Single Heater Test Research Articles

A field study for understanding thermally driven coupled processes in partially saturated fractured welded tuff

As part of a multi-laboratory team, we are carrying out two in situ thermal tests — the single Heater Test and Drift Scale Test, in an underground facility at Yucca Mountain, Nevada, USA, the proposed site for a high-level nuclear waste repository. Our objective in these tests is to gain a more in-depth understanding of the coupled thermal–hydrological–mechanical–chemical processes likely to exist in the fractured rock mass around a geological repository . These coupled processes are monitored continuously by numerous sensors emplaced in boreholes, while cross-hole radar tomography , neutron logging , electrical resistivity tomography, and interference air-permeability tests all serve to measure moisture change in the rock mass. Thermal–hydrological processes for both tests have been simulated (using a 3-D numerical model) and compared to the extensive data set. In this paper, we present examples to illustrate how an iterative approach requiring close integration of modeling and measurements enables us to track the complex coupled processes we seek to understand. The main manifestation of coupled thermal-hydrological processes is in the time evolution of the drying and condensation zones. Good agreement exists between model predictions and measurements, specifically the decrease in air-permeability values within zones of increased liquid saturation in the fractures and the increase of radar velocity in cross-hole radar survey in zones of decreased matrix liquid saturation. A heat-pipe signature in the temperature data arising from liquid–vapor counter-flow occurs in both the measurements and simulated results. The good agreement between predictions from the numerical simulations and measurements in the thermal tests indicates that our basic understanding of the thermal-hydrological processes in a potential repository at Yucca Mountain is sound. However, detailed behavior is impacted by site-specific heterogeneity, in the form of discrete fractures that are not likely to be predictable a priori. One emphasis of the on-going Drift Scale Test is to build on the present understanding and to assess the impact of heterogeneity to the repository performance.

Predictions and observations of the thermal–hydrological conditions in the Single Heater Test

The Single Heater Test (SHT) is one of two in-situ thermal tests included in the site characterization program for the potential underground nuclear waste repository at Yucca Mountain. Coupled thermal–hydrological–mechanical–chemical processes in the fractured rock mass around the heater were monitored by numerous sensors emplaced among 30 boreholes. Periodic active testing of cross-hole radar tomography, neutron logging, electrical resistivity tomography, and interference air permeability tests probed the change of moisture content in the rock mass. Thermal–hydrological processes in the SHT have been simulated using a 3-D numerical model and compared to the monitored data. The good agreement between the temperature data and simulated results indicates that the thermal–hydrological responses of the SHT in the 9 months of heating are well-represented by the coupled thermal–hydrological numerical model. The dominant heat transfer process is by conduction and the signature of vapor and liquid counter flow is subtle in the temperature data. The simulated result of a dry-out zone of about 1 m (at the end of the heating phase) around the heater hole, and a condensation zone of increased liquid saturation outside of the dry-out zone, is consistent with the radar tomography and air permeability data. Tomography data and post-test laboratory measurements indicate that the moisture content is larger below than above the heater horizon, suggesting gravity drainage of condensate in the fractures. Model studies show that gravity drainage occurs in simulations using the dual permeability conceptual model, but is absent in the effective-continuum model, where matrix and fractures are required to be in thermodynamic equilibrium at all times.

The Yucca Mountain Project Drift Scale Test

The Yucca Mountain Project is conducting a Single Heater Test (SHT) in the Exploratory Studies Facility at Yucca Mountain, NV to evaluate coupled thermal-mechanical-hydrologic-chemical processes in situ. This paper describes the thermomechanical aspects of the SHT and presents preliminary results from the test. The 5-m long heater deployed in the SHT was energized on 26 August 1996 with the heater power set at 4 kW. Thermomechanical instrumentation was installed within and on the rock mass surrounding the SHT. The thermomechanical instrumentation includes temperature measurements using thermocouples, RTDs, and thermistors; displacement measurements using multiple-point borehole extensometers (MPBXs), tape extensometers, and surface-mounted wire extensometers; load measurements from load cells on rock bolts installed both within thermally perturbed and ambient regions; and rock mass modulus measurements using the NX borehole jack. Pretest analyses of the thermomechanical response were conducted using the thermohydrologic code TOUGH2 and the thermomechanical structural code JAC3D. The test design, preliminary data, and comparisons between pretest predictions and early data from selected thermal and mechanical sensors are presented.

Post-test Comparison of Thermalhyrologic Measurements and Numerical Predictions for theIn-Situ Single Heater Test, Yucca Mountain, Nevada

The Single Heater Test (SHT) is a sixteen-month-long heating and cooling experiment begun in August, 1996, located underground within the unsaturated zone near the potential geologic repository at Yucca Mountain, Nevada. During the 9 month heating phase of the test, roughly 15 m{sup 3} of rock were raised to temperatures exceeding 100 C. In this paper, temperatures measured in sealed boreholes surrounding the heater are compared to temperatures predicted by 3D thermal-hydrologic calculations performed with a finite difference code. Three separate model runs using different values of bulk rock permeability (4 microdarcy to 5.2 darcy) yielded significantly different predicted temperatures and temperature distributions. All the models differ from the data, suggesting that to accurately model the thermal-hydrologic behavior of the SHT, the Equivalent Continuum Model (ECM), the conceptual basis for dealing with the fractured porous medium in the numerical predictions, should be discarded in favor of more sophisticated approaches.

