Thermo-hydro-mechanical Process Research Articles

Numerical analysis of thermo-hydromechanical process related to deep geological radioactive repository

This paper addresses numerical analysis of thermo-hydromechanical processes in the context of deep geological disposal of radioactive waste. The emphasis is put on modeling of damaged zones induced by excavation, pore pressure and temperature changes. The theoretical background of thermo-poroelasticity for saturated porous media is first recalled. The framework for modeling initiation and evolution of induced cracks is then presented by using a variational approach of phase-field method. A specific model with two crack phase fields and considering thermo-hydromechanical interaction is proposed. A particular attention is paid on the description of shear cracks. The proposed model is implemented in the standard finite element method. An example of application is finally presented on the analysis of thermo-hydromechanical responses and cracked zones evolution around a typical disposal repository in the context of French concept for high level waste disposal.

Numerical simulation of frost heave of saturated soil considering thermo‐hydro‐mechanical coupling

AbstractFrost heave can lead to both the ground uplifting and frost heave pressure under different circumstances, and cause many engineering problems. To describe the characteristics of frost heave under various thermo‐hydro‐mechanical (THM) coupling conditions and calculate both the frost heave amount and the frost heave pressure, the coupled THM process, as well as the phase change of the pore water, should be considered for freezing soil. In this paper, thermodynamic equilibrium conditions in saturated freezing soil were derived to account for the mechanical effect on the cryogenic suction and unfrozen saturation of pore water during phase transition. Then the governing equations were developed considering the water flow, heat transfer, stress equilibrium, phase change, ice segregation, and their complex interactions. Based on that, a numerical model considering fully THM coupling is presented within the framework of finite element provided by COMSOL. Both frost heave amount tests and frost heave pressure tests were simulated to verify the proposed model. Then the model was applied to subgrades with different types of soil and different boundary conditions, to reveal the characteristics of frost heave amount and frost heave pressure under different conditions, and at the same time to demonstrate the model's applicative prospect.

Three-dimensional Modeling of Cracking with Thermo-hydromechanical Process by Considering Rock Heterogeneity

Three-dimensional Modeling of Cracking with Thermo-hydromechanical Process by Considering Rock Heterogeneity

A thermo-hydro-mechanical model to evaluate the seismic properties of geothermal reservoirs

Fractured-vuggy thermal reservoirs with complex pore spaces (stiff pores, cracks, and fractures) are typical geothermal resources for development and utilization in China. The cyclic recovery of such thermal reservoirs involves a complex thermo-hydro-mechanical (THM) coupling process. Insights into the thermoelastic effects of heating-cooling cycles on the seismic response have great potential for seismic monitoring in the cyclic recovery, which remains largely unaddressed in the literature. We intend to fill this gap by applying the double-porosity thermoelasticity theory to interpret ultrasonic measurements on granite under water-cooling conditions. We consider an isotropic porous host embedded with fractures. A plane-wave analysis yields the classical P and S waves and three slow P waves, namely the slow (Biot) P1, the slow (Biot) P2, and a thermal P. We investigate the combined effect of temperature, porous structure, and pore fluid on the thermoelastic properties of the THM process for typical granite reservoirs that experience a cold-shock process. Fractures provide the main channels for heat exchange and fluid flow. Our THM thermoelastic model describes the reservoir properties as a function of temperature associated with thermal-induced cracking, where fracture porosity is more important than the stiff (host) pore to describe the reservoir quality. We find that the thermal conductivity and specific heat have negligible effects in the seismic frequency band for the temperature range of less than 400°C, whereas the crack density significantly affects the seismic response in the heating-cooling cycles because of the additional contribution of thermal- and cold-shock-induced cracks. We further determine that the P-wave velocity and attenuation due to thermal effects under water cooling offer an important index to monitor the thermal-induced cracking and operation efficiency of the enhanced geothermal system. The THM thermoelastic model lays the foundation for active (or passive) seismic monitoring of the cyclic recovery of thermal reservoirs.

