Thermal-hydro-mechanical Research Articles

Thermal management associated with geologic disposal of large spent nuclear fuel canisters in tunnels with thermally engineered backfill

Coupled thermal-hydro-mechanical (THM) numerical modeling is conducted to study thermal management associated with geologic disposal of spent nuclear fuel (SNF) in large dual-purpose canisters (DPCs). DPCs, are containers designed for SNF storage and transportation and if determined to be feasible for permanent geological disposal could provide a cost effective disposal solution. However, one of the challenges to direct disposal of DPCs is thermal management to avoid overheating of the Engineered Barrier System (EBS), including bentonite-backfill used as a protective buffer. The model simulations show that the use of a backfill that is thermally engineered for high heat conduction can reduce the EBS temperature to acceptable levels for disposal of large waste canisters in backfilled tunnels. On the other hand, the use of high heat conduction backfill will not reduce the far field rock peak temperature that can occur several thousand years after closure of the repository. This longer term host rock peak temperature generates thermal-poro-elastic stress and geomechanical changes that must be considered in the thermal management and design of a repository.

Numerical analysis of heat extraction efficiency in a multilateral-well enhanced geothermal system considering hydraulic fracture propagation and configuration

Studying the effects of physical-based hydraulic fracture configurations on heat extraction performance in a multi-lateral enhanced geothermal system (EGS) can provide meaningful suggestions for in-situ operations. In this work, we proposed a 3D thermal-hydro-mechanical (THM) coupled numerical model that is capable to simulate both hydraulic fracturing and heat extraction in dual porous and fracture medium. Using this model, the exploitation of deep geothermal energy in a multilateral-well EGS from creation and configuration of hydraulic fractures, to thermal extraction was simulated and comprehensively studied. Based on the results, the following conclusions were drawn: 1) configuration of fracture pattern in a multi-lateral well EGS was affected by in-situ geological conditions (e.g. in-situ stress state) and operation strategies (e.g. sequence of hydraulic fracturing operation, position of perforation for hydraulic fracturing etc.); 2) different fracture configurations lead to great differences in heat production. The final thermal power of the worst configuration was 5.55 MW, while that of the best configuration was approximately 7.25 MW in this study; 3) a zipper fracture configuration is advantageous for homogeneously heat reduction in the geothermal reservoir that is preferable for efficient heat extraction; 4) stress shadow induced by simultaneous hydraulic fracturing can be used to create a non-connected zipper fracture pattern that can improve the heat extraction performance in the fractures at the toe side.

Densified wood impregnated with phenol resin for reduced set-recovery

ABSTRACT Thermal-hydro-mechanical (THM) densification treatments were performed on Populus spp. (poplar), Picea abies (spruce), and Fagus sylvatica (beech). Prior to THM treatment, a low-molecular weight resin was impregnated into the wood structure. Densification results, mechanical characteristics (modulus of elasticity, rupture, and hardness), and set-recovery were assessed. All wood species were exposed to the same THM treatment and the specific anatomy, density, and physical behaviors yielded results specific to wood species making comparisons between species difficult. However, the mechanical performance of all samples was largely dictated by the specimens' surface density as seen in the density profiles. Spruce specimens had high levels of set-recovery after the THM treatment, which is believed to be caused by pit aspiration from kiln drying, limiting impregnation of the resin. The THM treatment used in this study was successfully applied to poplar, spruce, and beech. The resin impregnation combined with THM treatment led to densified wood with improved mechanical properties. The improvements were the greatest in poplar, including the largest reduction in set-recovery, which was the objective of this study. When comparing mechanical properties to commercially available structural wood-composites the results from this study indicate that resin impregnated THM treated wood could be used in this application.

