Thermo-hydro-mechanical Processes Research Articles

Disaster effects of climate change in High Mountain Asia: State of art and scientific challenges

High Mountain Asia (HMA) shows a remarkable warming tendency and divergent trend of regional precipitation with enhanced meteorological extremes. The rapid thawing of the HMA cryosphere may alter the magnitude and frequency of nature hazards. We reviewed the influence of climate change on various types of nature hazards in HMA region, including their phenomena, mechanisms and impacts. It reveals that: 1) the occurrences of extreme rainfall, heavy snowfall, and drifting snow hazards are escalating; accelerated ice and snow melting have advanced the onset and increased the magnitude of snowmelt floods; 2) due to elevating trigger factors, such as glacier debuttressing and the rapid shift of thermal and hydrological regime of bedrock/snow/ice interface or subsurface, the mass flow hazards including bedrock landslide, snow avalanche, ice-rock avalanches or glacier detachment, and debris flow will become more severe; 3) increased active-layer detachment and retrogressive thaw slumps slope failures, thaw settlement and thermokarst lake will damage many important engineering structures and infrastructure in permafrost region; 4) multi-hazards cascading hazard in HMA, such as the glacial lake outburst flood (GLOF) and avalanche-induced mass flow may greatly enlarge the destructive power of the primary hazard by amplifying its volume, mobility, and impact force; and 5) enhanced slope instability and sediment supply in the highland areas could impose remote catastrophic impacts upon lowland regions, and threat hydropower security and future water shortage. In future, ongoing thawing of HMA will profoundly weaken the multiple-phase material of bedrock, ice, water, and soil, and enhance activities of nature hazards. Compounding and cascading hazards of high magnitude will prevail in HMA. As the glacier runoff overpasses the peak water, low flow or droughts in lowland areas downstream of glacierized mountain regions will became more frequent and severe. Addressing escalating hazards in the HMA region requires tackling scientific challenges, including understanding multiscale evolution and formation mechanism of HMA hazard-prone systems, coupling thermo‒hydro‒mechanical processes in multi-phase flows, predicting catastrophes arising from extreme weather and climate events, and comprehending how highland hazards propagate to lowlands due to climate change.

Coupled Thermo-Hydro-Mechanical Processes in Fractured Rocks: Some Past Scientific Highlights and Future Research Directions

AbstractCoupled thermo-hydro-mechanical (THM) processes in fractured rocks have been a topic of intense scientific research for more than 30 years. The present paper takes a look into the past and highlights some scientific advances which are of an unusual “out-of-the-box” nature, and then looks forward and discusses possible directions of future research in this interesting field of study. Concerning future research directions, we see a trend from a focus on coupled THM processes in single fractures or a few interacting fractures, to the study of coupled THM behavior in complex fracture network systems where the fractures act collectively giving rise to local stress concentration points and points of large pressure gradients. Three examples of future research directions are presented. First is an effort towards identifying characterizing parameters of a fracture network that play a direct controlling role in major coupled THM phenomena (such as induced seismicity and flow channeling), rather than parameters of stochastic distributions of fractures in the network. The second example of research direction is accounting for the heterogeneity and hierarchy of fractures in a fault or fracture zone which has been associated with major THM events in a number of geo-energy projects. The third example is at the opposite end of the first; here it is recognized that in some cases, the coupled THM processes in fractured rocks may be controlled dominantly by only a few key bridges. Identification, characterization, and evaluation of these key bridges should be one of the important research directions in the coming days.