Thermo-hydro-mechanical (THM) evolution law and development of permeability and pore structure of enhanced geothermal systems at ultra-high temperatures

The evolution law and fluctuation mechanism of permeability and pore structure of stimulated reservoirs in hot dry rocks (HDR) under the coupled thermo-hydro-mechanical environment (THM) in ultra-high temperature are of great significance to the production capacity of enhanced geothermal systems (EGS). This paper designed a testing apparatus to take the effects of the THM processes into account by coupling high temperatures and the high triaxial stress. A series of flow tests were performed on a HDR under the coupled THM environment at ultra-high temperatures, i.e., temperature range of 25 to 650°C, triaxial stress of 25 MPa, and seepage pressure of 6 MPa, and ultra-high thermal treatments, i.e., temperature range from 25 to 650°C. Then, the micro-physical parameters of the specimens treated in various conditions were obtained via mercury intrusion porosimetry, thin-section petrography and micro X-ray computed tomography observations. The experimental results show that (1) in the temperature range of 25 to 400°C, the microscopic physical parameters increase monotonously with the increase of temperature. However, the microscopic physical parameters of specimens subjected to the coupled THM condition are lower than those obtained after considering only the thermal treatment, owing to the inhibition effect of the triaxial stress on the thermal expansion; (2) in the temperature range of 400 to 550°C, the microscopic physical parameters subjected to the coupled THM condition fluctuated slightly with the increase of temperature which is due to the strong thermo-elastic-plastic fluidity and the competitive effect of closing and re-opening of pores/fissures; (3) in the temperature range of 550 to 650°C, the microscopic physical parameters are much larger than those obtained after the thermal treatment owing to the severe deterioration of rock mass caused by the coupling THM process. The results obtained is of great significance for the assessment of stability, safety and site selection of high-temperature rock projects such as EGSs.

PORE-SCALE SIMULATION OF MULTIPHASE FLOW CONSIDERING THERMO-HYDRO-MECHANICAL COUPLING EFFECT IN POROUS MEDIA

Researches on the thermo-hydro-mechanical (THM) coupling effect in porous media from the perspective of pore-scale is of great significance for the study of enhanced oil recovery, nuclear storage, and geological sequestration of CO2. In order to study the migration of oil resources in porous media in the deep oil reservoir, we used a combined method to realize the THM process occurring in porous media. The Darcy-Brinkman-Biot method was applied to simulate the THM process and the calculation of thermal stress was achieved by adding the Duhamel-Neumann thermoelastic stress to the model. A water-oil two-phase flow process considering THM coupling effect in porous media was then realized. The model has simulated flow of multiphase fluid in the pore space by solving the Navier-Stokes equations and calculated flow of the fluid in the rock matrix by solving the Darcy equation. The two processes were coupled with a series of momentum exchange equations to obtain the displacement of solid particles, thus realizing the fluid-solid coupling effect. On this basis, a heat transfer model was added to numerical model to consider the influence of temperature field on the two-phase flow process. Temperature field acts on the matrix in the form of thermoelastic stress to realize the THM coupling process. Based on the model, we simulated the flow process of water-oil two-phase fluid in a two-dimensional porous media model. The results have shown that: (1) the direction of thermal stress was opposite to that generated by fluid-structure coupling effect, which made the total stress smaller than that under the fluid-structure coupling effect; (2) the porosity of the model increased with the increase of temperature, however, when the injection temperature difference reached 150 K, the porosity no longer increased significantly; (3) with the increase of temperature, the relative permeability of the water phase increased and the equal-permeability point shifted to the left.

Comparison of thermo-poroelastic and thermo-poroelastoplastic constitutive models to analyze THM process in clays

Thermal pore pressurization in soil media has been investigated for the past few decades. It has been shown that temperature variations may significantly affect thermal pore pressure in clay soils confined deep into the ground. Moreover, thermal loading may lead to stress change and thermal deformation. Thermo-poroelastic and advance thermo-poroelastoplastic constitutive models have been formulated and incorporated numerically to simulate the thermo-hydro-mechanical process. However, the accurate response of soil media during THM process has not been completely understood. Although numerical modelling reasonably predicts the experimental observations, they still could not be used to completely justify the field observations. In this study, the main features of the thermo-poroelastic model are incorporated in a thermo-poroelastoplastic constitutive model (ACMEG-T) to further investigate the effect of different thermal and hydraulic properties on thermo-hydro-mechanical (THM) response of the soil media.

Study on the changes in surface characteristics of Populus tomentosa due to thermo-hydro-process

The effects of high temperature, moisture, and mechanical action during the thermo-hydro-mechanical (THM) processing, on the changes in surface properties of poplar, namely, surface color, roughness, wettability, and microstructure, were investigated in this study. The correlation between observed changes in surface properties and chemical characteristics was also analyzed. Poplar woods with high moisture content were compressed using different pressures at temperature of 160 °C for four different periods. The wood surfaces became darker and smoother, and their surface free energy decreased significantly after the THM process. THM process markedly reduced surface hygroscopicity compared to the control wood. The cell lumens of THM wood became narrow with increasing compression ratio due to the enhancing high pressure. Collapse and fractures of cell walls developed during THM treatment. Furthermore, results indicated that a series of chemical reactions in different components of wood took place during THM process, such as degradation of hemicelluloses, condensation of lignin, and decomposition of extractives. In turn, these chemical modifications contributed to the darkening of color as well as the reduction of wettability and surface free energy of THM wood.