Coupled Numerical Simulation of Thermal-Reactive Flow and Geomechanics with Solid Mass Conversion

Summary An extension of the novel fully coupled thermal-hydromechanical (THM) open-system geomechanics (OSG) model is developed for the analysis of coupled flow and oil-shale geomechanics. The model is cast within the framework of Biot's elasticity and classical thermoelasticity. A new term in the equations accounting for the effects of pyrolysis is included in the present model to capture the removal of mass from the reactive solid phase because of the kerogen-conversion process modeled through a simplified chemical-reaction model. The proposed novel formulation approach and its numerical implementation differ from traditional methods and offer a step improvement in the geomechanical modeling of thermally reactive porous media, such as oil shales. The numerical implementation of the OSG model, a first in the literature, is developed within the framework of a proprietary coupled thermal-reactive flow and geomechanics simulator, which was extensively validated in a previous publication. In this paper, we compare the thermal-reactive OSG model against experimental measurements. Numerical results from the validation test show that the model captures the fundamental physical behavior of oil-shale geomechanics realistically and correctly. Parametric analyses of the OSG model indicate that the chemical conversion term is the critical term that dictates the magnitude of compaction in the solid equation, and a further investigation illustrates the importance of the mobility term in the pore-pressure buildup. It is also noticed that the initiation and the rate of compaction of the oil-shale sample are governed by the chemical activation energy and the reaction-rate constant.

Crack initiation rate of brittle rock under thermal-hydro-mechanical coupling condition

Abstract A calculation formula of thermal-hydro-mechanical (THM) coupling crack initiation rate for brittle rock was derived based on the energy conservation law. The self-designed THM coupling fracture test with conductive adhesive electrical measurement method was applied to measuring the THM coupling crack propagation rate of brittle rock continuously. Research results show that both calculation and test results of crack initiation rate increased with increase of the temperature and the hydraulic pressure. They are almost in good agreement, which can prove validity of the calculation formula of THM coupling crack initiation rate.

Comprehensive stability analysis of an inclined wellbore embedded in Colorado shale formation for thermal recovery

Thermal processes have been widely used to recover heavy oil from oil-sands reservoirs, where the overburden shale undergoes cyclic heating and cooling processes. During thermal operations, thermally induced pore pressure can be developed in the shale formation around a thermal well and thermal pore pressure may lead to plastic yielding in the shale. In this study, integrated characterization results on anisotropic thermal-hydro-mechanical (THM) properties of Colorado shale with different clay fractions are presented. Low strength shale intervals along a borehole are identified and the characterized results are used to conduct a stability analysis on an inclined wellbore during thermal operations. The effect of thermally induced pore pressure in Colorado shale on the integrity of a thermal well is investigated using 3D THM-coupled finite element analysis with the consideration of shale's transversely isotropic (TI) elastic characteristics. The results indicate that the wellbore can lose the confinement in high clay fraction shale intervals due to the thermal operation. The mechanisms can be caused by the thermal pore pressure-induced shear plastic yielding behavior in the Colorado shale formation. Tensile fracturing behavior is less likely to happen. The consideration of anisotropy in elastic properties is important in characterizing the shale's stress path in the analysis. Compared with the result based on the TI elastic model, the result based on the isotropic elastic model gives less plastic yielding zone which underestimates the risk.

Bonding performance and mechanism of thermal-hydro-mechanical modified veneer

In the present study, fast-grown western hemlock (Tsuga heterophylla) was subjected to a thermal-hydro-mechanical (THM) treatment. Bond-line shear strength and durability were evaluated, and three parameters (substrate density, adhesive type and amount of adhesive spread) were investigated. To understand how THM processing affects bond performance, surface roughness and cell morphology of wood, before and after THM treatment, were observed. Adhesive penetration depths and patterns in untreated wood and THM-treated wood were also examined. In addition, Fourier transform infrared (FTIR) spectroscopy analysis of bond-line and microscopic observation of interphase region were conducted. Results revealed that polymeric diphenylmethane diisocyanate (PMDI) and polyurethane (PUR) had better bonding performance than phenol formaldehyde resin (PF). THM treatment improved bond durability, and THM plywood met the adhesive bond requirements of exterior plywood. THM treatment reduced surface roughness and porosity of wood. Adhesive penetration patterns were different in untreated wood and THM-treated wood and were affected by amount and type of adhesive. Pathways for adhesive flow into western hemlock were primarily tracheid lumens, ray cell lumens and the interconnecting pits. PF is the only adhesive that showed no evidence of flow via ray tissues or cell wall penetration. Since bond-line shear strength largely depends upon the weakest link across the laminas, ultimate strength or percent wood failure does not always depend merely on adhesive penetration. In addition, FTIR analysis showed different chemical reactions and bonding mechanisms, which provides a possible explanation of THM-caused changes in bonding performance at the molecular level.

