Thermo-hydro-mechanical-chemical Research Articles

A Numerical Simulation Study on the Migration of the 90Sr Nuclide of Buffer Material Under the Coupling Effect of Multiple Factors

With the development of nuclear energy in China, the geological disposal of high-level radioactive waste (HLW) is increasingly receiving national attention. Among them, the study of nuclide migration is an important and complex technical system, which requires continuous in-depth research. Under the decay heat, radiation, and groundwater effects of HLW, buffer materials generate complex coupled thermo-hydro-mechanical-chemical (THMC) processes. The migration and diffusion of nuclides in buffer materials are controlled by the coupling effect of THMC. It is of great significance for the long-term safety of a HLW repository to study the long-term retarding effect of buffer material on nuclide strontium under the coupling effect of multiple factors. This study leverages the solving advantages of COMSOL Multiphysics 5.6, using a combination of the self-developed Mock-up experimental device as a geometric model and numerical simulations to study the multi-field coupling performance and radionuclide migration evolution characteristics of THMC buffer materials, which overcomes the difficulties due to the limitations of the experimental time and spatial scale. The simulation results can predict the migration range and distance variation of strontium in buffer materials at different time scales. In the initial stage, the migration and diffusion of nuclide in buffer materials are relatively slow, and the migration distance increases by about 0.03 m with time. In the mid-to-late stage, the migration distance increases by about 0.05 m over time; to ensure that, in 1000 years, core strontium does not penetrate the buffer material and migrate into the surrounding rock groundwater of the disposal facility, a buffer material thickness of 0.3 m needs to be set. The construction of THMC control equations for the migration and diffusion of nuclides in buffer materials under multi-field coupling conditions has been revealed, providing an important reference for a deeper understanding of the risk analysis of radionuclide contamination in disposal environments.

Atomistic origin of montmorillonite clay subjected to freeze-thaw hysteresis

The freeze-thaw cycles of frozen soil could significantly affect its thermo-hydro-mechanical-chemical (THMC) properties, causing the frost heaving and thawing settlement. The microscale essence is the water-ice phase transition, but the microscale details are still poorly understood, especially at ultra-low temperatures. Nuclear magnetic resonance (NMR) technology and molecular dynamics (MD) simulation methods were performed to explore the freeze-thaw behaviors of montmorillonite clay under temperature of 210 - 293 K. Then, the water-ice phase transition, freeze-thaw hysteresis, ice nucleation mechanism, and surface effect of clay at an atomistic level were discussed. A classification method of different types of unfrozen water through NMR experiment was proposed, including bulk, capillary, and bound water. Here, it is found that: (1) the freeze-thaw process of frozen soil at the macroscale was essentially the occurrence of ice-water phase transition at the microscale. (2) The freeze-thaw hysteresis was caused by different growth and melting rates of ice crystals, where the ice growth/nucleation on clay surface was more difficult to develop. (3) The surface effect of clay was essential for the ice nucleation and the existence of bound water. For example, little unfrozen water still existed in unfrozen soil even at 213 K. (4) For unsaturated frozen soil, the quasi-liquid water was an essential component of unfrozen water that cannot be ignored. This work could provide an atomistic insight to unravel the atomistic origin of the freeze-thaw mechanism of montmorillonite clay and complement relevant experimental evidence.

Numerical simulation for subsidence control in CO2 storage and methane hydrate extraction

