Results for “THERMAL STRESSES” — Page 91

DOE OSTI2016

Applications of Transient Grating Spectroscopy to Radiation Materials Science

Radiation damage is an inherently multi-scale phenomenon, with much of the key material evolution occurring at the mesoscale. Current techniques for the direct interrogation of material property changes due to irradiation either function with very small volumes, such as in-situ TEM, cannot cover the full range of defect populations produced, such as in positron annihilation spectroscopy, or requires destruction of the test material, such as in standard or micromechanical testing. In addition, the ultimate quantities of interest to radiation materials scientists are the changes in key material properties with irradiation. These include stiffness, ductility, and thermal conductivity for structural materials, and optical properties such as index of refraction or changes in transmittance spectra for optically active materials. The latter of these are often correlated directly to stress- or thermally-induced changes, themselves a function of the specific defect populations responsible for their change. The rapid study of such a mesoscale phenomenon requires an inherently mesoscale technique, where multiple quantities of interest can be accurately measured in very short times. Determining the sensitivity of any technique with known certainly is of critical importance to qualifying a new technique to measure in-situ material property changes during irradiation. In this paper, we present the new applicability of an existing technique, known as transient grating (TG) spectroscopy, to rapidly and accurately study key problems in radiation materials science. Results quantifying the absolute sensitivity of the technique are presented, alongside molecular dynamics (MD) simulations of the TG spectroscopy process to gauge direct agreement between theory, simulation, and experiment. An example problem of interest is demonstrated as an illustration of what is possible, showing elastic and thermal changes in irradiated silicon carbide (SiC) films. Finally, future work to quantify the effects of experimentally-simulated radiation defects is proposed, in support of building a database to use TG spectroscopy to deconvolve irradiated material property changes in-situ in terms of the defect populations responsible for their evolution. The application of the TG spectroscopy technique to in-situ radiation materials science has the potential to answer long-standing questions in the field, relating to ion beam rastering, the onset of void swelling, and direct measurement of radiation-created point defect concentrations. Before it can do so, the technique must carefully be analyzed and tuned to increase its sensitivity, the certainty of its results, and the ability to predict radiation-induced changes by simulations must be developed. Making such measurements in-situ with the aid of predictive MD simulations has the potential to elucidate mechanisms of radiation damage accumulation, as well as more rapidly qualify radiation-resistant alloys for service in nuclear systems. Work is underway to build this database of experimentally-simulated radiation-produced defects, which will be combined and compared with actual irradiated materials for eventual in-situ deconvolution of the TG signal into the defect populations responsible for irradiation-induced microstructural change. (authors)

36 MATERIALS SCIENCE↗

DOE OSTI2024

Inelastic Deformation in Methylammonium Lead Iodide Perovskite and Mitigation by Additives during Thermal Cycling

Metal halide perovskite thin films are promising materials for next-generation photovoltaic applications, but the thermomechanical instabilities of these materials are critical barriers to device longevity. In this study, we measure the evolution of stresses in methylammonium lead iodide (MAPbI3) films thermally cycled from 25 to 105 °C using a multibeam optical stress sensor (MOSS) system. We demonstrate that these films undergo residual stress buildup due to inelastic deformation as well as a simple mitigation strategy by incorporating low concentrations of 5-aminovaleric acid hydrochloride (5-AVACl) as an additive. Adding 5-AVACl increases the tensile stresses measured at room temperature but beneficially decreases the biaxial modulus from 9.60 ± 0.44 GPa to 7.95 ± 0.55 GPa, resulting in greater accommodation of thermal stresses. The hysteresis loop is ~63% smaller with the inclusion of 5-AVACl, and the stress relaxation at 50 °C decreases from 86.46 to 51.49 MPa. The larger stress relaxation in pristine MAPbI 3 is correlated with grain boundary opening after five cycles, while the inclusion of 5-AVACl mitigates this degradation. Furthermore, our findings underscore the importance of studying the dynamic response of perovskite films during thermal cycling and lay the foundation for further exploration into the mechanisms governing the thermomechanical behavior of perovskite thin films.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1990

Thermal effects on fiber optic strain sensors embedded in graphite-epoxy composites

Smart structures deployed in low earth orbit will be exposed to a hostile environment which will include temperature extremes during each orbit as the platform moves from the day to the night side. The stresses due to thermal cycling may compromise the performance of embedded fiber optic strain sensors because of differential thermal expansion of the fiber and the composite material. The effects of elevated temperature and thermal cycling on the performance of a fiber optic strain sensor embedded in a graphite-epoxy tube have been investigated for temperatures from 65 to 220 F. The results indicate that the fiber optic strain sensor measurements correlate well with conventional resistance strain gages attached to the tube.