A separate-ice based solution for frost heaving-induced pressure during coupled thermal-hydro-mechanical processes in freezing soils

A frost heaving-induced pressure (FHIP) solution is developed in this paper based on the proposed separate-ice frost heave model. A coupled thermal-hydro-mechanical (THM) model, describing the growth of a single ice lens under the restraint, is firstly constructed by considering the relationship between soil porosity and effective stress. The new ice lens formation criterion indicates that the formation of a new ice lens occurs when the disjoining pressure at the ice-water interface exceeds the summation of the external pressure and the critical pressure of soil strength. With consideration of the influence of the external pressure on the deformation of unfrozen soil, equivalent water pressure and critical separation pressure for ice segregation, the FHIP model is established by combining the growth model of a single ice lens and the formation criterion of new ice lenses. The freezing tests of Xuzhou (China) silty clay under different restraints were conducted to verify our numerical results, and the observed consistency thus validates the FHIP model. Our numerical results demonstrate the stress induced deformation of the soil column and indicate that the FHIP increases with the restrained stiffness. Particularly, with the decrease of the restrained stiffness, the FHIP decreases following an exponential function. The prediction of the FHIP using the proposed method improves our understanding of the characterized behavior of the FHIP under various restraints, and thus contributes to the analysis of the coupled thermal-hydro-mechanical processes in freezing soils.

Discrete element analysis of the mechanical properties of deep-sea methane hydrate-bearing soils considering interparticle bond thickness

Due to increasing global energy demands, research is being conducted on the mechanical properties of methane hydrate-bearing soils (MHBSs), from which methane hydrate (MH) will be explored. This paper presents a numerical approach to study the mechanical properties of MHBSs. The relationship between the level of MH saturation and the interparticle bond thickness is first obtained by analyzing the scanning electron microscope images of MHBS samples, in which is the bridge connecting the micromechanical behavior captured by the DEM with the macroscopic properties of MHBSs. A simplified thermal-hydromechanical (THM) bond model that considers the different bond thicknesses is then proposed to describe the contact behavior between the soil particles and those incorporated into the discrete element method (DEM). Finally, a series of biaxial compression tests are carried out with different MH saturations under different effective confining pressures to analyze the mechanical properties of deep-sea MHBSs. The results of the DEM numerical simulation are also compared with the findings from triaxial compression tests. The results show that the macromechanical properties of deep-sea MHBSs can be qualitatively captured by the proposed DEM. The shear strength, cohesion, and volumetric contraction of deep-sea MHBSs increase with increasing MH saturation, although its influence on the internal friction angle is obscure. The shear strength and volumetric contraction increase with increasing effective confining pressure. The peak shear strength and the dilation of MHBSs increase as the critical bond thickness increases, while the residual deviator stress largely remains the same at a larger axial strain. With increasing the axial strain, the percentage of broken bonds increases, along with the expansion of the shear band.

A New Approach to the Modeling of Hydraulic-Fracturing Treatments in Naturally Fractured Reservoirs

Summary A fully coupled thermal/hydromechanical (THM) model for hydraulic-fracturing treatments is developed in this study. In this model, the mixed finite-volume/finite-element method is used to solve the coupled system, in which the multipoint flux approximation L-method is used to calculate interelement fluid and heat flux. The Gu et al. (2011) crossing criterion is extended to a 3D scenario to delineate the crossing behaviors as hydraulic fractures meet inclined natural fractures. Moreover, the modified Barton et al. (1985) model proposed by Asadollahi et al. (2010) is used to estimate the fracture aperture and model the shear-dilation effect. After being (partially) verified by means of comparison with results from the literature, the developed model is used to investigate complex-fracture-network propagation in naturally fractured reservoirs. Numerical experiments show that the key factors controlling the complexity of the induced-fracture networks include stress anisotropy, injection rate, natural-fracture distribution (fracture-dip angle, strike angle, spacing, density, and length), fracture-filling properties (the degree of cementation and permeability), fracture-surface properties (cohesion and friction angle), and tensile strength of intact rock. It is found that the smaller the stress anisotropy and/or the lower the injection rate, the more complex the fracture network; a high rock tensile strength could increase the possibility of the occurrence of shear fractures; and under conditions of large permeability of fracture filling combined with small cohesive strength and friction coefficient, shear slip could become the dominant mechanism for generating complex-fracture networks. The model developed and the results presented can be used to understand the propagation of complex-fracture networks and aid in the design and optimization of hydraulic-fracturing treatments.