A fully coupled thermo-hydro-mechanical elastoplastic damage model for fractured rock

A detailed and fully coupled thermo-hydro-mechanical (THM) model for fractured rock is presented. This model distinguishes itself by harmoniously integrating elastoplastic material behaviour with a continuum damage mechanics framework. Solid matrix displacement, pore and fissure water pressures and temperature of the solid are introduced as the primary nodal variables. The ingenuity of this research is embedded in the intricate coupling of THM processes with plastic deformation and damage mechanics in a double porous medium, a venture that significantly broadens the remit of existing methodologies. The model is implemented using finite element method (FEM) and validation is achieved by comparing the FEM results against existing literature numerical outcomes describing linear and elastoplastic continuum damage behaviour of fractured rock. The model also exhibits an extraordinary proficiency in reproducing experimental triaxial test results, using THM components conjoined with elastoplastic bounding surface aspects and inherent hardening effects. It is imperative, nonetheless, to underscore the model's sensitivity to certain material properties, inclusive of strength parameters, leakage coefficients, and permeability attributes. This fully coupled THM model provides a comprehensive and sophisticated tool for investigating the behaviour of fractured rock under various loading conditions. It can help us better understand the physics of fractured rock behaviour and contribute to the development of more accurate and reliable models for engineering applications, such as CO2 injection.

Influence of anionic polyacrylamide on the freeze–thaw resistance of silty clay

Freeze–thaw (FT) alternation can severely affect the properties of fine–grained soils, often leading to the failure of foundation backfill projects, such as open canals and roads in cold regions. To address these problems, a water–soluble anionic polyacrylamide (APAM) was used to improve the properties of fine–grained soils, particularly silty clay. This study aims to analyze the effects of the APAM polymer on the physico–mechanical and microstructural properties of silty clay under both unfrozen and FT cycles. For this purpose, different addition dosages and FT cycle numbers were determined, and then a series of zeta potential, thermal conductivity, permeability, triaxial compression, and scanning electron microscopy (SEM) tests were conducted on the soil samples. Additionally, a unidirectional freezing test was constructed to investigate the coupled thermo–hydro–mechanical processes of APAM–treated soil samples under subzero temperature conditions. The results showed that the addition of the APAM polymer significantly enhanced the macroscopic engineering properties and microstructural characteristics of silty clay. This improvement was attributed to the charge neutralization, adsorption bridging effect, and hydrophobic interaction provided by the APAM polymer. However, all the soil samples showed a significant deterioration in their engineering properties during FT cycles, especially after the third FT cycle. It was notable that APAM–treated soil samples were superior to the untreated sample in terms of FT resistance. During the unidirectional freezing process, the pore water migrated from the unfrozen zone towards the freezing front due to the temperature gradient. The treated sample's pore water migration volume was considerably lower than that of the untreated sample, resulting in a 55.25% reduction in total frost heave when the APAM dosage was 0.30%. The findings of this paper may be utilized to mitigate the risk of frost damage to foundation backfill projects in cold regions, as well as to get a better understanding of the stabilization mechanism of the eco–friendly APAM polymer.

Phase-Field Modeling of Coupled Thermo-Hydromechanical Processes for Hydraulic Fracturing Analysis in Enhanced Geothermal Systems

Abundant geothermal energy can be harvested from deep, low-permeability rocks using an enhanced geothermal system (EGS) that relies on an artificially created permeable fracture network through cold water injection. In this study, a two-way coupling technique for thermo-hydromechanical modeling was used to simulate hydraulic fracture propagation in an EGS. The transient heat and fluid flow in porous media were modeled using the finite volume method, while hydraulic fracture propagation and interaction with natural fractures were modeled using the variational phase-field method. Our findings underscore the importance of poroelastic and thermoelastic effects on fracture propagation in naturally fractured reservoirs. The results of this study demonstrate that thermo-hydromechanical processes are crucial elements in simulating and optimizing an enhanced geothermal reservoir system.