Gas hydrates are increasingly viewed as a promising alternative to traditional fossil fuels. However, their extraction process poses risks to structural integrity, potentially causing significant subsidence. In this study, we developed a Thermo-Hydro-Mechanical-Chemical (THMC) model to analyze the impact of gas hydrate extraction on seabed subsidence. Our investigation focused on the influence of bottom hole flowing pressure, initial hydrate concentration, gas saturation, permeability, porosity, and rock thermal conductivity on subsidence during gas hydrate extraction via depressurization.The results show that seabed subsidence is affected by various factors such as bottom hole flowing pressure, initial hydrate concentration, gas saturation, permeability, porosity, and rock thermal conductivity. It was noted that significant subsidence is associated with low initial hydrate concentration, high permeability, porosity, low gas saturation, low rock thermal conductivity, and a notable pressure drop of 79.31%.To address this issue, we propose a seabed subsidence mitigation strategy involving CO2 injection. This approach not only safeguards offshore infrastructure and coastal communities but also helps reduce CO2 emissions, aligning with global climate change mitigation efforts. In our model, CO2 injection occurs in the subsurface reservoir at the interface between the free water zone and hydrate-bearing formations. The CO2 hydrates formation process releases heat, which dissociates methane hydrates, allowing the methane to be replaced by CO2 molecules and move towards the production well.Our analysis reveals that increasing injection temperature and rate significantly reduces subsidence. Additionally, the range of investigated injection pressures, which included pressures equal to and more than double the initial reservoir pressure, showed inconsequential impacts on seabed subsidence.The effectiveness of subsidence reduction is significantly enhanced by injecting a CO2/N2 mixture compared to pure CO2 injection. The most substantial reduction in subsidence occurred when a mixture of CO2 and N2 in a 50/50 vol/vol ratio was injected at a high rate.These findings offer crucial insights for optimizing the efficiency and control of gas hydrate extraction methods. They emphasize the importance of employing balanced injection strategies to minimize environmental risks and ensure sustainable energy extraction.

State of the Art of Coupled Thermo–hydro-Mechanical–Chemical Modelling for Frozen Soils

AbstractNumerous studies have investigated the coupled multi-field processes in frozen soils, focusing on the variation in frozen soils and addressing the influences of climate change, hydrological processes, and ecosystems in cold regions. The investigation of coupled multi-physics field processes in frozen soils has emerged as a prominent research area, leading to significant advancements in coupling models and simulation solvers. However, substantial differences remain among various coupled models due to the insufficient observations and in-depth understanding of multi-field coupling processes. Therefore, this study comprehensively reviews the latest research process on multi-field models and numerical simulation methods, including thermo-hydraulic (TH) coupling, thermo-mechanical (TM) coupling, hydro-mechanical (HM) coupling, thermo–hydro-mechanical (THM) coupling, thermo–hydro-chemical (THC) coupling and thermo–hydro-mechanical–chemical (THMC) coupling. Furthermore, the primary simulation methods are summarised, including the continuum mechanics method, discrete or discontinuous mechanics method, and simulators specifically designed for heat and mass transfer modelling. Finally, this study outlines critical findings and proposes future research directions on multi-physical field modelling of frozen soils. This study provides the theoretical basis for in-depth mechanism analyses and practical engineering applications, contributing to the advancement of understanding and management of frozen soils.

Numerical analysis on the effects of hydrothermal development on the permeability of porous reservoir based on coupled THMC modeling

As a clean and renewable energy, geothermal is gaining widespread attention worldwide. The development of hydrothermal resources is accompanied by the coupling processes of thermo-hydro-mechanical-chemical (THMC) caused by production and injection, which have a complex effect on the porosity and permeability of reservoir. Exploring the effects of these processes on the changes in various physical fields within the reservoir is of great significance for the efficient and sustainable development of geothermal energy. Based on the fully coupled framework of fluid flow and transfer in TOUGH2, a method that considered both mechanical and chemical processes was proposed and taken in this study, to analyze the effects of long-term injection and production on reservoir porosity and permeability. The results show that the effective stress changes under fluid pressure and thermal stress, as well as mineral dissolution and precipitation under chemical action, jointly led to the variation of reservoir porosity and permeability. The overall variation trend was mainly controlled by the mechanical action, as the influence of fluid pressure was more obvious and rapid, which dominated the mechanical changes of the whole reservoir in the early stage of injection (within 3.5 years). With the continuous injection, thermal stress played a more obvious role around the cooling area. By 30 years, the significant change in effective stress has extended to the entire reservoir. In the later stage of operation (after 20 years), as the temperature field of the reservoir gradually stopped changing, porosity began to increase under the dominance of fluid pressure. The effect of chemical action was long-term and lag in time. Mineral precipitation only occurred near the injection well, and the precipitation range and amount increased gradually with the continuous injection. By 30 years, the precipitation of dolomite injected into the grid reached 0.06%.

