capture and storage (CCS) in subsurface reservoirs has been identified as a potentially cost-effective way to reduce CO2 emissions to the atmosphere. Global emissions reductions on the gigatonne scale using CCS will require regional or basin-scale deployment of CO2 storage in saline aquifers. Thus the evaluation of both the dynamic and ultimate CO2 storage capacity of formations is important for policy makers to determine the viability of CCS as a pillar of the greenhouse gas mitigation strategy in a particular region. We use a reservoir simulation model representing the large-scale Bunter Sandstone in the UK Southern North Sea to evaluate the dynamics and sensitivities of regional CO2 plume transport and storage. At the basin-scale, we predict hydrogeological changes in the storage reservoir in response to multiple regional carbon sequestration development scenarios. We test the sensitivity of injection capacity to a range of target CO2 injection rates and fluctuations in CO2 supply. Model sensitivities varying the target injection rates indicate that in the absence of pressure management up to 3.7 Gt of CO2 can be stored in the Bunter region over 50 years given the pressure constraints set to avoid fracturing the formation. Long-term (approx. 1000 years), our results show that up to 16 Gt of CO2 can be stored in the Bunter region without pressure management. With pressure management, the estimate rises to 32 Gt. However, consideration must be given to the additional operational and economic requirements of pressure management using brine production

The Importance of Thermal Loading Conditions to Waste Package Performance at Yucca Mountain

Temperature and relative humidity are primary environmental factors affecting waste package corrosion rates for the potential repository in the unsaturated zone at Yucca Mountain, Nevada. Under ambient conditions, the repository environment is quite humid. If relative humidity is low enough (<70%), corrosion will be minimal. Under humid conditions, corrosion is reduced if the temperature is low (<60 C). Using the V-TOUGH code, the authors model thermo-hydrological flow to investigate the effect of repository heat on temperature and relative humidity in the repository for a wide range of thermal loads. These calculations indicate that repository heat may substantially reduce relative humidity on the waste package, over hundreds of years for low thermal loads and over tens of thousands of year for high thermal loads. Temperatures associated with a given relative humidity decrease with increasing thermal load. Thermal load distributions can be optimized to yield a more uniform reduction in relative humidity during the boiling period

Probabilistic Simulation of Subsurface Fluid Flow: A Study Using a Numerical Scheme

There has been an increasing interest in probabilistic modeling of hydrogeologic systems. The classical approach to groundwater modeling has been deterministic in nature, where individual layers and formations are assumed to be uniformly homogeneous. Even in the case of complex heterogeneous systems, the heterogeneities describe the differences in parameter values between various layers, but not within any individual layer. In a deterministic model a single-number is assigned to each hydrogeologic parameter, given a particular scale of interest. However, physically there is no such entity as a truly uniform and homogeneous unit. Single-number representations or deterministic predictions are subject to uncertainties. The approach used in this work models such uncertainties with probabilistic parameters. The resulting statistical distributions of output variables are analyzed. A numerical algorithm, based on axiomatic principles of probability theory, performs arithmetic operations between probability distributions. Two subroutines are developed from the algorithm and incorporated into the computer program TERZAGI, which solves groundwater flow problems in saturated, multi-dimensional systems. The probabilistic computer program is given the name, PROGRES. The algorithm has been applied to study the following problems: one-dimensional flow through homogeneous media, steady-state and transient flow conditions, one-dimensional flow through heterogeneous media, steady-state and transient flow conditions, and two-dimensional steady-stte flow through heterogeneous media. The results are compared with those available in the literature

Near-Field Thermal-Hydrological Behavior for Alternative Repository Designs at Yucca Mountain

three-dimensional calculations that explicitly represent a realistic mixture of waste packages (WPs) are used to analyze decay-heat-driven thermal-hydrological behavior around emplacement drifts in a potential high-level waste facility at Yucca Mountain, Calculations, using the NUFT code, compare two fundamentally different ways that WPs can be arranged in the repository, with a focus on temperature, relative humidity, and liquid-phase flux on WPs. These quantities strongly affect WP integrity and the mobilization and release of radionuclides from WPs. Point-load spacing, which places the WPs roughly equidistant from each other, thermally isolates WPs from each other, causing large variability in temperature, relative humidity, and liquid-phase flux among the drifts. Line-load spacing, which WPs nearly end to end in widely spaced drifts, results in more locally intensive and uniform heating along the drifts, causing hotter, drier and more uniform conditions. A larger and more persistent reduction in relative humidity on WPs occurs if the drifts are backfilled with a low thermal conductivity granular material with hydrologic properties that minimize moisture wicking

Using geologic CO2 storage for enhanced geothermal energy and water recovery and energy storage