Thermo-Hydro-Mechanical Modeling of Brine Migration in a Heated Borehole Test in Bedded Salt

AbstractThis research paper focuses on the thermo-hydro-mechanical (THM) modeling of brine migration in a heated borehole test conducted as part of the ongoing Brine Availability Test in Salt (BATS) at the Waste Isolation Pilot Plant (WIPP) in New Mexico. It is a component of the international collaboration project DECOVALEX-2023 (DEvelopment of COupled models and their VALidation against EXperiments), which aims to understand the THM processes governing brine flow in heated rock salt repositories through collaborative analysis by multiple research teams. Using the TOUGH–FLAC simulator, THM simulations were performed and compared with data from the BATS phase 1a. This experiment involved two identical horizontal-borehole arrays, one heated and one serving as a control, both equipped with sensor arrays. Analysis of measurements revealed water flow rate surges during heater power transitions, with the highest jump observed during cooling. Acoustic emission activity exhibited distinct patterns in response to heater power changes, suggesting that damage to rock salt is particularly pronounced during the cooling phase. The THM simulations successfully captured these phenomena, highlighting the significance of thermal effects, brine migration, and mechanical behavior in predicting brine availability in heated and damaged rock salt. Our modeling also revealed the critical interplay between heating and cooling-induced damage and its influence on flow properties, particularly affecting brine inflow estimation. Notably, we found that cooling-induced brine inflow spikes result from increased permeability due to tensile dilatancy. These findings have important implications for the development of robust containment strategies and enhance our understanding of the complex processes involved in repository performance.

Transient temperature and water distributions in compacted MX80 bentonite under high temperature gradients

Numerous element-scale and full-scale tests have been carried out on compacted bentonite to investigate its coupled thermo-hydro-mechanical (THM) behavior when used as a buffer material in high-level nuclear waste geological repositories, such as the Full-scale Engineered Barriers Experiment (FEBEX) and its forerunner laboratory mock-up test [5]. In these past studies the applied temperatures were below 100°C, while new repository conditions being explored in the recently started HotBENT project at the Grimsel Test Site involve waste canister temperatures up to 200°C [2]. Recently, Lu and McCartney [3] conducted a tank scale test where a temperature of 200°C was maintained by a cylindrical heating element having a diameter of 12.5 mm at the center of a compacted MX80 bentonite layer. A notable temperate drop in temperature was observed with distance from the small diameter heating element. A similar phenomenon was also reported by Gens et al. (2020) for heating tests on MX80 pellets during heating to 140°C. Measurements from the FEBEX project indicate a more distributed temperature decay away from the heater [3]. This discrepancy between laboratory and field tests provided the motivation for better understanding the temperature and water distribution near a heater under high temperature gradients to better guide the design of laboratory-scale experiments in the future.
 A one-dimensional heating test was performed in this study to investigate the coupled THM response of a MX80 bentonite layer in unconstrained conditions during basal heating. Measurements include the transient temperature and volumetric water content redistribution at different vertical distances from the flat plate heater under high-temperature gradients as well as global volumetric strains. Three dielectric sensors (Decagon 5TM) were placed into a compacted MX80 layer having an initial gravimetric water content of 12.2% and a dry density of 1.15 Mg/m3 within a PVC modified Proctor mold (inner diameter of 152 mm and height of 178 mm), as shown in Figure 1(a). Low-density polyethylene sheeting was stretched across the top and bottom of the soil layers to minimize any global loss of water from the compacted bentonite layer during heating. A temperature-controlled hot plate was used as the basal heat source, and a dial indicator was mounted on the top of the soil layer to measure volume changes. Fiberglass insulation was wrapped around the cell to minimize lateral heat loss from the side boundary. A basal temperature of 128°C was maintained during a heating stage lasting 4000 h to ensure THM equilibrium, after which three short-term (96 h for each) cooling-heating cycles were applied. Results indicate that soil temperatures increase rapidly in the first 20 h of heating then gradually stabilize (Figure 1(b)). The volumetric water content increases at the beginning of heating then slowly decreases (Figure 1(c)). This increasing-decreasing trend is due to the upward movement of a wetting front away from the heater due to coupled heat transfer and water flow. An initial increase in volumetric strain (swelling) was followed by a decrease (contraction) as heating progressed (Figure 1(d)). This is consistent with the observed thermal response of normally consolidated expansive soils (e.g., [4]). A sharp monotonic decrease in volumetric strain occurred during the three short-term cooling-heating cycles. The temperature and water content follow nearly identical trajectories in each cycle. The temperature distribution in the bentonite with distance from the heating element after 4000 h of heating is smoother than the sharp drop-off in temperature from the small-diameter heater observed by Lu and McCartney [3], as shown in (Figure 1(f)). The main implication of the results presented in this study is that the surface area of the heater in contact with the surrounding bentonite plays a major role in the temperature distribution and the associated coupled water flow. This is a critical finding that may help guide the geometric design of small-scale laboratory mock-up experiments used to study THM processes in controlled laboratory conditions.