Hydromechanical characterization of gas transport amidst uncertainty for underground nuclear explosion detection

Given the challenge of definitively discriminating between chemical and nuclear explosions using seismic methods alone, surface detection of signature noble gas radioisotopes is considered a positive identification of underground nuclear explosions (UNEs). However, the migration of signature radionuclide gases between the nuclear cavity and surface is not well understood because complex processes are involved, including the generation of complex fracture networks, reactivation of natural fractures and faults, and thermo-hydro-mechanical-chemical (THMC) coupling of radionuclide gas transport in the subsurface. In this study, we provide an experimental investigation of hydro-mechanical (HM) coupling among gas flow, stress states, rock deformation, and rock damage using a unique multi-physics triaxial direct shear rock testing system. The testing system also features redundant gas pressure and flow rate measurements, well suited for parameter uncertainty quantification. Using porous tuff and tight granite samples that are relevant to historic UNE tests, we measured the Biot effective stress coefficient, rock matrix gas permeability, and fracture gas permeability at a range of pore pressure and stress conditions. The Biot effective stress coefficient varies from 0.69 to 1 for the tuff, whose porosity averages 35.3% ± 0.7%, while this coefficient varies from 0.51 to 0.78 for the tight granite (porosity <1%, perhaps an underestimate). Matrix gas permeability is strongly correlated to effective stress for the granite, but not for the porous tuff. Our experiments reveal the following key engineering implications on transport of radionuclide gases post a UNE event: (1) The porous tuff shows apparent fracture dilation or compression upon stress changes, which does not necessarily change the gas permeability; (2) The granite fracture permeability shows strong stress sensitivity and is positively related to shear displacement; and (3) Hydromechanical coupling among stress states, rock damage, and gas flow appears to be stronger in tight granite than in porous tuff.

The impact of thermal transport on reactive THMC model for carbon capture and storage

Addressing current limitations in Thermo-Hydro-Mechanical-Chemical (THMC) models is crucial for accurately simulating complex physical and chemical interactions in carbon capture and storage (CCS) processes. This paper presents a new fully coupled THMC model, specifically focusing on investigating thermo-chemo coupling (dissolution) effects for CCS applications. The model is a further extension of the mixture coupling theory, which incorporates entropy and mass balance equations, phenomenological equations, and Maxwell's relations to describe the intricate coupling between mechanical, hydraulic, chemical, and thermal properties. The model considers a two-phase flow, liquid and supercritical. By deriving new coupling terms from the phenomenological equations, the model provides capacity to account for energy losses due to friction between the two phases, along with energy losses arising from the transport of the solute and the evolution of porosity/permeability. Furthermore, the model incorporates an additional coupling term in the mechanical equation to represent the chemical dissolution effect on volumetric strain relief. Two terms added in the liquid equation account for the friction of newly dissolved minerals particles on liquid transport and the effects of porosity changes on liquid flow through dissolution. Finally, Thermal transport coupling is integrated into the model, as it is a crucial part of CCS applications. Numerical results demonstrate that increasing temperature significantly decreases porosity due to thermal expansion, but thermal energy triggers chemical dissolution which in the long run increases both porosity and permeability. Chemical dissolution has the most significant incremental effect on porosity over time, while thermal expansion has the greatest impact until thermal equilibrium is reached, with the direct impacts of pressure and strain on porosity falling in between. This research presents a comprehensive coupling framework that accounts for two-phase friction, the dissolution process, and changing permeability, which gives it potential utility in the planning process for CCS applications.

Numerical Analysis on Gas Production and Geomechanical Responses of Natural Gas Hydrate Reservoirs.

