THMC Processes Research Articles

Model-based performance assessment of repository-induced effects: contribution to the site-selection process in Switzerland

Abstract. The procedure for selecting repository sites in Switzerland is specified in the Sectoral Plan for a deep geological repository (DGR), which consists of three stages. In the last Stage 3, three candidate siting regions, Jura Ost, Nördlich Lägern and Zürich Nordost in northern Switzerland, were examined in detail. For each site, the performance of the geological barrier was evaluated based on the characteristics at the time of repository closure considering a certain degree of uncertainty associated with the exploration and characterization of the geology as well as the long-term evolution of the site. Furthermore, performance assessment includes an evaluation of the safety-relevant repository-induced effects on the disposal system, such as thermal, gas, chemical and mechanical (THMC) effects introduced by the interaction of the waste with the near field. A criterion-based indicator approach was adopted to ensure a traceable implementation of the site-selection process. The complexity of the involved THMC processes calls for model-supported workflows to reach a quantitative assessment of the barrier performance and an adequate evaluation of the associated conceptual and parametric uncertainties. The first task of the workflow was dedicated to the elaboration of preliminary site-descriptive models for the proposed repository configurations at the candidate sites. Three-dimensional and two-dimensional representations of entire repositories and individual repository components were established and incorporated into the site-descriptive models to provide preliminary site-specific assessment models for the evaluation of coupled THMC effects. The models accounted for the entire life cycle of the repositories from construction to the post-closure phase. Safety-relevant indicators were developed to quantify the performance of different repository configurations and to establish potential discriminators for the site selection. The workflow integrated deterministic and probabilistic assessments on the repository and component scales. As a final step, the data acquired with the latest site investigations (i.e., new data from seismic and deep borehole campaigns) were integrated into the existing models for the elaboration of the site-specific analyses. The indicators were evaluated by integrating a wide range of information, including regional stress conditions, tectonic structures, stratigraphy, lithology, mineralogy, and the impact from effects induced by the repository for the entire spectrum of repository configurations. The spatial maps of indicators depicting the impact from repository-induced effects were synthesized in the site-selection process as aggregated georeferenced performance measures to evaluate the performance at each site with regards to the regulatory criteria.

Thermo-hydro-mechanical-chemical coupling in chalk reservoirs: Impact on fluid flow and deformation during water injection

Reliable prediction of the rate- and fluid-dependent evolution of elasto-plastic strain in chalk is essential to minimize the risk associated with sea-floor subsidence and subsurface deformation. Experimental observations show that physicochemical interaction between the aqueous solutions and rock surface causes the change of mechanical properties in chalk. Here, we quantify the experimental observations of hydrostatic pore collapse strength and bulk modulus of water-saturated chalk specimens as a function of temperature and sulfate concentration. Further, a wrapper is developed to couple the non-isothermal transport of the multi-phase flow simulator with the geomechanics simulator to consider the impact of the coupled interactions on fluid transport and reservoir deformation. In this study, the interplay between thermo-hydro-mechanical-chemical processes in chalk reservoirs is investigated by means of numerical experiments. Computational results suggest that reservoir pressure depletion, temperature changes, and water weakening play significant roles in controlling the reservoir's deformation and production. Our study also confirms that accounting explicitly for the coupled THMC processes in reservoir simulations of water flooding in chalk is required for reliable prediction of reservoir responses.

EURADWASTE’22 Paper – Host rocks and THMC processes in DGR

Deep geological disposal aims to contain and isolate radioactive waste from the biosphere. Repository systems are made of multiple barriers working together, typically comprising the natural geological barrier provided by the repository host rock and its surroundings and an engineered barrier system. Due to their excellent properties for the confinement of contaminants, including low permeability, high sorption capacity, and swelling/self-sealing capacity, clayey materials are considered as engineered and/or natural barriers in most repository designs under development in Europe. During the lifetime of the repository, clay barriers will be exposed to perturbations, among which those are resulting from gas and heat production within the system. It is important to verify that these perturbations will not be detrimental to the good functioning of these barriers. In this paper, it is shown how the two EURAD R&D work packages, GAS and HITEC use a combination of experimental and modelling approaches to increase the understanding and predictability of the impact on clay barriers of the fundamental processes and their couplings related to gas and heat transport respectively, providing building blocks to support the evaluation of the robustness of the repository concepts.