Comparison of two approaches for modelling fracture opening due to cold water injection in geothermal reservoir

Heat and electricity production from deep hot reservoirs through hydrothermal or petrothermal system requires to inject cold fluid in a naturally or artificially fractured medium. Cold water injection in a hot reservoirs causes thermo-hydro-mechanical (THM) coupled processes that may have several influences on operations and long term production [1]. Cold water has a higher viscosity, which means injection can become difficult due to higher flow impedance, while thermal diffusion in the rock matrix causes shrinkage and thus an increasing fracture aperture over time that can lead to flow channeling and a reduction of operation time. A deep understanding of the contribution of coupled THM processes to injection into fractured media is thus important to predict the long-term performance of a geothermal power plant. In this study, coupled THM processes in a single fracture are numerically investigated with two different approaches to model the discontinuity.
 The first numerical approach proposes modelling an implicit fracture in a fracture zone using solid elements (Fig.1 (a)). The impact of the fracture is introduced by means of a stress dependent fracture aperture, which in turn determines the fracture transmissivity. This function allows recovery of the opening, and thus the model replicates opening and closure of the fracture. Then, an equivalent continuum permeability [m2] is obtained as follows:
 where [m] is the elastic opening, br and bmax [m] are the residual and maximum apertures, respectively, a is the stress dependency coefficient; [Pa] is effective normal stress, and [m]the fracture spacing. The combination of the continuum elements to model an implicit fracture embedded in a fracture zone and the evolving permeability has been used successfully used to model elastic opening in volcanic rock [2] and granodiorite rock [3, 4].
 In the second approach, zero-thickness interface elements are used to explicitly model the opening of an existing fracture (Fig.1 (b)), following the work by Liaudat et al. [5]. A constitutive law that is capable of describing the fracture initiation, propagation, closing and opening is implemented in this approach, considering heat transfer in the discontinuity. In this approach, the cubic law is used to estimate the longitudinal transmissivity [m3] as a function of the normal fracture aperture:
 where [m] is the normal separation and [m3] is a constant value which makes it possible to assign an initial longitudinal transmissivity even if the fracture is mechanically closed.
 In both approaches, the simulation of cold water injection requires to consider the heat and hydraulic flows in the fracture alongside mechanical behaviour, since changes in pore water pressure and temperature influence the fracture aperture, thus modifying the fracture transmissivity.
 In order to compare both approaches, a simple synthetic model is studied. The model with its boundary and initial conditions are shown in Fig.2. Water with a temperature of 20 °C and pressure varying from 4 MPa – 10 MPa is injected at the centering point of the fracture. Figure 3 shows the injection pressure profile. Fracture permeability evolution is compared in Fig.4, which shows the two different methods capture different characteristics of the natural fracture behaviour. In the continuum method, the fracture is modelled as a continuum element, thus any pressure or stress change will result in corresponding changes in the fracture permeability, which is reflected in Fig.4 showing that the permeability changes with the injection pressure changing, In contrast, in the interface element, Fig.4 shows only when the injection pressure reaches 10 MPa, the permeability increases sharply from the initial value to the peak value. This is because penalty method is used to allow the small overlapping of the two faces of the discontinuity, thus keeping negligible permeability variation, before it opens. The result in Fig.4 also illustrates the effect of thermal stress on the fracture opening, considering the total vertical stress is 10 MPa while injection pressure is below or equal to 10 MPa.
 In conclusion, though the peak permeability predicted from two methods are consistent, both around 1.5 x 10 -12 D, the behaviour before fracture opens in these two approaches are quite different. For the interface method, permeability variations keep negligible before fracture opens, while for the continuum method, permeability changes with corresponding pressure or stress changes without dependence on the opening of the fracture.