Reductions in CO2 emissions at a scale consistent with limiting the increase in the global average temperature to below 2oC above pre-industrial levels requires a range of measures, including increased use of renewable and low-carbon energy and reduced CO2 intensity of fossil energy use, with each of these measures having major deployment barriers. The variability of the predominant renewable resources (wind and solar) requires major advances in utility-scale diurnal-to-seasonal energy storage. Base-load energy, such as nuclear, that cannot be cycled during periods of over-generation will have difficulty co-existing on electric grids with a large presence of variable renewables. Major deployment barriers for CO2 capture, utilization, and storage (CCUS) in saline reservoirs include: (1) net cost (after accounting for utilization benefits); (2) water intensity of CO2 capture, and (3) overpressure, which is fluid pressure that exceeds the original reservoir pressure due to CO2 injection, because it drives key storage risks: induced seismicity, caprock fracture, and CO2 leakage. We present a synergistic approach to CCUS in sedimentary basins designed to address each of these deployment barriers. Our approach uses the huge fluid and thermal storage capacity of the subsurface, together with overpressure driven by CO2 storage, to harvest, store, and dispatch energy from subsurface (geothermal) and surface (solar, nuclear, fossil) thermal resources, as well as excess energy from electric grids. Captured CO2 is injected into saline reservoirs to store pressure, generate artesian flow of brine, and provide a supplemental working fluid for efficient heat extraction and power conversion. Concentric rings of injection and production wells create a hydraulic divide to confine the stored pressure, CO2, and thermal energy below the caprock seal that overlies the CO2 storage reservoir. This energy storage can take excess power from the grid and excess/waste thermal energy from thermal power plants, and dispatch that energy when it is demanded and thus enable higher penetration of variable renewables, while utilizing thermal energy that would otherwise be wasted. CO2 stored in the subsurface functions as a cushion gas to provide enormous pressure-storage capacity and displace large quantities of brine, which will flow under artificially-created artesian pressure up production wells. Geothermal power generated from produced CO2 and brine and energy-storage applications may generate enough revenues to compensate for (or to even exceed) CO2 capture and storage costs. To address the CCUS deployment barrier of overpressure, we apply a pressure-management strategy that diverts a portion of the produced brine once a target overpressure is reached at the injection wells. The target overpressure is that determined to be low enough to reduce the risk of induced seismicity, caprock fracture, and CO2 leakage. Diverted brine is available for beneficial consumptive use, such as for power-plant cooling, or it can be used to generate fresh water using desalination technologies, such as reverse osmosis. The benefit of water generation can be particularly valuable in water-stressed regions. Our analyses indicate that only a small portion (\u3c 5% unless CO2is stored at a very high rate) of the produced brine needs to be diverted for the injection wells to remain below the target overpressure. Because the required recovery factor for desalination is relatively small ( Our approach has several advantages over conventional (e.g., hydrothermal) and enhanced geothermal energy systems (EGS). CO2 is a very efficient geothermal working fluid. Combined with the benefits of harnessing the overpressure driven by CO2 storage and the greater lateral extent, permeability, and porosity of sedimentary basins, compared to hydrothermal upflows or artificially-created EGS reservoirs, it allows for much greater spacing between injection and production wells. This efficient use of wells enables utilizing resources with lower temperatures than those of typical geothermal systems, resulting in wider deployment potential. The added benefit of bulk energy storage (BES) creates an arbitrage opportunity that enhances economic viability. Our analyses show that BES achieved by time-shifting the parasitic load of pressurizing our system does not reduce the efficiency of driving fluid recirculation; hence our approach is more efficient than other BES technologies, such as lithium-ion batteries or pumped hydro. Because the primary cost of BES is that associated with oversizing the pumps for fluid reinjection, the capital cost is much less than that of other BES approaches. Moreover, the huge capacity of the subsurface can enable seasonal energy storage, while most other approaches are limited to diurnal storage. This study was funded by the U.S. Department of Energy (DOE) Geothermal Technologies Office (GTO) under grant number DE-FOA-0000336, managed by Elisabet Metcalfe and Sean Porse, and a U.S. National Science Foundation (NSF) Sustainable Energy Pathways (SEP) grant (1230691). This work was performed under the auspices of the USDOE by LLNL under contract DE-AC52-07NA2734