Sensitivity analysis of coupled processes and parameters on the performance of enhanced geothermal systems

3-D modeling of coupled thermo-hydro-mechanical (THM) processes in enhanced geothermal systems using the control volume finite element code was done. In a first, a comparative analysis on the effects of coupled processes, operational parameters and reservoir parameters on heat extraction was conducted. We found that significant temperature drop and fluid overpressure occurred inside the reservoirs/fracture that affected the transport behavior of the fracture. The spatio-temporal variations of fracture aperture greatly impacted the thermal drawdown and consequently the net energy output. The results showed that maximum aperture evolution occurred near the injection zone instead of the production zone. Opening of the fracture reduced the injection pressure required to circulate a fixed mass of water. The thermal breakthrough and heat extraction strongly depend on the injection mass flow rate, well distances, reservoir permeability and geothermal gradients. High permeability caused higher water loss, leading to reduced heat extraction. From the results of TH vs THM process simulations, we conclude that appropriate coupling is vital and can impact the estimates of net heat extraction. This study can help in identifying the critical operational parameters, and process optimization for enhanced energy extraction from a geothermal system.

Effect of thermo-hydro-mechanical densification on microstructure and properties of poplar wood (Populus tomentosa)

This study aimed at developing a thermo-hydro-mechanical (THM) processing to compress poplar wood and investigating the effects of high temperature, moisture, and pressure during the THM processing on the changes in microstructure, porosity, mechanical properties, and dimensional stability of compressed poplar wood. The variations in these properties were correlated and their mathematical relations were determined. Poplar woods with high moisture content were compressed using different pressures at a temperature of 160 °C for different periods. The compression level was characterized by the volume compression ratio (CR), which is defined as the ratio of the compression volume and the original volume of sample before and after THM processing. The obtained results indicated that the high pressure of THM process caused the collapsing of wood cell lumens and the developing of a certain amount of fractures in the cell wall. The damage level of wood cells increased with increasing pressure and time. Moreover, the pressure narrowed the cell lumens, which decreased significantly the pore volume in wood substrate. The pore size distribution shifted from the level of macropores to those of mesopores and micropores after THM process. The THM process created superior mechanical property, especially for those with higher CR. Besides, it was revealed that the process decreased dramatically the set recovery of treated woods and improved their dimensional stability. A significant improvement was achieved in terms of the mechanical and physical properties of compressed poplar wood via the structural reformation during the THM process.

Numerical modeling of heating and hydration experiments on bentonite pellets

In a common, designed disposal system for high-level radioactive waste, bentonite is used worldwide as a buffer material due to its favorable thermodynamic properties. In-situ experiments at different scales are being conducted in several underground laboratories to investigate the long-term coupled thermo-hydro-mechanical (THM) and chemical behavior of bentonite. Simultaneously, numerous laboratory experiments under well-defined conditions were performed to determine the material properties of bentonite. Since 2012, a laboratory heating and hydration test on MX-80 bentonite has been performed by CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas). The obtained data during the test were analyzed by different research teams within the international DECOVALEX project in order to understand the coupled THM behavior of bentonite and furthermore to determine the thermal-mechanical properties of bentonite. This paper presents a numerical model for fully-coupled THM processes in bentonite based on the finite element method program OpenGeoSys (Kolditz et al., 2012a). In this model, the description of heat conduction is based on Fourier's law, and advection is also considered in the heat transfer. As an important coupling factor from hydraulic process to thermal process, the dependency of thermal conductivity on the water saturation is intensively analyzed. A multiphase flow model considering water evaporation and vapor diffusion is used to describe the hydraulic process. Special attention was also paid on the analysis of the water retention behavior. An elastic constitutive model based on generalized Hook's law was applied to describe the material's mechanical behavior complemented with consideration of the thermally induced strain and the swelling deformation. Good agreement between the calculated and measured data has been achieved concerning temperature, relative humidity, total stress, and water intake. The main coupling processes and bentonite behavior in the experiment could be captured and well analyzed in this model. However, further investigations are also needed, especially for the long-term THM process with respect to water intake.