Numerical modeling on nuclide transport around a nuclear waste repository under coupled thermo-hydro-mechanical condition

Coupled thermo-hydro-mechanical (THM) processes may raise the uncertainty of nuclide transport behavior in long-term operation of nuclear waste repository. A far-field modeling framework on nuclide transport around a nuclear waste repository under coupled THM conditions was developed and verified. Base on conceptual models of high-level waste repository in fractured rocks, a three-dimensional repository model including a vertical fault and a horizontal fracture was built, and influence of coupled THM processes on nuclide transport is numerically investigated. Effects of repository depth, fracture inclination, and internal aperture heterogeneity were investigated using the developed model. The results show that the coupled THM processes significantly affect nuclide transport behavior, especially in the early stages of nuclide leaching. The thermal stress induced by heat release of nuclear waste causes the reduction in fracture aperture around the repository. The nuclide transport under THM conditions is retarded before 103 years than that under hydraulic condition. The deeper locations of the candidate repository exhibit the lower rate of nuclide transport. The inclined fracture induces opposite flow directions between two fractures, which causes the delaying of nuclide transport. Fractures with heterogeneous apertures have preferential flow paths, which results in early arrivals of nuclide transport than fractures with homogenous apertures.

Open-source Python library for modeling coupled thermo-hydro-mechanical (THM) processes

Abstract. The interactions between temperature, fluids, and mechanical properties in a repository system are essentially described scientifically by coupled thermo-hydro-mechanical (THM) processes. THM modeling, i.e., the prediction of the behavior of materials under different conditions, is the fundamental numerical tool here. The confident handling and deep understanding of numerical computation methods is thus the prerequisite for performing and evaluating preliminary safety analyses in the site selection process. At the Federal Office for the Safety of Nuclear Waste Management (BASE), a software library for the simulation of coupled THM processes is currently being developed. The main goal of the in-house development is an open-source toolbox in the scripting language Python and is motivated by several long-term sub-goals: targeted development of expertise within BASE regarding numerical modeling of safety-relevant aspects in the long-term safety analyses; diversification of the testing capabilities regarding the preliminary safety investigations by means of an in-house, independent modeling software; foundation of a library of known benchmarks and evaluation examples for the comparison of different software tools; documentation and processing of basic THM scenarios for internal or, if necessary, public technical training. The focus of this development is on creating a toolbox that is easy to use and at the same time highly flexible. The main methodical aspects are as follows: building a new library based on the pyGIMLi pre- and postprocessing framework (Rücker et al., 2017); creating a finite-element reference implementation in the Python scripting language for maximal transparency; creating an easy-to-use interface to the solution of the weak formulation for the finite-element theory with expressions of a symbolic manner allowing maximal flexibility; defining an interface to allow for the integration of alternative, third-party high-performance libraries; creating a collection of Jupyter notebooks of well-documented test cases and benchmarks. Choosing the open-source approach ensures the best possible transparency and, in the long term, also allows the provision of appropriately quality-assured and documented simulation tools to the public. The presented poster shows the current development status of the software library and the currently implemented quality assurance concepts and gives an outline of the potential applications of the library.