Natural gas hydrate (NGH) has attracted considerable global attention as a promising energy resource in recent years. To acquire valuable insights into regarding the interplay between mechanical properties and production outcomes during the production, in this study, a fully coupled thermo-hydro-mechanical-chemical (THMC) model based on the geological features of reservoirs in the Shenhu area of the South China Sea (SCS) was developed to analyze the response characteristics of various physical fields within the reservoir during the exploitation. Furthermore, the study examined the influence of mechanical behavior on hydrate exploitation and investigated the effects of varying initial hydrate saturation and seawater depth on production efficiency and reservoir deformation. The simulation results indicated that neglecting the impact of solid mechanics in the analysis of hydrate productivity can result in overestimated results, particularly during the initial production stage. Reservoirs with higher hydrate saturation experience lower initial production rates due to the influence of permeability and capillary force. Moreover, reservoirs with high hydrate saturation exhibit greater compression but lower wellhead subsidence during the long-term development. The impact of seawater depth on production capacity primarily arises from the pressure's influence on the gas-water ratio, where greater seawater depth corresponds to increased reservoir compression and wellhead subsidence.

Integration of subsurface dynamic coupled modelling and monitoring technologies for CCS: examples from existing projects

&#x0D; Introduction&#x0D; Host rock injectivity and storage complex integrity (containment) for CO2 sequestration are critical factors to ensure that capacity targets are met while preventing CO2 leakage over time and confirm permanent storage.&#x0D; &#x0D; Figure 1 sketches the typical CO2 injectivity problem. Evaluating injectivity, capacity and containment requires assessing key phenomena such as: seal and faults capillary controls, non-isothermal reactive flow, geomechanical failure induced in the seal, along faults and/or fractures and in the vicinity of the wells. Additional analyses will also be needed to evaluate the completion integrity of the wells (injectors, monitoring and existing wells) under variable load conditions in terms of pressure, temperature and chemistry. The listed phenomena, and their likely effects, are analysed by means of dynamic coupled modelling, considering co-existing (geo)mechanical and dynamic flow simulations. Therefore, coupled modelling improves the risk analysis allowing to evaluate the Thermo-Hydro-Mechanical-Chemical (THMC) processes associated to CO2 injection.&#x0D; Analyses&#x0D; In this presentation we will provide some examples of integration of subsurface dynamic coupled modelling and monitoring technologies for CCS. Some of these examples are derived from existing projects.&#x0D; &#x0D; Injectivity impairment during CO2 injection is a well-known issue often observed at well and field scale (e.g. Snøhvit, Ketzin, Decatur projects [1], [2]). Loss of injectivity is governed by the interactions between CO2-brine phase behavior, CO2-solid phase (rock) behavior, salt precipitation, multi-phase flow. Characterizing zonal injectivity and developing mitigation strategies will be therefore important for project success. Figure 2 (left and center) shows an example of time-lapse CO2 saturation evolution during injection and consecutive non-isothermal processes associated to Joule-Thompson effect (H2O vaporization, cooling and induced salt precipitation due to dry-out). Temperature and pressure changes can lead to near wellbore failure (thermal and pressure induced fracturing) impacting injection performance due to near wellbore fracturing and consecutive permeability changes (Figure 2 right).&#x0D; To close the evaluation loop, the integrated subsurface characterization workflow is completed by measurements to image the subsurface before and after CO2 injection. This provides essential insights into the storage complex quality before injection and effective monitoring during injection and permanent storage, tracking the position of the CO2 pressure and concentration plumes, assessing surface heave, and detecting any risks or potential leakage paths. There is a tight link between subsurface dynamic coupled modelling and existing monitoring technologies. This link expands the well-known MMV (Measurement, Monitoring and Verification) process into a more integrated 3MV process (Modelling, Measurement, Monitoring and Verification), opening to the definition of a subsurface digital twin supporting successful CCS operations. We will briefly introduce methods and technologies enabling the subsurface dynamic modelling and monitoring of CO2 injection and storage. Seismic and borehole petrophysical and geophysical measurements for initial subsurface characterization and consecutive monitoring will be described.