CO2 Permeability Analysis of Caprock Containing a Single Fracture Subject to Coupled Thermal-Hydromechanical Effects

Coupled THM (thermal-hydromechanical) processes have become increasingly important in studying the issues affecting subsurface flow systems. CO2 permeability of the fracture in caprock is a key factor that affects sealing efficiency of caprock. A new model associated with coupled THM processes that shows a good reliability was derived. Then, based on the COMSOL multiphysics software, a series of numerical calculations were performed on caprock models with a single fracture subject to coupled THM effects. Transmissivity of the fracture as a function of fracture angle, overburden pressure, fluid pressure difference, injected CO2 temperature, and the initial fracture aperture was elucidated, respectively. Average transmissivity of the fracture undergoes an increase by 1.74 times with the fracture angle (45°–90°), 2-3 orders of magnitude with the fluid pressure difference (5–30 MPa), and 4-5 orders of magnitude with the initial fracture aperture (0.05–0.5 mm), while it decreases by 3-4 orders of magnitude as overburden pressure increases from 30 to 80 MPa. Injected CO2 temperature has a small impact on the fracture permeability. This work provides an alternative tool to enrich the numerical modeling for the assessment of CO2 caprock sealing efficiency.

A Pore-Scale Simulation on Thermal-Hydromechanical Coupling Mechanism of Rock

Thermal-hydromechanical (THM) coupling process is a key issue in geotechnical engineering emphasized by many scholars. Most existing studies are conducted at macroscale or mesoscale. This paper presents a pore-scale THM coupling study of the immiscible two-phase flow in the perfect-plastic rock. Assembled rock matrix and pore space models are reconstructed using micro-CT image. The rock deformation and fluid flow are simulated using ANSYS and CFX software, respectively, in which process the coupled physical parameters will be exchanged by ANSYS multiphysics platform at the end of each iteration. Effects of stress and temperature on the rock porosity, permeability, microstructure, and the displacing mechanism of water flooding process are analyzed and revealed.

Numerical modeling of coupled thermal-hydro-mechanical behavior of GMZ bentonite in the China-Mock-up test

Gaomiaozi (GMZ) bentonite is currently considered as the most suitable buffer material for high-level radioactive waste geological repository in China. To investigate the long-term performance of GMZ bentonite under repository conditions, a large scale mock-up test has been carried out since 2011. In the test, a hydration system and an electrical heater were employed to simulate the Thermal-Hydro-Mechanical (THM) coupled conditions near a repository. In this work, a numerical study is performed to reproduce the experimental data obtained in the first three years of the China-Mock-up experiment. The principle THM characteristics and parameters of the bentonite are presented. On this basis, a THM coupled constitutive model is introduced. With the proposed model, numerical simulations of the China-Mock-up test are carried out using LAGAMINE. The numerical data of temperature, relative humidity, and swelling pressure of the GMZ compacted bentonite are compared with the experimental ones. We discuss the effect of the balance between the drying effect and water penetration process on the THM behavior of the GMZ bentonite. The good agreement between the predicted and experimental results suggests that the proposed model can reproduce the fundamental behavior of GMZ compacted bentonite under coupled THM conditions.

Degradation of the mechanical properties imaged by seismic tomography during an EGS creation at The Geysers (California) and geomechanical modeling

Using coupled thermal-hydro-mechanical (THM) modeling, we evaluated new seismic tomography results associated with stimulation injection at an EGS demonstration project at the Northwest Geysers geothermal steam field, California. We studied high resolution seismic tomography images built from data recorded during three time periods: a period of two months prior to injection and during two consecutive one month periods after injection started in October 2011. Our analysis shows that seismic velocity decreases in areas of most intense induced microseismicity and this is also correlated with the spatial distribution of calculated steam pressure changes. A detailed analysis showed that shear wave velocity decreases with pressure in areas where pressure is sufficiently high to cause shear reactivation of pre-existing fractures. The analysis also indicates that cooling in a liquid zone around the injection well contributes to reduced shear wave velocity. A trend of reducing compressional wave velocity with fluid pressure was also found, but at pressures much above the pressure required for shear reactivation. We attribute the reduction in shear wave velocity to softening in the rock mass shear modulus associated with shear dislocations and associated changes in fracture surface properties. Also, as the rock mass become more fractured and more deformable this favors reservoir expansion caused by the pressure increase, and so the fracture porosity increases leading to a decrease in bulk density, a decrease in Young modulus and finally a decrease in Vp.