Numerical simulation of long-term performance of deep geological repository placement rooms in crystalline and sedimentary rocks

Evaluation of the stability of placement rooms and damages in the geosphere surrounding a deep geological repository (DGR) for high-level nuclear waste are essential components for assessing the overall repository performance. Assessments of the performance of placement rooms for the duration of the regulatory period (typically 1 million years) usually involves simulations using numerical codes capable of modeling coupled thermo-hydro-mechanical (THM) processes induced by different natural and repository-induced perturbations over different time and length scales. The numerical constitutive model used to represent the host medium should be capable of capturing highly nonlinear responses to stress changes, including development of damage and fracturing. In this study, a novel approach for simulation of damage evolution and fracturing around the repository rooms, based on the bonded block model (BBM) devised for analyzing coupled THM processes, is used to perform a study of the long-term geomechanical performance of the Nuclear Waste Management Organization hypothetical DGR configurations in sedimentary and crystalline geological settings. The perturbation effects of different loading conditions, including in-situ stress changes by room excavation, heat generated from the used fuel canisters, and glaciation were analyzed. Each THM analysis was conducted using a 3D continuum numerical code to generate evolving temperature and pore pressure fields in the rock mass. The temperature and pore pressure data were then imported in the 2D BBM model for simulating evolution of damage around the placement rooms. Sensitivity analysis was conducted to investigate the effect of different poro-mechanical parameters, rock mass permeabilities, and different approaches for calibrating the BBM, on predictions of room stability and extent of the excavation damage zone around the placement room. The modeling results indicate that the confinement provided by the bentonite backfill inside the room clearly prevents any unravelling but also limits the extent of the damage to less than 1.6 m into the room walls in any of the analyzed cases considering different loading conditions and uncertainty of the material properties.

Physics-Informed Neural Network Solution of Thermo–Hydro–Mechanical Processes in Porous Media

Physics-informed neural networks (PINNs) have received increased interest for forward, inverse, and surrogate modeling of problems described by partial differential equations (PDEs). However, their application to multiphysics problem, governed by several coupled PDEs, presents unique challenges that have hindered the robustness and widespread applicability of this approach. Here we investigate the application of PINNs to the forward solution of problems involving thermo–hydro–mechanical (THM) processes in porous media that exhibit disparate spatial and temporal scales in thermal conductivity, hydraulic permeability, and elasticity. In addition, PINNs are faced with the challenges of the multiobjective and nonconvex nature of the optimization problem. To address these fundamental issues, we (1) rewrote the THM governing equations in dimensionless form that is best suited for deep learning algorithms, (2) propose a sequential training strategy that circumvents the need for a simultaneous solution of the multiphysics problem and facilitates the task of optimizers in the solution search, and (3) leveraged adaptive weight strategies to overcome the stiffness in the gradient flow of the multiobjective optimization problem. Finally, we applied this framework to the solution of several synthetic problems in one and two dimensions.

An evaluation framework for production performance of high-temperature fractured and karstified geothermal reservoirs: Production mechanism, sensitivity study, and key parameters ranking

High-temperature (>150 °C) geothermal resources have great potential for sustainable power generation. These geothermal resources can be exploited efficiently through highly permeable geological structures, i.e. tectonic fractures and dissolution caves, which thus are categorized as fractured and karstified geothermal reservoirs. However, the study of long-term cold fluids injection into these high-temperature geothermal systems is faced with significant unsolved challenges, i.e. the simulation of coupled porous media flow and free flow in multiscale and heterogeneous fracture-cave networks with distinct thermo-hydro-mechanical (THM) coupling effects. To address these challenges, we develop a novel coupled THM model based on the discrete fracture-cave network approach. We then apply this model to perform the analysis of THM coupling mechanism, sensitivity study, and key parameters ranking to identify the development characteristics of high-temperature geothermal reservoirs. Results show that with respect to coupled THM processes, the fluid flow and heat transfer are dominated by connected fracture clusters and are strongly influenced by the inclusion of caves. However, the behavior of solid deformation is governed by the distribution of caves. As to the reservoir thermal performance under different working fluids, i.e. water and CO2, the water-based geothermal reservoir exhibits longer service life, greater global thermal power, and higher heat extraction rate than the CO2-based one. Regarding sensitivity analysis and key factors ranking, fracture attributes (fracture length and density) are the two most important parameters in regulating the geothermal extraction performance. Caves fall out as the third most significant parameter. However, the operating parameters (i.e. injection temperature, injection pressure, production pressure, well length, and well distance) play a limited role in influencing thermal performance compared with geometrical parameters of fractures and caves. The developed model and insights obtained from our research have crucial implications for understanding and optimizing the thermal production of high-temperature fractured and karstified geothermal reservoirs.