Rogowski, R. S.↗

DOE OSTI2015

Direct measurements of irradiation-induced creep in micropillars of amorphous Cu{sub 56}Ti{sub 38}Ag{sub 6}, Zr{sub 52}Ni{sub 48}, Si, and SiO{sub 2}

We report in situ measurements of irradiation-induced creep on amorphous (a-) Cu{sub 56}Ti{sub 38}Ag{sub 6}, Zr{sub 52}Ni{sub 48}, Si, and SiO{sub 2}. Micropillars 1 μm in diameter and 2 μm in height were irradiated with ∼2 MeV heavy ions during uniaxial compression at room temperature. The creep measurements were performed using a custom mechanical testing apparatus utilizing a nanopositioner, a silicon beam transducer, and an interferometric laser displacement sensor. We observed Newtonian flow in all tested materials. For a-Cu{sub 56}Ti{sub 38}Ag{sub 6}, a-Zr{sub 52}Ni{sub 48}, a-Si, and Kr{sup +} irradiated a-SiO{sub 2} irradiation-induced fluidities were found to be nearly the same, ≈3 GPa{sup −1} dpa{sup −1}, whereas for Ne{sup +} irradiated a-SiO{sub 2} the fluidity was much higher, 83 GPa{sup −1} dpa{sup −1}. A fluidity of 3 GPa{sup −1} dpa{sup −1} can be explained by point-defect mediated plastic flow induced by nuclear collisions. The fluidity of a-SiO{sub 2} can also be explained by this model when nuclear stopping dominates the energy loss, but when the electronic stopping exceeds 1 keV/nm, stress relaxation in thermal spikes also contributes to the fluidity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1984

Shell tile thermal protection system

A reusable, externally applied thermal protection system for use on aerospace vehicles subject to high thermal and mechanical stresses utilizes a shell tile structure which effectively separates its primary functions as an insulator and load absorber. The tile consists of structurally strong upper and lower metallic shells manufactured from materials meeting the thermal and structural requirements incident to tile placement on the spacecraft. A lightweight, high temperature package of insulation is utilized in the upper shell while a lightweight, low temperature insulation is utilized in the lower shell. Assembly of the tile which is facilitated by a self-locking mechanism, may occur subsequent to installation of the lower shell on the spacecraft structural skin.

Macconochie, I. O.↗

NASA NTRS2024

Evaluating Liftoff Debris for NASA’s Space Launch System (SLS) Prior to the Artemis I Launch

The SLS Artemis I launch vehicle is the first of several planned Artemis launch vehicles, with a number of design differences from earlier NASA missions that incur liftoff debris risk to the mission. As a test vehicle, the Artemis I hardware also endured environments and tests not planned for future missions, which led to several additional factors contributing to an evolving liftoff debris risk to the SLS vehicle. This paper will summarize these risk factors and address the processes used to evaluate and communicate the risks to support a successful Artemis I launch. It will discuss how the evolving risks that were quantified and evaluated by a Cross-Program team of debris Subject Matter Experts to mitigate liftoff debris hazards and communicate updated risk to the SLS vehicle. This process was performed through the inaugural use of an SLS debris day-of-launch (DOL) standard operating procedure that will be used for subsequent Artemis missions. This paper addresses the risk of liftoff debris, debris released by the vehicle or from the launch pad during liftoff through vehicle tower clear. Expected liftoff debris is well understood from previous NASA programs’ experience and from tests of materials, processes and functions that are known to release liftoff debris. These expected sources were assessed and cleared well ahead of launch day. However, given the ever-changing schedules and environments, processes were in place to evaluate any additional potential liftoff debris risks identified during launch countdown. Although many of the Artemis vehicle hardware components are similar to those on the NASA Shuttle Program, there are important differences in the architecture of the Artemis I vehicle which require new assessments of liftoff debris risk for the Artemis missions. The more favorable Artemis crew module location and surfaces are far less vulnerable to debris impacts; however, the longer vehicle can result in higher liftoff debris impact energies to those components on the aft end of the vehicle. Additionally, the positional change of the RS-25 liquid engines to nearer the Booster nozzle exit plane along with the change in Booster throat plug design is a disadvantage to the overall liftoff debris risk which resulted in additional test and analysis efforts for evaluating the integrated vehicle debris risk. In spite of the comprehensive tests and analyses of Artemis I expected liftoff debris, a number of additional tests/processes were completed prior to the Artemis I mission that were required to support a complete understanding of a new launch vehicle, but increased the risk of releasing liftoff debris. The hardware endured several additional cryogenic loading cycles, including the Green Run tests at Stennis Space Center, Wet Dress Rehearsals at Kennedy Space Center, and multiple launch attempts. Each of these cycles induced stresses in the thermal protection system (TPS) materials, increasing the risk of damage to and release of the TPS. Additionally, induced and weather environmental factors that could increase the likelihood of debris release were significant. Vibrations and stresses in the TPS were induced by a required roll-back to the Vehicle Assembly Building before Hurricane Ian to protect the vehicle from damage by high winds. Wind damage and potential internal stresses to several outer mold line materials on the integrated SLS vehicle and mobile launcher were caused by weathering Hurricane Nicole at Pad 39B the week before launch. A thorough imagery scan of the vehicle was performed after each event and the damage observed was repaired, removed, or assessed and the risk to the mission evaluated. Mitigation of debris risk can occur by tests and analyses to show debris impacted components as damage tolerant, by new/improved processes for prevention of debris availability, or redesign. Risk mitigation processes for Artemis I-specific liftoff debris events and the development and use of the SLS debris day of launch (DOL) procedures that will be used for subsequent Artemis missions will be described.