Numerical simulation on the cracking and failure law of compacted clay lining in landfill closure cover system

SUMMARYAccording to the mass conservation equation of solid phase, water transport, air migration and enthalpy balance equations in a deformable and unsaturated porous medium, and the mechanics model, a thermal‐hydro‐mechanical (THM) coupling mathematical model is developed in order to accurately predict the crack propagation of the compacted clay lining (CCL) in a landfill closure cover system. Numerical simulation analysis of the cracking and failure of the CCL in a landfill closure cover system under complex conditions (landfill gas pressure, variation of soil surface temperature, moisture loss of coverage soil, and uneven settlement of landfilled MSW (municipal solid waste)) is performed. The results show that the cracks are discovered firstly on the soil surface and then extend to the deeper cover system and became bigger. The cracks had broken through the closure cover system at 30 days. Over the depth range of 0.06 m, the change rate of settlement is found to be the highest because of the strong influence of temperature. The rise of temperature around the cracks is fast when the CCL is broken through by the cracks. There is a hollow area near the cracks over the depth range from 0.15 to 0.7 m, and the temperature stabilize at around 11°C without any obvious reduction. Under the condition of evaporation, the main crack in the CCL is similar to a pumping well, which causes more rapid moisture content reduction around the cracks. Vertical deformation increases linearly with depth in the process of deformation and cracking of CCL induced by uneven settlement of landfilled MSW. Copyright © 2014 John Wiley & Sons, Ltd.

Thermal-hydro-mechanical coupling damage model of brittle rock

Based on fluid mechanics, thermodynamics and damage mechanics, thermal-hydro-mechanical (THM) coupling damage model of brittle rock is established by analyzing THM coupling mechanism, where THM coupling damage variable DTHM is dominated by TH coupling damage variable DTH, TM coupling damage variable DTM and HM coupling damage variable DHM, and DTH is firstly expressed in term of dimensionless total thermal conductivity of the water Nu. Permeability test, uni-axial compression test and THM coupling test are conducted to measure the permeability, elastic modulus and THM coupling stress-strain curves of brittle rock. The tested values of THM coupling elastic modulus ETHM′ are in good agreement with the predicted values of THM coupling elastic modulus ETHM, which can verify the newly established THM coupling damage model.

Investigation of the thermal-hydro-mechanical (THM) behavior of GMZ bentonite in the China-Mock-up test

In this paper, the large scale China-Mock-up test, intended to study the thermal-hydro-mechanical (THM) behavior of Gaomiaozi (GMZ) bentonite under relevant repository condition, is presented. In accordance with the current disposal concept of High-level Radioactive Waste (HLW) disposal in China, the facility is designed as a vertical cylindrical tank, filled with compacted GMZ-bentonite. A heater, which simulates the HLW canister, is placed inside the compacted bentonite blocks and pellets. The installed hydration system on the exterior surface simulates the supply of groundwater. The operational phase started on 1st April 2011. Based on the current recorded experimental data, several aspects are addressed in the paper, including the observed THM behavior of the compacted bentonite, the displacement of the electrical heater, and also the influence of unexpected interruptions of electric power supply. The saturation process of the compacted bentonite is strongly influenced by the competing mechanisms of drying effect induced by the heater and the wetting effect by water penetration. As a result, the desiccation–saturation process was observed in the zone close to the heater. An upward displacement of the heater was also noticed, which verified the potential influence of buffer material on the stability of canister. The experiment is a valuable step in establishing the viability of the reference concept, and making progress in the understanding of the behavior of buffer material under thermal-hydro-mechanical (THM) coupled condition.