Thermal‒hydraulic‒mechanical‒chemical coupling analysis of enhanced geothermal systems based on an embedded discrete fracture model

Enhanced geothermal system (EGS) is subject to the comprehensive effects of multiple physical fields during the long-term heat extraction process, including hydraulic (H), thermal (T), mechanical (M) and chemical (C) fields. The embedded discrete fracture model (EDFM) can effectively simulate the variations of flow, temperature, mechanical and concentration fields in fractured reservoirs. At present, however, the thermo-hydro-mechanical-chemical (THMC) coupling model based on EDFM is less researched. In this paper, the THMC coupling model of fractured reservoir is established based on EDFM by considering the changes in reservoir heterogeneity and physical properties as well as water–rock reactions. Then, the spatiotemporal evolution of flow, temperature, displacement and concentration fields in the operation process of EGS is simulated and analyzed. And the following research results are obtained. First, when the permeability of the basement rock is low, the production temperature decrease during exploitation is gradual, allowing EGS to maintain a high exploitation temperature for an extended period. However, lower permeability may result in a decrease in the quality flow rate from production wells, thereby affecting net heat extraction power. Second, when fracture permeability or fracture opening changes, EGS can output higher temperature stably for a certain period and then the temperature decreases at different amplitudes. When the fracture permeability increases to a certain value or the fracture opening decreases to a certain value, the influence of the change in fracture parameters on production temperature gets weak. Third, After 40 years of EGS operation, considering variable property fluids results in a 22 °C lower exploitation temperature compared to using constant property fluids, and considering water–rock reactions results in a 15 °C lower exploitation temperature, with a 12.5 % increase in reservoir average porosity. In conclusion, when researching a long-term operating EGS, it is necessary to comprehensively consider the influences of reservoir rock parameters, physical properties of injected fluid, water–rock reaction and other factors. And in the future, attention shall be paid to the two-way coupling of chemical reaction and mechanical deformation of other mineral compositions in the reservoir to the hydro-thermo-chemical field influence, so as to provide more accurate and reliable prediction for the engineering development and utilization of EGS reservoirs.

The Harpers THMC flow bench: A triaxial multi-reactor setup for the investigation of long-term coupled thermo-hydro-mechanical-chemical fluid-rock interaction.

The scientific analysis and interpretation of coupled thermo-hydro-mechanical-chemical (THMC) processes in rocks requires complex and diverse instrumentation. In this study, we introduce the "Harpers THMC Flow Bench," a multi-cell, flow-through reactor system that allows long-term testing on rock plugs and powdered samples. The setup consists of four small triaxial cells that can hold confining and pore pressure of up to 20 MPa and an axial load of up to 300 MPa. Axial deformation of the samples is measured with linear variable differential transducers. The cells can be heated to 90 °C, and effluents (gas, water, and brine) can be sampled for compositional analysis. An additional Hastelloy-autoclave enables fluid mixing and saturation with gas prior to injection into the samples. Each cell can be operated individually, allowing independent experiments over long testing periods. The sample holders were designed such that they are transparent for X-rays during X-ray computer tomography, minimizing sample handling effects on the imaging results. To demonstrate examples of the capabilities of the flow bench, we present case studies on Carnmenellis granite (Cornwall, UK) and Castlegate sandstone (Utah, USA) samples. Permeability measurements are shown using fractured granite undergoing periodic loading of effective pressure. To demonstrate chemical measurement capabilities, we used deionized water to leach elements from granite powders. We then analyzed effluent compositions using inductively coupled plasma optical emission spectroscopy. Finally, we conducted a strength test and a cyclic differential stress test on sandstone to demonstrate the mechanical testing capabilities of the setup.