The Coupled THMC finite-element modeling of hydrothermal systems: Insights into the Jiama porphyry metallogenic system

The formation and localization of hydrothermal ore deposits largely depend on coupled thermal–hydraulic–mechanical–chemical processes (called coupled THMC processes) in a porphyry metallogenic system. However, these processes and the hydrodynamics of porphyry metallogenic systems remain unclear. In this study, we developed two-dimensional coupled THMC finite-element models to simulate the spatiotemporal evolutions and interactions among the temperature, pore pressure, deformation, and migration of Cu in the Jiama Cu-polymetallic porphyry metallogenic system and to investigate the hydrodynamic mechanisms based on the geology of this deposit. The results show that the low-permeability upper layer of the wall rock, inhomogeneous thermal expansion, and continuous and multiple intrusions of magma induce an abnormally high pore fluid overpressure zone above the Jiama porphyry intrusion. The high overpressure zone and its evolution above the porphyry intrusion play key roles in the formation of the hydrothermal metallogenic system. First, the evolution of the overpressure changes the fluid flow patterns during different periods of the hydrothermal metallogenic system, that is, the outward flow from the porphyry in the early stage and the backflow towards the porphyry in the late stage. These two different flow patterns are consistent with the observations of mineral growth bounds and unidirectional solidification textures. Second, the high overpressure leads to the appearance and development of fractures above the intrusion. These fractures conform to the ore-bearing spaces of the hornfels and porphyry orebodies above the intrusion. Finally, the evolution of the overpressure can be used to predict the Cu concentration in interlayer detachment zones, which is consistent with geological observations based on which the interlayer detachment zones contain skarn orebodies. In this study, an overpressure-driven hydrodynamic process of a hydrothermal metallogenic system is illustrated, physical and mechanical explanations for geological observations in the Jiama porphyry metallogenic system are provided, and the advantages of numerical simulations in studies of complex multi-field coupled geological systems are highlighted.

A generic framework for overpressure generation in sedimentary sequences under thermal perturbations

Abstract Abnormal pore pressures in excess of the hydrostatic equilibrium are observed in natural and artificial sediment systems worldwide. Such overpressure exerts a fundamental control over the diagenesis, stability, and productivity of both sedimentary reservoirs. The basin dynamics for overpressure generation involve complex THMC processes during sediment accumulation. In this paper, we develop a generic framework for thermo-chemical consolidation in natural and artificial sedimentary sequences by extending Biot’s classical poroelasticity. The proposed theoretical model is then conveniently recast into an explicit form for one-dimensional sedimentation. The resulting formulation essentially extends the original Gibson’s theory by incorporating coupled THMC effects in general sedimentary sequences, and a hierarchical scrutiny of different overpressuring mechanisms thus becomes straightforward. Moreover, a closed-form equation is derived from the new chemo-thermo-poroelasticity model for accreting sediments, and the critical temperature for pore pressure changes via hydration/dehydration is thus determined. The model demonstrates that overpressuring and depressuring are not mutually exclusive because of the competition between water volume changes and thermal strains induced by chemical reactions. By using evolutive properties of a reactive artificial sediment, our model calculations have highlighted the critical influences of the sediment and environment temperatures and deposition rate on the contribution of each overpressuring mechanism.

A coupled THMC modeling application of cemented coal gangue-fly ash backfill

Cemented coal gangue-fly ash backfill (CGFB) is prepared by mixing coal gangue, fly ash, cement and water. The CGFB mixtures are filled into mined-out areas of coal mines for ground control and waste disposal. Once placed underground, the performance of CGFB is subjected to the thermal (T), hydraulic (H), mechanical (M) and chemical (C) processes and THMC coupling effects. Understanding the coupled THMC influence on CGFB is crucial for predicting the behavior of CGFB. This paper develops a THMC coupling model to describe and analyze the coupled THMC processes that occur in CGFB. The developed model considers the courses of thermal conduction and convection, fluid flow, suction development, chemical shrinkage, thermal expansion, and binder hydration. The simulation and prediction results of the developed model are compared with the experimentally tested data from 3 case studies. Good agreement is found between the modeling results and experimental data, validating the capability of the developed model to well characterize the properties of CGFB under coupled THMC loadings. The modeling results can also contribute to a better design of stable, durable and environment-friendly CGFB structures.