Space Launch System↗

NASA NTRSDate not supplied

Evaluating Liftoff Debris for NASA’s Space Launch System (SLS) Prior to the Artemis I Launch

The SLS Artemis I launch vehicle is the first of several planned Artemis launch vehicles, with a number of design differences from earlier NASA missions that incur liftoff debris risk to the mission. As a test vehicle, the Artemis I hardware also endured environments and tests not planned for future missions, which led to several additional factors contributing to an evolving liftoff debris risk to the SLS vehicle. This paper will summarize these risk factors and address the processes used to evaluate and communicate the risks to support a successful Artemis I launch. It will discuss how the evolving risks that were quantified and evaluated by a Cross-Program team of debris Subject Matter Experts to mitigate liftoff debris hazards and communicate updated risk to the SLS vehicle. This process was performed through the inaugural use of an SLS debris day-of-launch (DOL) standard operating procedure that will be used for subsequent Artemis missions. This paper addresses the risk of liftoff debris, debris released by the vehicle or from the launch pad during liftoff through vehicle tower clear. Expected liftoff debris is well understood from previous NASA programs’ experience and from tests of materials, processes and functions that are known to release liftoff debris. These expected sources were assessed and cleared well ahead of launch day. However, given the ever-changing schedules and environments, processes were in place to evaluate any additional potential liftoff debris risks identified during launch countdown. Although many of the Artemis vehicle hardware components are similar to those on the NASA Shuttle Program, there are important differences in the architecture of the Artemis I vehicle which require new assessments of liftoff debris risk for the Artemis missions. The more favorable Artemis crew module location and surfaces are far less vulnerable to debris impacts; however, the longer vehicle can result in higher liftoff debris impact energies to those components on the aft end of the vehicle. Additionally, the positional change of the RS-25 liquid engines to nearer the Booster nozzle exit plane along with the change in Booster throat plug design is a disadvantage to the overall liftoff debris risk which resulted in additional test and analysis efforts for evaluating the integrated vehicle debris risk. In spite of the comprehensive tests and analyses of Artemis I expected liftoff debris, a number of additional tests/processes were completed prior to the Artemis I mission that were required to support a complete understanding of a new launch vehicle, but increased the risk of releasing liftoff debris. The hardware endured several additional cryogenic loading cycles, including the Green Run tests at Stennis Space Center, Wet Dress Rehearsals at Kennedy Space Center, and multiple launch attempts. Each of these cycles induced stresses in the thermal protection system (TPS) materials, increasing the risk of damage to and release of the TPS. Additionally, induced and weather environmental factors that could increase the likelihood of debris release were significant. Vibrations and stresses in the TPS were induced by a required roll-back to the Vehicle Assembly Building before Hurricane Ian to protect the vehicle from damage by high winds. Wind damage and potential internal stresses to several outer mold line materials on the integrated SLS vehicle and mobile launcher were caused by weathering Hurricane Nicole at Pad 39B the week before launch. A thorough imagery scan of the vehicle was performed after each event and the damage observed was repaired, removed, or assessed and the risk to the mission evaluated. Mitigation of debris risk can occur by tests and analyses to show debris impacted components as damage tolerant, by new/improved processes for prevention of debris availability, or redesign. Risk mitigation processes for Artemis I-specific liftoff debris events and the development and use of the SLS debris day of launch (DOL) procedures that will be used for subsequent Artemis missions will be described.