Thermal-hydraulic-mechanical-chemical modeling and simulation of an enhanced geothermal system based on the framework of extended finite element methods - Embedded discrete fracture model

The thermo-hydro-mechanical-chemical (THMC) coupling process plays a crucial role in Enhanced Geothermal System (EGS), particularly during long-term heat extraction. However, most models for fractured reservoirs are computationally expensive, with very dense grids required, especially near fractures, limiting their practical use in EGS research. The embedded discrete fracture model (EDFM) can construct fracture and matrix grids independently, significantly reducing computational complexity while maintaining high accuracy. Although it is suitable for efficiently evaluating the long-term performance of heat extraction in fractured reservoirs, there are few THMC-coupled models based on EDFM, and THMC coupling effects have often been overlooked in evaluations of the environmental benefits of EGS. To address this, a fully THMC-coupled model based on EDFM and the extended finite element method (XFEM) is proposed and verified. The evolution of heat production and environmental benefits of an EGS over 30 years are presented. The results show that the production temperature and electric power remain highly efficient in the first 5 years and decrease by 7.8% and 15.4% respectively over 15 years. Moreover, the total amount of extracted heat during this period is 6.52 × 1012 kJ, which is equivalent to saving 22.26 × 108 kg of coal consumption and reducing CO2 emissions by 5.83 Mt, SO2 emissions by 189.22 × 105 kg, and NOx emissions by 166.96 × 105 kg. These findings highlight the significant potential of geothermal energy for energy supply and environmental benefits. Additionally, the study investigates the effects of critical operating and physical parameters on the production performance of EGS, including injection temperature, injection flow rate, initial reservoir temperature, and initial reservoir permeability. This work serves as a reference for both the theoretical THMC coupled model and practical EGS engineering.

Numerical Simulating the Influences of Hydrate Decomposition on Wellhead Stability

Natural gas hydrate reservoir has been identified as a new alternative energy resource which has characteristics of weak cementation, low reservoir strength and shallow overburden depth. Thus, the stability of subsea equipment and formation can be affected during the drilling process. To quantitatively assess the vertical displacement of the formation induced by hydrate decomposition and clearly identify the influence laws of various factors on wellhead stability, this study established a fully coupled thermo-hydro-mechanical-chemical (THMC) model by using ABAQUS software. The important factor that affects the wellhead stability is the decomposition range of hydrates. Based on this, the orthogonal experimental design method was utilized to analyze the influence laws of some factors on wellhead stability, including the thickness of hydrate formation, initial hydrate saturation, overburden depth of hydrate sediment, and mudline temperature. The results revealed that the decomposition of hydrate weakens the mechanical properties of the hydrate formation, thus leading to the compression of the hydrate formation, further causing the wellhead subsidence. When the duration of drilling operations was 24 h and no decomposition of natural gas hydrate occurs, the wellhead subsidence is recorded at 0.053 m, this value increases with an increase in drilling fluid temperature. The factors were listed in descending order as following, according to their significance of influences on wellhead stability: the thickness of hydrate formation, initial hydrate saturation, overburden depth of hydrate sediment, and mudline temperature. Among the above factors, statistical significance of the mudline temperature was less than 15% confidence level, suggesting that the effect of mudline temperature on wellhead stability is negligible. These findings not only confirm the influence of hydrate decomposition on wellhead stability, but also suggest important implications for the drilling of hydrate-bearing formation.

Rapid testing and prediction of soil–water characteristic curve of subgrade soils considering stress state and degree of compaction

The subgrade soil is generally in saturated or unsaturated condition. To analyze complex thermo-hydro-mechanical-chemical (THMC) behaviors of subgrade, it is essential to determine the soil–water characteristic curve (SWCC) that represents the relationship between matric suction and moisture content. In this study, a full-automatic rapid stress-dependent SWCC pressure-plate extractor was developed. Then, the influences of overburden stress and degree of compaction on the SWCC of subgrade soil such as high liquid limit silt (MH) and low liquid limit clay (CL) were analyzed. Accordingly, a new model taking into account the influences of overburden stress and degree of compaction based on the well-known Van Genuchten (VG) SWCC fitting model was presented and validated. The results show that with the increase of the degree of compaction and overburden stress, the saturated moisture content of subgrade soil decreases, while the air-entry value increases and the transition section curve becomes flat. The influences of the degree of compaction and overburden stress on the SWCC of MH is greater than that of CL. Meanwhile, there was a satisfactory agreement between the prediction and measurement, indicating a good performance of the new model for predicting the SWCC.