Mathematical Model of Seismoexplosion in Tunnel Surrounding

The construction of objects of critical infrastructures such as a new roads, utility tunnels, communication networks, underground parks and other structures is associated by the way with generation of vibrations in the vicinity of the underground structures, and not only human lives can be lost but also extensive material damage can be expected due to technological and natural hazards based on vibrations. This paper presents the general governing equations for coupled thermo-hydromechanical process in the tunnel surrounds. Modeling and analysis of laminated composite cylinder applied for the objective analysis of shock wave propagation in the soils and possibilities to estimates of the tunnel structure damage.

Simulation of coupled THM process in surrounding rock mass of nuclear waste repository in argillaceous formation

To investigate and analyze the thermo-hydro-mechanical (THM) coupling phenomena of a surrounding rock mass in an argillaceous formation, a nuclear waste disposal concept in drifts was represented physically in an in-situ test way. A transversely isotropic model was employed to reproduce the whole test process numerically. Parameters of the rock mass were determined by laboratory and in-situ experiments. Based on the numerical simulation results and in-situ test data, the variation processes of pore water pressure, temperature and deformation of surrounding rock were analyzed. Both the measured data and numerical results reveal that the thermal perturbation is the principal driving force which leads to the variation of pore water pressure and deformations in the surrounding rock. The temperature, pore pressure and deformation of rock mass change rapidly at each initial heating stage with a constant heating power. The temperature field near the heater borehole is relatively steady in the subsequent stages of the heating phase. However, the pore pressure and deformation fields decrease gradually with temperature remaining unchanged condition. It also shows that a transversely isotropic model can reproduce the THM coupling effects generating in the near-field of a nuclear waste repository in an argillaceous formation.

A thermo‐hydro‐mechanical model for unsaturated soils based on the effective stress concept

AbstractA simple thermo‐hydro‐mechanical (THM) constitutive model for unsaturated soils is described. The effective stress concept is extended to unsaturated soils with the introduction of a capillary stress. This capillary stress is based on a microstructural model and calculated from attraction forces due to water menisci. The effect of desaturation and the thermal softening phenomenon are modelled with a minimal number of material parameters and based on existing models. THM process is qualitatively and quantitatively modelled by using experimental data and previous work to show the application of the model, including a drying path under mechanical stress with transition between saturated and unsaturated states, a heating path under constant suction and a deviatoric path with imposed suction and temperature. The results show that the present model can simulate the THM behaviour in unsaturated soils in a satisfactory way. Copyright © 2010 John Wiley & Sons, Ltd.

Modelling the effect of temperature on unsaturated soil behaviour

A simple thermohydromechanical (THM) constitutive model for unsaturated soils is described. The effective stress concept is extended to unsaturated soils with the introduction of a capillary stress. This capillary stress is based on a microstructural model and calculated from attraction forces due to water menisci. The effect of desaturation and the thermal softening phenomenon are modelled with a minimal number of material parameters and based on existing models. THM process is qualitatively and quantitatively modelled by using experimental data and previous work to show the application of the model, including a drying path under mechanical stress with transition between saturated and unsaturated states, a heating path under constant suction and a deviatoric path with imposed suction and temperature. The results show that the present model can simulate the THM behaviour in unsaturated soils in a satisfactory way.

Analysis of thermo–hydro–mechanical process in the engineered barrier system of a high-level waste repository

This paper presents a simulation of the KAERI's engineering scale experiment, KENTEX which is to study the thermal, hydrological and mechanical (THM) processes occurring in the engineered barrier system of a high-level waste repository. The simulation was performed by using the computer code, TOUGH2, which analyzes the multi-dimensional fluid and heat flows of multiphase, multicomponent fluid mixture in unsaturated medium. The detailed geometry of KENTEX was incorporated into the model, and the laboratory experiments were carried out to determine the thermal, hydrological and mechanical properties of bentonite, which were used as input parameters for the simulation. The calculated results on the temperature, water content, and total pressure distribution throughout the bentonite buffer were compared with the experimental ones.

Thermophysical Models of Underground Coal Gasification and FEM Analysis

In this study, mathematical models of the coupled thermohydromechanical process of coal rock mass in an underground coal gasification panel are established. Combined with the calculation example, the influence of heating effects on the observed values and simulated values for pore water pressure, stress, and displacement in the gasification panel are fully discussed and analyzed. Calculation results indicate that 38, 62, and 96 days after the experiment, the average relative errors for the calculated values and measured values for the temperature and water pressure were between 8.51–11.14% and 3–10%, respectively; with the passage of gasification time, the calculated errors for the vertical stress and horizontal stress gradually declined, but the simulated errors for the horizontal and vertical displacements both showed a rising trend. On the basis of the research results, the calculated values and the measured values agree with each other very well.