Space Launch System↗

DOE OSTI2025

Evaporator Temperature Transient Testing of a High-Performance Sodium Filled Heat Pipe

Heat pipes are two-phase heat transfer devices that enable passive removal of heat from the reactor core to the power conversion system in heat pipe-cooled microreactor designs. Experimental investigations of heat pipe transients are needed for technology demonstration, verification and validation of numerical codes, and the establishment of regulatory requirements. The Single Primary Heat Extraction and Removal Emulator (SPHERE) facility at Idaho National Laboratory (INL) serves as a platform for evaluating the dynamic response of high-temperature heat pipes under a variety of operating conditions. The present work details the experimental investigation of a high-performance, defined as over 2 kW sodium heat pipe subjected to rapid input power fluctuations induced by sudden changes in the evaporator temperature setpoint. In addition, the heat pipe was subjected to an asymmetrical heat load where a subset of heaters operated at 30% and 70% below their nominal power. These experimental conditions were chosen to simulate thermal and operational stresses expected to be encountered in microreactors to provide data on heat pipe behavior during such important transient events. Key data and performance metrics, including time series of temperatures and strains, axial temperature profiles, thermal response times, and heat transfer capabilities, the thermal output over thermal input, were reported and discussed. The results highlight the resilience of heat pipes, revealing their potential to maintain thermal stability and efficiency under varying power loads. Lastly, the paper concludes with a discussion on the significance of the results and their implications for future research.

22 - GENERAL STUDIES OF NUCLEAR REACTORS↗

DOE OSTI2020

Modeling The Effects of Loading Scenario and Thermal Expansion Coefficient on Potential Failure of Cryo-compressed Hydrogen Vessels

A multiscale thermomechanical model for a simplified Type-3 cryogenic compressed hydrogen (H2) storage vessel is developed in this paper. The model accounts for the temperature-dependent elastic-plastic behavior of the vessel carbon/epoxy composite overwrap and aluminum alloy liner. The homogenized thermo-elastic-plastic behavior for the individual laminas of the vessel layup is obtained by an incremental Eshelby-Mori-Tanka approach associated with a micromechanical failure criterion to predict lamina failure while a standard elastic-plastic constitutive model is used to describe the behavior of a typical aluminum alloy assumed for the liner. The vessel response to external loadings is achieved by a finite element method. Four loading scenarios representing four thermomechanical cycles applied to the vessel are analyzed to evaluate constituent and lamina stresses as well as the associate failure criterion during the cycle according to these scenarios. The model can provide helpful guidance to mitigate thermal stresses by an adequate selection of loading scenario, optimizing the layup and by tailoring thermomechanical properties of the resin matrix.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Thermophysical property and pore structure evolution in stressed and non-stressed neutron irradiated IG-110 nuclear graphite

The nuclear graphite, IG-110, was irradiated with and without a compressive load of 5 MPa at ~400 *C up to 9.3E25 n/m2 (E > 0.1 MeV). Following irradiation physical properties were studied to compare the effect of graphite irradiation on microstructure developed under compression and in stress-free conditions. Properties included: dimensional change, thermal conductivity, dynamic modulus, and CTE. The effect of stress on open internal porosity was determined through nitrogen adsorption. The IG-110 graphite experienced irradiation-induced creep that is differentiated from irradiation-induced swelling. Irradiation under stress resulted in somewhat greater thermal conductivity and coefficient of thermal expansion. While a significant increase in dynamic modulus occurs, no differentiation between materials irradiated with and without compressive stress was observed. Nitrogen adsorption analysis suggests a difference in pore evolution in the 0.3e40 nm range for graphite irradiated with and without stress, but this evolution is seen to be a small contributor to the overall dimensional change.