Improving heat extraction performance of enhanced geothermal systems: Insights from critical fracture network parameter and multi-objective optimization method

• An integrated workflow is proposed for improving thermal recovery of hot dry rocks. • Network connectivity is a key fracture parameter in dominating thermal efficiency. • Network connectivity governs thermo-hydro-mechanical processes of fractured rocks. • Fractures with connectivity around 15–35 are better for production optimization. The generation of a highly permeable fracture network in Enhanced Geothermal Systems (EGSs) is a prerequisite to develop such geothermal reservoirs efficiently. However, it lacks a quantitative geometrical metric to evaluate what kind of fracture network is preferable for geothermal production. To address this problem, we use our previously developed high-fidelity Thermo-Hydro-Mechanical (THM) coupling model to investigate the joint influence of fracture geometry and thermal stress on the THM behaviors of EGSs. Results show that the geometrical connectivity of fracture networks plays a dominant role in determining the THM processes and the thermal performance of EGSs, in which the network connectivity integrates multiple properties of fracture networks including the fracture length, intensity, location and orientation. Satisfactory thermal performance tends to be achieved when the connectivity of fracture systems ranges from 15 to 35. We focus on such fracture networks and design an appropriate production scenario for optimizing multiple objectives (i.e. maximum EGS service life, heat extraction rate, and global thermal power). The overall heat extraction performance of EGSs under the optimum scenario gets improved by 77.7% compared with the base case. This research provides an effective workflow and feasible methods for evaluating and optimizing energy exploitation efficiency of EGSs.

Simulations and global sensitivity analysis of the thermo-hydraulic-mechanical processes in a fractured geothermal reservoir

A fractured reservoir can support and sustain long-term geothermal energy extraction operations. The process of injecting cold water to extract hot water from a fractured reservoir result in thermal and poroelastic stress alteration in the rock matrix and the fracture. The thermo-hydro-mechanical (THM) processes govern the efficiency of an enhanced geothermal system (EGS) operation. The THM processes are controlled by various rock and fluid parameters and the initial and boundary conditions of the model set-up. In this paper, we have identified and analyzed 22 parameters. Due to this large number of involved parameters, it is a rigorous process to accurately estimate the relative importance of individual parameters. Thermal breakthrough time, mass flux and overall energy recovery are the three key aspects of operating a geothermal reservoir efficiently and economically. We have performed an extensive sensitivity analysis using the design of the experiments method (Plackett-Burman) and identified key parameters influencing these three key aspects. For a given discrete fracture network, the fracture aperture, the rock matrix permeability, and the wellbore radius are the most influential parameters controlling the thermal breakthrough time, production well mass flux, and overall energy recovery. Accurate estimation of these influencing parameters is important before a field investigation.