Site-specific Thermal and Mechanical Property Characterizations ofIn-Situ Thermal Test Areas at Yucca Mountain, Nevada

The US Department of Energy (DOE) is investigating Yucca Mountain, Nevada as a potential site for the disposal of high-level nuclear waste. The site is located near the southwest corner of the Nevada Test Site (NTS) in southern Nye County, Nevada. The underground Exploratory Studies Facility (ESF) tunnel traverses part of the proposed repository block. Alcove 5, located within the ESF is being used to field two in situ ESF thermal tests: the Single Heater Test (SHT) and the Drift Scale Test (DST). Laboratory test specimens were collected from three sites within Alcove 5 including each in situ field test location and one additional site. The aim of the laboratory tests was to determine site-specific thermal and mechanical rock properties including thermal expansion, thermal conductivity, unconfined compressive strength, and elastic moduli. In this paper, the results obtained for the SHT and DST area characterization are compared with data obtained from other locations at the proposed repository site. Results show that thermal expansion, and mechanical properties of Alcove 5 laboratory specimens are slightly different than the average values obtained on specimens from surface drillholes.

Preliminary thermomechanical results of a heater test in welded tuff

The Yucca Mountain Project is conducting a Single Heater Test (SHT) in the Exploratory Studies Facility at Yucca Mountain, NV to evaluate coupled thermal-mechanical-hydrologic-chemical processes in situ . This paper describes the thermomechanical aspects of the SHT and presents preliminary results from the test. The 5-m long heater deployed in the SHT was energized on 26 August 1996 with the heater power set at 4 kW. Thermomechanical instrumentation was installed within and on the rock mass surrounding the SHT. The thermomechanical instrumentation includes temperature measurements using thermocouples, RTDs, and thermistors; displacement measurements using multiple-point borehole extensometers (MPBXs), tape extensometers, and surface-mounted wire extensometers; load measurements from load cells on rock bolts installed both within thermally perturbed and ambient regions; and rock mass modulus measurements using the NX borehole jack. Pretest analyses of the thermomechanical response were conducted using the thermohydrologic code TOUGH2 and the thermomechanical structural code JAC3D. The test design, preliminary data, and comparisons between pretest predictions and early data from selected thermal and mechanical sensors are presented.

Coupled HTM modelling of cold water injection in fractured hydrocarbon reservoirs

The Yucca Mountain Project is conducting a Single Heater Test (SHT) in the Exploratory Studies Facility at Yucca Mountain, NV to evaluate coupled thermal-mechanical-hydrologic-chemical processes in situ . This paper describes the thermomechanical aspects of the SHT and presents preliminary results from the test. The 5-m long heater deployed in the SHT was energized on 26 August 1996 with the heater power set at 4 kW. Thermomechanical instrumentation was installed within and on the rock mass surrounding the SHT. The thermomechanical instrumentation includes temperature measurements using thermocouples, RTDs, and thermistors; displacement measurements using multiple-point borehole extensometers (MPBXs), tape extensometers, and surface-mounted wire extensometers; load measurements from load cells on rock bolts installed both within thermally perturbed and ambient regions; and rock mass modulus measurements using the NX borehole jack. Pretest analyses of the thermomechanical response were conducted using the thermohydrologic code TOUGH2 and the thermomechanical structural code JAC3D. The test design, preliminary data, and comparisons between pretest predictions and early data from selected thermal and mechanical sensors are presented.

Thermal—mechanical—hydrological—chemical responses in the single heater test at the ESF

Mapping microearthquakes induced by hydraulic fracturing can delineate fracture systems produced by the stimulation, and can provide spatial resolution as fine as 10 to 20 m. Microseismicity provides the only detailed 3-D data about the fracture system away from boreholes. Yet, since the microseismicity results from slip on joints or fractures, it may not provide a reliable view of the conductive fractures or joints. A more direct approach to identifying fluid pathways is needed. The presence of fluids within the rock reduces the seismic wave velocities, particularly for shear waves, and can provide a direct seismic indication of major fluid paths. Although seismic tomographic techniques can determine the velocity structure of rock masses, only relatively coarse spatial resolution of perhaps 100–200 m has been achieved because of the limited number of observation sites. This resolution is not adequate for modeling fracture systems. Finer spatial resolution may be achieved by imaging of seismic scatterers using their signatures in the waveforms of the microearthquakes. This approach exploits secondary arrivals contained within the seismic coda of the microearthquakes. These secondary arrivals are presumed to arise because of scattering of seismic waves by some of the major individual fluid paths within the fracture system. Imaging of the strong scattering features may allow identification of the major fluid paths.

Thermal-mechanical-hydrological-chemical responses in the single heater test at the ESF

On-going Single Heater Tests (SHTs) are conducted in the Exploratory Studies Facility (ESF) to study the thermal-mechanical responses of the rock mass. In addition to the heater hole and those used for Sandia National Laboratory's (SNL's) thermal-mechanical studies, a set of 14 boreholes were drilled in the test region to conduct a scoping test of the coupled thermal-mechanical-hydrological-chemical (TMHC) processes. For the TMHC tests, the boreholes are equipped with electrical resistivity tomography (ERT), neutron logging/temperature, hydrological, and optical multiple point borehole extensometer measurement devices. The heater hole contains a 4-kW heater, which was energized on August 26, 1996. Our results show that some movements of the water around the heater have occurred, which might be associated with a possible dry-out region near the heater, and the water appears to be more dilute than the in situ ground water, except for the concentration of Ca. This indicates that fractures are the major water pathways, and the displaced water may have reached an equilibrium with carbonate minerals on the fracture surfaces. No mechanical-hydrological coupling has been observed yet.