Snead, Lance↗

NASA NTRS1981

A test and instrumentation system for the investigation of degradation of electrical insulating materials

A comprehensive test apparatus has been developed for the evaluation of the effects of thermal cycling, high temperatures and high electric fields on the quality and stability of electrical insulating materials used in spacecraft systems. The system includes a test chamber in which the specimen can be subjected to thermal and electrical stresses encountered in the space environment (temperatures from -50 to 200 C and voltages from 0 to 10 kV rms, respectively) simultaneously. The system instrumentation includes provisions for the measurement of the change in capacitance, the thermal dissipation factor of the material, and complex dielectric constants with changes in temperature, corona intensity and voltages at frequencies from 22 kHz to 70 MHz, as well as the volume and surface resistivities of the materials.

Doreswamy, C. V.↗

NASA NTRS2000

Thermoelastic Stress Analysis: An NDE Tool for the Residual Stress Assessment of Metallic Alloys

During manufacturing, certain propulsion components that will be used in a cyclic fatigue environment are fabricated to contain compressive residual stresses on their surfaces because these stresses inhibit the nucleation of cracks. Overloads and elevated temperature excursions cause the induced residual stresses to dissipate while the component is still in service, lowering its resistance to crack initiation. Research at the NASA Glenn Research Center at Lewis Field has focused on employing the Thermoelastic Stress Analysis technique (TSA, also recognized as SPATE: Stress Pattern Analysis by Thermal Emission) as a tool for monitoring the residual stress state of propulsion components. TSA is based on the fact that materials experience small temperature changes when they are compressed or expanded. When a structure is cyclically loaded (i.e., cyclically compressed and expanded), the resulting surface-temperature profile correlates to the stress state of the structure s surface. The surface-temperature variations resulting from a cyclic load are measured with an infrared camera. Traditionally, the temperature amplitude of a TSA signal has been theoretically defined to be linearly dependent on the cyclic stress amplitude. As a result, the temperature amplitude resulting from an applied cyclic stress was assumed to be independent of the cyclic mean stress.

Gyekenyesi, Andrew L.↗

NASA NTRS1969

Convection and stress propagation in the upper mantle.

Dynamic processes in upper earth mantle, discussing thermal convection currents and stress propagation

Elsasser, W. M.↗

NASA NTRS1986

Thermal barrier coating life prediction model development

A methodology is established to predict thermal barrier coating life in a environment similar to that experienced by gas turbine airfoils. Experiments were conducted to determine failure modes of the thermal barrier coating. Analytical studies were employed to derive a life prediction model. A review of experimental and flight service components as well as laboratory post evaluations indicates that the predominant mode of TBC failure involves thermomechanical spallation of the ceramic coating layer. This ceramic spallation involves the formation of a dominant crack in the ceramic coating parallel to and closely adjacent to the topologically complex metal ceramic interface. This mechanical failure mode clearly is influenced by thermal exposure effects as shown in experiments conducted to study thermal pre-exposure and thermal cycle-rate effects. The preliminary life prediction model developed focuses on the two major damage modes identified in the critical experiments tasks. The first of these involves a mechanical driving force, resulting from cyclic strains and stresses caused by thermally induced and externally imposed mechanical loads. The second is an environmental driving force based on experimental results, and is believed to be related to bond coat oxidation. It is also believed that the growth of this oxide scale influences the intensity of the mechanical driving force.

Demasi, J. T.↗

NASA NTRS2011

Simulating Thermal Cycling and Isothermal Deformation Response of Polycrystalline NiTi

A microstructure-based FEM model that couples crystal plasticity, crystallographic descriptions of the B2-B19' martensitic phase transformation, and anisotropic elasticity is used to simulate thermal cycling and isothermal deformation in polycrystalline NiTi (49.9at% Ni). The model inputs include anisotropic elastic properties, polycrystalline texture, DSC data, and a subset of isothermal deformation and load-biased thermal cycling data. A key experimental trend is captured.namely, the transformation strain during thermal cycling is predicted to reach a peak with increasing bias stress, due to the onset of plasticity at larger bias stress. Plasticity induces internal stress that affects both thermal cycling and isothermal deformation responses. Affected thermal cycling features include hysteretic width, two-way shape memory effect, and evolution of texture with increasing bias stress. Affected isothermal deformation features include increased hardening during loading and retained martensite after unloading. These trends are not captured by microstructural models that lack plasticity, nor are they all captured in a robust manner by phenomenological approaches. Despite this advance in microstructural modeling, quantitative differences exist, such as underprediction of open loop strain during thermal cycling.