Coupled THM analysis of a full-scale test for high-level nuclear waste and spent fuel disposal under actual repository conditions during 18 years of operation

This study focuses on the thermo-hydro-mechanical (THM) analysis of the behaviour of a full-scale engineered barriers experiment designed to study the long-term response of a disposal concept for high-level radioactive waste (HLW) and spent fuel (SF). Crucially, the test involved two dismantling operations, a partial one after 5 years of heating and a final one, after 18·3 years of operation, allowing the direct observation of the state of the barrier at two different times. The coupled THM modelling discussed in this paper mainly focuses on the last 13 years of test operation and also on the final dismantling of the barrier system. The numerical model was unchanged from the early stages of the test, so the results presented can be considered as blind predictions. The numerical model predicted reasonably well the state of the barrier and its evolution, especially in terms of the distribution of the bentonite dry density, confirming the predictive capability of the model to capture the transient THM processes. The combination of experimental and numerical investigations, involving one of the most long-lasting full-scale in situ experiments monitoring the behaviour of the barrier components of an HLW/SF repository, has led to a better understanding of the THM behaviour of this complex system.

A simple computational framework for THM processes in saturated geomaterials

Investigations of thermo-hydro-mechanical (THM) processes in geomaterials have become a continuing concern due to the increasing interests in the application of geothermal energy. In the last two decades, a number of THM coupled constitutive models were proposed for soils and rocks based on either the elastoplastic theory or the thermodynamics. The practical application of the constitutive models inevitably requires computational approaches such as finite element or finite difference methods. In this paper, a simple computational framework for THM processes in saturated geomaterials is proposed. An effort is made to explain the details and major concerns related to the governing equations of three physical fields. The computational framework is validated through the benchmark problem of 1D THM processes in saturated linear elastic material by comparing with the analytical solution. We then further investigate some key factors of the multiphysical processes and their influence on the time-dependent evolution of excess pore pressure, effective stress and strain. The flexibility of the framework allows the effect of some coupling components, such as the temperature-dependent dynamic viscosity of pore fluids, which are often ignored in previous studies, to be examined. The framework serves as the basis for the next-step in-depth analysis of theoretical multiphysical problems and practical design of energy foundations.

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.

Experimental Study on the Permeability of Quartz Sandstone under Coupled Thermo-Hydromechanical Loading

Abstract This paper focuses on the influence of coupled thermo-hydromechanical processes on the permeability of quartz sandstone. The permeability has been studied under five different confining pressures, three different temperatures, and three fluid pressures. The experimental results exhibit that the permeability of quartz sandstone decreases with the increase of confining pressure while it increases with temperature and fluid pressure. The identification of permeability under fully coupled thermo-hydromechanical conditions is also studied via the realization of four coupled tests. One observes that the temperature plays a more important role on the permeability with respect to the fluid pressure. Moreover, the influence of fluid pressure on the permeability of studied sandstone has been amplified by the temperature. The obtained experimental results allow us to get a good understanding of the permeability of quartz sandstone in petroleum engineering and can help us to guarantee the long-term structural stability.

Numerical Prediction of the Large-Scale in Situ PRACLAY Heater Test in the Boom Clay

The large-scale PRACLAY heater test has been running since November 2014 in the HADES URF (Belgium) to examine the combined impact on the host Boom Clay of the thermo-hydro-mechanical (THM) perturbations due to gallery construction and a large-scale thermal load which simulates the heat dissipation of radioactive waste. To get an insight into the expected evolution of the test, which will be helpful to guide the test, numerical prediction of the PRACLAY heater test had been performed before the heater was switched on. The numerical prediction, based on the existing related knowledge before the switch-on of the heater test, took into account the complex coupled THM processes and interactions in all the involved materials of the experimental set-up (Boom Clay, bentonite, concrete lining, backfill sand, etc.) and the test phases from the gallery excavation till the end of heating which is planned in 2025. A large number of constitutive models and associated parameters required to define the numerical problem had been identified and calibrated on the basis of literature review, results of extensive laboratory tests and back analysis of many relevant in situ measurements from the HADES URF. The results of the numerical prediction are constantly and extensively compared with the in situ measured data obtained since the switch-on of the heater till 4-year stationary heating. Overall, it can be stated that the general Boom Clay THM responses in the PRACLAY heater test are in good agreement with the numerical prediction, which gives good indications on the model capacity and confirms the existing knowledge of the Boom Clay THM behaviour.