The value of CO2-geothermal bulk energy storage to CO2

Two primary challenges for modern societies are to reduce the amount of carbon dioxide (CO2) that is emitted to the atmosphere and to increase the penetration of renewable energy technologies into electricity systems. CO2-bulk energy storage (CO2-BES) is a CO2 capture and storage (CCS) approach that can address both of these challenges by using CO2 emitted from large point sources (e.g., fossil fuel power plants, cement manufacturers) that is sequestered in sedimentary basin geothermal resources to take power from, and deliver power to, electricity grids. Electricity can be generated by wind and solar energy technologies regardless of whether there is demand for that electricity because wind and sunlight are variable resources. When over-generation occurs, the excess electricity can be used to compress and inject CO2 into sedimentary basin geothermal resources. Electricity can then be dispatched when needed by producing the pressurized and geothermally-heated CO2 from the storage reservoir and converting the heat to electricity in a CO2-geothermal power plant. In this way, CO2-BES can time-shift excess electricity that is generated by wind and solar energy facilities to when there is demand for that electricity. This ability can increase the utilization of installed wind and solar energy capacity. Thus, CO2-BES can (1) directly reduce CO2 emissions to the atmosphere by isolating them in porous and permeable subsurface reservoirs and (2) indirectly reduce CO2 emissions by displacing electricity from power plants that emit CO2 (e.g., fossil fuel plants) with electricity from wind and solar energy facilities. We present an approach to estimate the value of these direct and indirect benefits. Our approach uses an optimization model that we developed to determine the cost-minimizing dispatch of electricity-generating facilities to meet diurnal demand in regional electricity systems. In our analysis, electricity can be generated by base load and variable load power plants, wind- and solar-energy technologies, and CO2-BES facilities. We varied prices on CO2 emissions (e.g., a CO2 emissions tax) in order to determine the optimal CO2-BES storage capacity for each CO2 price. This method allows us to assign a monetary value to the optimized energy storage capacity. We use time increments of one hour, during which we assume electricity generation and demand are constant. Initial results using hypothetical but realistic scenarios for electricity demand and electricity generation by solar energy technologies suggest that the optimal energy storage capacity of CO2-BES is sensitive to a range of CO2 prices. That is, a small increase in the price on CO2 emissions can cause substantial change in the optimal distribution of electricity generation and the energy storage capacity of CO2-BES. Thus, independent system operators (ISOs) could dispatch CO2-BES without needing additional ancillary service compensation schemes if CO2 emissions were modestly taxed. This work was funded by the U.S. National Science Foundation Sustainable Energy Pathways program (grant 1230691)

Two-Stage, Integrated, Geothermal-CO2 Storage Reservoirs: An Approach for Sustainable Energy Production, CO2-Sequestration Security, and Reduced Environmental Risk

We introduce a hybrid two-stage energy-recovery approach to sequester CO{sub 2} and produce geothermal energy at low environmental risk and low cost by integrating geothermal production with CO{sub 2} capture and sequestration (CCS) in saline, sedimentary formations. Our approach combines the benefits of the approach proposed by Buscheck et al. (2011b), which uses brine as the working fluid, with those of the approach first suggested by Brown (2000) and analyzed by Pruess (2006), using CO{sub 2} as the working fluid, and then extended to saline-formation CCS by Randolph and Saar (2011a). During stage one of our hybrid approach, formation brine, which is extracted to provide pressure relief for CO{sub 2} injection, is the working fluid for energy recovery. Produced brine is applied to a consumptive beneficial use: feedstock for fresh water production through desalination, saline cooling water, or make-up water to be injected into a neighboring reservoir operation, such as in Enhanced Geothermal Systems (EGS), where there is often a shortage of a working fluid. For stage one, it is important to find economically feasible disposition options to reduce the volume of brine requiring reinjection in the integrated geothermal-CCS reservoir (Buscheck et al. 2012a). During stage two, which begins as CO{sub 2} reaches the production wells; coproduced brine and CO{sub 2} are the working fluids. We present preliminary reservoir engineering analyses of this approach, using a simple conceptual model of a homogeneous, permeable CO{sub 2} storage formation/geothermal reservoir, bounded by relatively impermeable sealing units. We assess both the CO{sub 2} sequestration capacity and geothermal energy production potential as a function of well spacing between CO{sub 2} injectors and brine/CO{sub 2} producers for various well patterns and for a range of subsurface conditions

On the movement of a liquid front in an unsaturated, fractured porous medium, Part 1

The primary aim of this paper is to present approximate analytical solutions of the fracture flow which gives the position of the liquid fracture front as a function of time. These solutions demonstrate that the liquid movement in the fracture can be classified into distinctive time periods, or flow regimes. It is also shown that when plotted versus time using a log-log scale, the liquid fracture front position asymptotically approaches a series of line segments. Two-dimensional numerical simulations were run utilizing input data applicable to the densely welded, fractured tuff found at Yucca Mountain in order to confirm these observations. 19 refs., 15 figs., 8 tabs