The coupled non-isothermal, multiphase-multicomponent flow and reactive transport simulator OpenGeoSys–ECLIPSE for porous media gas storage

Numerical simulations are a viable tool to gain insights into complex coupled THMC processes prevailing in many geoscientific applications. In this work, a coupling approach for OpenGeoSys and ECLIPSE is presented, which combines the multiphase flow simulations of ECLIPSE with heat and reactive geochemical component transport simulations of OpenGeoSys. The coupled simulator is capable of dealing with multiphase-multicomponent systems with no specific limitations regarding the components used. Furthermore, thermal effects like the Joule–Thomson effect and geochemical feedback on fluid flow and mass transport are accounted for by the coupled simulator. The developed coupled code is validated in a series of benchmarks. It is found that the results of the coupled simulator are in very close agreement with those obtained from the reference simulations with the relative errors being smaller than 0.00001, 0.0002 and 0.003 % for phase pressures, saturations and component concentrations, respectively. Validation of the thermal coupling of the simulators shows the same good agreement, if no thermal feedback on fluid flow is considered with a maximum relative error of 0.0015 %. Including thermal feedback on fluid flow shows increased relative differences of up to 0.3 % due to the slightly different equations of states used in the simulators. Given the good accuracy of the validation runs, the coupled code can thus now be applied for reservoir simulations of coupled processes occurring in the subsurface.

Evolution of the transport properties of fractures subject to thermally and mechanically activated mineral alteration and redistribution

AbstractStrong feedbacks link temperature (T), hydrologic flow (H), mechanical deformation (M), and chemical alteration (C) in fractured rock. These processes are interconnected as one process affects the initiation and progress of another. Dissolution and precipitation of minerals are affected by temperature and stress, and can result in significant changes in permeability and solute transport characteristics. Understanding these couplings is important for oil, gas, and geothermal reservoir engineering, for CO2 sequestration, and for waste disposal in underground repositories and reservoirs. To experimentally investigate the interactions between THMC processes in a naturally stressed fracture, we report on heated (25°C up to 150°C) flow‐through experiments on fractured core samples of Westerly granite. These experiments examine the influence of thermally and mechanically activated dissolution of minerals on the mechanical (stress/strain) and transport (permeability) responses of fractures. The evolutions of the permeability and relative hydraulic aperture of the fracture are recorded as thermal and stress conditions' change during the experiments. Furthermore, the efflux of dissolved mineral mass is measured periodically and provides a record of the net mass removal, which is correlated with observed changes in relative hydraulic fracture aperture. During the experiments, a significant variation of the effluent fluid chemistry is observed and the fracture shows large changes in permeability to the changing conditions both in stress and in temperature. We argue that at low temperature and high stresses, mechanical crushing of the asperities and the production of gouge explain the permeability decrease although most of the permeability is recoverable as the stress is released. While at high temperature, the permeability changes are governed by mechanical deformation as well as chemical processes, in particular, we infer dissolution of minerals adjacent to the fracture and precipitation of kaolinite.

Modeling of coupled THMC processes in porous media

For landfill monitoring and aftercare, long-term prognoses of emission and deformation behaviour are required. Landfills may be considered as heterogeneous porous soil-like structures, in which flow and transport processes of gases and liquids interact with local material degradation and mechanical deformation of the solid skeleton. Therefore, in the framework of continuous porous media mechanics a model is developed that permits the investigation of coupled mechanical, hydraulical and biochemical processes in municipal solid waste landfills.

Coupled thermo-hydro-mechanical–chemical behaviour of cemented paste backfill in column experiments: Part II: Mechanical, chemical and microstructural processes and characteristics

The evolution of coupled thermal (T), hydraulic (H), mechanical (M) and chemical (C) (THMC) properties of underground cemented paste backfill (CPB) has been studied by means of experiments with insulated–undrained high columns. This paper which compliments Part I (Ghirian and Fall, 2013), presents the mechanical, chemical and microstructural processes and characteristics. Two columns have been built and filled with a specific CPB mix and equipped with different sensors. The vertical deformation and drying shrinkage for a period of 150days are measured. Also, four other columns are cured for 7, 28, 90 and 150days and then small samples are taken out from the columns to investigate the evolution of unconfined compressive strength (UCS), shear strength parameters, microstructural properties, and pore fluid chemistry. The combined results of the two part experiment show that strongly coupled THMC processes control CPB behavior. This study has demonstrated that mechanical properties are coupled to chemical reactions due to cement hydration and temperature changes inside the columns. Also, suction development due to self-desiccation can significantly increase the uniaxial compressive strength values with time. Chemical analysis including ion concentration measurements with time has revealed that change in pore fluid concentration affects the refinement of pore voids, and thereby results in improvement in hydro-mechanical performance as well as microstructural evolution of the CPB. External environmental loading, such as surface evaporation, can affect the durability performance of CPB structures. The results show that there is degradation of strength following surface shrinkage as well as an increase in saturated hydraulic conductivity due to existence of micro-cracks. The obtained results support that coupled THMC effects should be taken into account in field conditions to understand CPB behavior where stronger interplay reactions take place.