The influence of thermal-hydro-mechanical processing on chemical characterization of Tsuga heterophylla

Viscoelastic thermal compression (VTC) is a type of thermal-hydro-mechanical (THM) processing that requires only a short processing time. THM processing causes some chemical transformations, the nature and extent of hydro-thermolysis depends on the special treatment conditions and the chemical nature of wood species. In the present study, the chemical transformations of the cell wall components and wood extractives during VTC treatment were investigated, and correlation between chemical characterizations and observed property changes was analyzed. For this purpose, the content of extractives and pH values were determined, and FTIR analysis was performed on extractable substances, extract-free wood, holocellulose, α-cellulose and lignin. Two temperatures and two steam exposure times were adopted to determine the influence of processing conditions on chemical characterization of Tsuga heterophylla. The results revealed that THM treatment caused a series of chemical reactions in extractives. Treatment temperature and conditioning time have significant influence on chemical changes of extractives. For all of the VTC treatments used in this study, no significant changes occurred in the lignin and α-cellulose components. The only significant chemical changes occurred in the hemicelluloses, which were primarily reduction of carbonyl and acetyl functional groups. This study also confirmed that the chemical transformation of wood correlates with property changes of VTC wood.

Numerical analysis of THM coupling of a deeply buried roadway passing through composite strata and dense faults in a coal mine

Because of the reduction in shallow-buried coal resources, increasingly coal mines in China extend to deep resources. The coupling of thermal-hydro–mechanical (THM) fields is much more complicated in deep strata, especially when the roadway passes through composite strata and dense faults. High ground pressures, high ground temperature and high seepage pressure are most important features of deep strata. In order to investigate the THM coupling mechanism of deeply buried roadways, this article presents the geological features of deep strata and analyzes the interactions among thermal, hydraulic and mechanical fields considering the water-weakening effect of the surrounding rock. Finally, the THM coupling process of a deeply buried roadway passing through composite strata and dense faults is simulated. The deformation characteristics and distribution of the stress field are described in the results.

Äspö 원형 처분장에 대한 열-수리-역학적 모델링 연구: 열적 거동 해석

고준위폐기물처분장에 대한 열-수리-역학적 거동 모델링은 처분장의 성능 및 안전성 평가를 위해서 선행되어야 할 중요한 연구과제이다. 본 연구에서는 <TEX>$\ddot{A}$</TEX>sp<TEX>$\ddot{o}$</TEX> 원형처분장의 열적 거동을 해석하고, 현장 실험데이터와의 비교를 통해 모델 계산결과의 타당성을 검증하였다. 모델 시뮬레이션에서는 처분공과 처분터널 및 그 주변 암반에 대한 온도분포를 분석하였다. 현장 실험데이터와의 비교는 처분공 DH-6를 대상으로 수행하였다. 그 결과 모델 계산치는 측정위치에 따라 실험치 보다 약 2-<TEX>$5^{\circ}C$</TEX> 정도 높은 온도값을 보였으나, 온도변이 곡선은 비슷한 패턴을 보여 주었다. 실험치와 모델 계산치의 이러한 차이는 모델에 의한 열적거동 해석에서 암반을 제외한 완충재, 벤토나이트 펠렛, 뒷채움재 내의 수리학적 및 역학적 거동을 고려하지 않았기 때문으로 판단되었다. Thermal-hydro-mechanical (THM) modeling is a critical R&D issue in the performance and safety assessment of a high-level waste repository. With an <TEX>$\ddot{A}$</TEX>sp<TEX>$\ddot{o}$</TEX> prototype repository, its thermal behavior was analyzed and then compared with in-situ experimental data for its validation. A model simulation was used to calculate the temperature distributions in the deposition holes, deposition tunnel, and surrounding host rock. A comparison of the simulation results with the experimental data was made for deposition hole DH-6, which showed that there was a temperature difference of <TEX>$2{\sim}5^{\circ}C$</TEX> depending on the location of the measuring points, but there was a similar trend in the evolution curves of temperature as a function of time. It was expected that the coupled modeling of the thermal behavior with the hydro-mechanical behavior in the buffer and backfill of the <TEX>$\ddot{A}$</TEX>sp<TEX>$\ddot{o}$</TEX> prototype repository would give a better agreement between the experimental and model calculation results.