Manchiraju, Sivom↗

DOE OSTI2024

Environmentally Assisted Fatigue in Light Water Reactor Environment

This report summarizes the Environmentally Assisted Fatigue (EAF) research conducted at ANL under the US DOE Light Water Reactor Sustainability (LWRS) program. Starting from a rich background in theoretical and experimental EAF, ANL previously developed an approach to evaluate fatigue performance of reactor materials in light water reactor environments with the correction factor F en . The approach was based on a large body of experimental work performed at ANL and elsewhere, and was consistent with American Society of Mechanical Engineers (ASME)’s methodology governing the design and construction of reactor components. In recent years, the program was focused on component fatigue prediction and made several major and fundamental contributions in this area. These accomplishments help meet the needs identified by the industry concerning component level fatigue predictions in complex, transient conditions. The main contribution of the ANL program involved the development of a system-level model for estimating residual strain and life of nuclear reactor coolant system components under connected-system-thermal-mechanical boundary conditions. The goal was to predict the stress hotspots, strain residuals, strain amplitudes and the resulting fatigue lives. Thermal-mechanical stress analysis was performed considering thermal stratification and a design-basis reactor loading cycle. Based on the finite element (FE) model results, the strain residuals, strain amplitudes and resulting fatigue lives of reactor coolant system (RCS) components were predicted. The results show that some of the RCS components can have significantly different strain amplitudes, residual strain, and fatigue lives, despite having similar geometry and material. In addition, the simulated component-level strain profile can guide the selection of appropriate test inputs for conducting laboratory-scale EAF tests. Building upon the system-level model, ANL developed a digital twin (DT) framework to predict the structural states and associated fatigue life of components in real-time. This framework is a comprehensive system designed to predict the structural states and fatigue lives of reactor components. It includes multiple models and integrates artificial intelligence (AI), machine learning (ML), and FE based modeling tools to evaluate the structural states and fatigue lives.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗

NASA NTRS1982

Failure mechanisms of thermal barrier coatings exposed to elevated temperatures

The failure of a ZrO2-8%Y2O3/Ni-14% Al-0.1% Zr coating system on Rene 41 in Mach 0.3 burner rig tests was characterized. High flame and metal temperatures were employed in order to accelerate coating failure. Failure by delamination was shown to precede surface cracking or spalling. This type of failure could be duplicated by cooling down the specimen after a single long duration isothermal high temperature cycle in a burner rig or a furnace, but only if the atmosphere was oxidizing. Stresses due to thermal expansion mismatch on cooling coupled with the effects of plastic deformation of the bond coat and oxidation of the irregular bond coat are the probable life limiting factors. Heat up stresses alone could not fail the coating in the burner rig tests. Spalling eventually occurs on heat up but only after the coating has already failed through delamination.

Miller, R. A.↗

DOE OSTI2015

Numerical modeling of injection, stress and permeability enhancement during shear stimulation at the Desert Peak Enhanced Geothermal System

Creation of an Enhanced Geothermal System relies on stimulation of fracture permeability through self-propping shear failure that creates a complex fracture network with high surface area for efficient heat transfer. In 2010, shear stimulation was carried out in well 27-15 at Desert Peak geothermal field, Nevada, by injecting cold water at pressure less than the minimum principal stress. An order-of-magnitude improvement in well injectivity was recorded. In this work, we describe a numerical model that accounts for injection-induced stress changes and permeability enhancement during this stimulation. We use the coupled thermo-hydrological–mechanical simulator FEHM to (i) construct a wellbore model for non-steady bottom-hole temperature and pressure conditions during the injection, and (ii) apply these pressures and temperatures as a source term in a numerical model of the stimulation. A Mohr–Coulomb failure criterion and empirical fracture permeability is developed to describe permeability evolution of the fractured rock. The numerical model is calibrated using laboratory measurements of material properties on representative core samples and wellhead records of injection pressure and mass flow during the shear stimulation. The model captures both the absence of stimulation at low wellhead pressure (WHP ≤1.7 and ≤2.4 MPa) as well as the timing and magnitude of injectivity rise at medium WHP (3.1 MPa). Results indicate that thermoelastic effects near the wellbore and the associated non-local stresses further from the well combine to propagate a failure front away from the injection well. Elevated WHP promotes failure, increases the injection rate, and cools the wellbore; however, as the overpressure drops off with distance, thermal and non-local stresses play an ongoing role in promoting shear failure at increasing distance from the well.

15 GEOTHERMAL ENERGY↗