A numerical study on THM coupled behavior in the high-level radioactive waste disposal system

It is essential to securely isolate high-level radioactive waste from the biosphere, and geologic disposal of it at a deep underground repository is considered the most effective method. Therefore, it is crucial to research the complex thermo-hydro-mechanical-chemical (THMC) coupled behavior in geological disposal systems parallel with numerical simulation. DECOVALEX is an international cooperating project to efficiently develop numerical methods and models and validate through test results for predicting the THMC interactions in the disposal systems. In Task C of DECOVALEX-2023, the modeling teams focus on understanding pore pressure development and THM interactions in the host rock and buffer material during the FE experiment, a 1:1 scale in-situ heater test based on Nagra’s Reference repository design. We used OGS-FLAC3D for the numerical simulation, combining OpenGeoSys for TH simulation and FLAC3D for M simulation. At the first phase of the task, a simple two-dimensional benchmark problem was defined to set up the numerical model. THM coupled processes in the bentonite were simulated with a two-phase flow system, and we investigated the temperature and pressure variations on the given monitoring position. Vaporization induced by a temperature increase in the bentonite was observed at the heater’s vicinity, and flow occurred by capillarity, and pressure difference was well simulated. Additionally, the flow process was dominant in the near field of engineering barrier, while we observed thermal pressurization in the far-field area. We plan to apply the developed model to a full-scale three-dimensional numerical simulation for the next phase.

Coupled thermo-hydro-mechanical-chemical modeling of fines migration in hydrate-bearing sediments with CFD-DEM

Fines migration associated with the multiphase flow in the exploitation of hydrate-bearing sediments (HBS) usually induces local clogging and sand production around wells, and thus its behavior with multi-field coupling is of vital importance but still poorly discovered. This paper establishes a coupled thermo-hydro-mechanical-chemical (THMC) model incorporating fines migration in HBS from micro- to macro-scale. Two typical hydrate pore habits, i.e., grain coating and pore filling, are simulated with the discrete element method (DEM) under different depressurization modes, water flow is simulated using computational fluid dynamics (CFD), and heat transfer and chemical reactions are also considered in coupled CFD-DEM simulations. Two distinct fines migration modes and their consequence on the mechanical and hydromechanical properties are revealed. For the grain-coating habit, the coarse particle size reduction induced by hydrate dissociation under an intense depressurization decreases the constriction size, increasing the local pore-clogging probability and reducing the growth rate of the hydraulic conductivity. Conversely, the fine particle size reduction in the pore-filling habit facilitates fines migration and thus sand production, with hydromechanical properties of HBS evolving oppositely compared to the clogging case.

Numerical simulation on the evolution of physical and mechanical characteristics of natural gas hydrate reservoir during depressurization production

Changes in the physical and mechanical characteristics of the natural gas hydrate reservoir during depressurization production can affect safe and efficient production. In order to reveal the evolution law of reservoir physical and mechanical characteristics, based on the establishment of thermo-hydro-mechanical-chemical (THMC) multi-field coupling theoretical model, taking SH2 drilling platform in Shenhu sea area of the South China Sea as an example, COMSOL multiphysics is used to simulate the processes of depressurization production with a single horizontal well. The results show that, after the bottom hole pressure began to decrease, the gas and water production rates immediately increased from zero to their respective peaks, and then decreased rapidly. The decomposition of hydrate is an endothermic process. The changes of temperature and pressure conditions have a significant impact on the decomposition of hydrate. Effective stress and Mises stress appear to be concentrated in the area of complete hydrate decomposition. Mises stress rises sharply at the location of the leading edge of decomposition, which needs to be alert to the risk of landslide. In the process of depressurization production, the top of the reservoir gradually appears settlement behavior. The upper area of the horizontal well has a large amount of subsidence, and reservoir modification can be implemented during production to improve the mechanical stability of the reservoir. The results are an important guide to achieve stable and continuous gas production.

Hydro-thermal impacts on near-field flow and transport in a shale-hosted nuclear waste repository

Performance of geologic radioactive waste repositories depends on near-field and far-field processes, including km-scale flow and transport in engineered and natural barriers, that may require simulations of up to 1 M years of regulatory period. For a relatively short time span (less than 1000 years), the coupled thermo-hydro-mechanical–chemical (THMC) processes caused by heat from the waste package will influence near-field multiphase flow, chemical/reactive transport, and mechanical behaviors in the repository system. This study integrates the heat-driven perturbations in THM characteristics into THC simulations using PFLOTRAN to reduce dimensionality and improve computational efficiency by implementing functions of stress-dependent permeability and saturation-temperature-dependent thermal conductivity. Our reduced-order THMC simulation of a single waste package in a shale-hosted repository reveals three physical mechanisms associated with heat pulse emitted from the waste package: (1) increase of liquid mobility that accelerates the inflow and solute transport from the fully saturated host rock, (2) production of thermal gradients that results in outward flow within the buffer and disturbed rock zone (DRZ) nearest the waste package, and (3) reduction of thermal conductivity that confines heat within the engineered barrier system (EBS). These combined hydro-thermal mechanisms influence the rate of re-saturation process and transport/exchange of corrosive species in the repository system.

Contributions of thermo-poroelastic and chemical effects to the production of enhanced geothermal system based on thermo-hydro-mechanical-chemical modeling

Geothermal energy is gaining attention as an environmentally friendly and sustainable alternative to fossil fuels. There exist complicated coupling processes among fluid flow, heat transfer, mechanical deformation, and chemical reaction in fractured geothermal reservoirs. Understanding of thermo-hydro-mechanical-chemical (THMC) coupled process is of significance for the efficient development of enhanced geothermal systems. This research aims to quantify the contributions of mechanical and chemical behaviors to reservoir feature variation during geothermal production. Herein, a fully coupled THMC model is developed for the production of an enhanced geothermal system based on fracture aperture alternation. The key improvement lies in the inclusion of mechanical-chemical (MC) coupling where chemical behaviors cause stress to be redistributed. On a geographical and temporal scale, the unique proportion technique is employed to study ratios of mechanical to chemical effects and poroelasticity to thermoelasticity. The findings emphasize the necessity of incorporating the MC coupling when analyzing reservoir characteristic variations. The mechanical effect plays a major role near the injection well and in a short term. The chemical effect is dominated far from the injection well and its time scale of action is long-term. The intensity of thermoelasticity is stronger than that of poroelasticity. Moreover, the proportion analysis reveals that when stable production is achieved, there are primarily thermo-driven and chemical-driven regions. To improve production performance and adjust geothermal reservoir features, injection temperature and solute concentration should be optimized. This paper serves as a valuable reference for both the theoretical multi-physical coupling model and practical geothermal production.

DECOVALEX-2019: An international collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems

The DECOVALEX Project is an international research collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems. DECOVALEX stands for “DEvelopment of COupled Models and VALidation against EXperiments”. The creation of this international initiative, now running for almost 30 years, was motivated by the recognition that prediction of these coupled effects is an essential part of the performance and safety assessment of geologic disposal systems for radioactive waste and spent nuclear fuel, and also for a range of other sub-surface engineering activities. DECOVALEX emphasizes joint analysis and comparative modeling of state-of-the-art field and laboratory experiments, across a range of host rock options and repository designs. Participating research teams are from radioactive waste management organizations, national research institutes, regulatory agencies, universities, and consulting groups, providing a wide range of perspectives and solutions to these complex problems. The most recent phase of the DECOVALEX Project, here referred to as DECOVALEX-2019, started in 2016 and ended in 2019. Modeling teams from 13 international partner organizations participated in the comparative evaluation of seven modeling tasks involving complex field and/or laboratory experiments. This Virtual Special Issue on DECOVALEX-2019 provides an in-depth overview of these collaborative research efforts and how these have advanced the state-of-the-art of understanding and modeling coupled THMC processes.