Modeling of Coupled Thermo-Hydro-Mechanical Processes with Links to Geochemistry Associated with Bentonite-Backfilled Repository Tunnels in Clay Formations

This paper presents simulation results related to coupled thermal-hydraulic-mechanical (THM) processes in engineered barrier systems (EBS) and clay host rock, in one case considering a possible link to geochemistry. This study is part of the US DOE Office of Nuclear Energy's used fuel disposition campaign, to investigate current modeling capabilities and to identify issues and knowledge gaps associated with coupled THMC processes and EBS- rock interactions associated with repositories hosted in clay rock. In this study, we simulated a generic repository case assuming an EBS design with waste emplacement in hor- izontal tunnels that are back-filled with bentonite-based swelling clay as a protective buffer and heat load, derived for one type of US reactor spent fuel. We adopted the Barcelona basic model (BBM) for modeling of the geo- mechanical behavior of the bentonite, using properties corresponding to the FEBEX bentonite, and we used clay host rock properties derived from the Opalinus clay at Mont Terri, Switzerland. We present results related to EBS host-rock interactions and geomechanical performance in general, as well as studies related to peak temperature, buffer resaturation and thermally induced pressurization of host rock pore water, and swelling pressure change owing to variation of chemical composition in the EBS. Our ini- tial THM modeling results show strong THM-driven interactions between the bentonite buffer and the low- permeability host rock. The resaturation of the buffer is delayed as a result of the low rock permeability, and the fluid pressure in the host rock is strongly coupled with the temperature changes, which under certain circumstances could result in a significant increase in pore pressure. Moreover, using the BBM, the bentonite buffer was found to have a rather complex geomechanical behavior that eventually leads to a slightly nonuniform density distribu- tion. Nevertheless, the simulation shows that the swelling of the buffer is functioning to provide an adequate increase in confining stress on the tunnel wall, leading to a stabil- ization of any failure that may occur during the tunnel excavation. Finally, we describe the application of a pos- sible approach for linking THM processes with chemistry, focusing on the evolution of primary and secondary swelling, in which the secondary swelling is caused by changes in ionic concentration, which in turn is evaluated using a transport simulation model.

Coupled thermo-hydro-mechanical–chemical behaviour of cemented paste backfill in column experiments. Part I: Physical, hydraulic and thermal processes and characteristics

Cemented paste backfill (CPB) technology has been widely used to fill underground mine voids for underground mine support and/or tailings disposal. Once placed, the CPB structure is subjected to strong coupled thermal (T), hydraulic (H), mechanical (M) and chemical (C) processes. In this paper, an experiment with insulated-undrained high columns has been carried out to understand these THMC processes. A CPB mixture is loaded into two columns and instrumented with various sensors to monitor the evolution of temperature, pore water pressure and suction for a period of 150days. In addition, four other CPB columns are cured at 7, 28, 90 and 150days, and then extensive laboratory testing is carried out on the CPB samples with regards to their thermal conductivity, saturated hydraulic conductivity, water retention properties, and mechanical and physical properties. Also, the rate of evaporation is monitored in a cylindrical CPB sample for the entire period of study. The results obtained show that a higher temperature can result in faster microstructural refinement and hence lower fluid transport ability. Reduction in the degree of saturation decreases thermal conductivity. The hydraulic properties are strongly coupled to chemical and mechanical factors. Mechanical deterioration following surface shrinkage and cracks can increase the saturated hydraulic conductivity. Also, changes in pore fluid chemistry affect microstructural evolution and can influence physical properties, such as void ratio and degree of saturation. Moreover, the obtained results support that the THMC properties of CPB are strongly coupled due to several internal mechanisms, such as heat of hydration, self-desiccation, suction development and cement hydration. The findings can contribute to a better understanding on the behaviour of CPB and thus towards the designing of more cost-effective and durable CPB structures. The results of the evolution of the mechanical, chemical and microstructural processes are discussed in a companion paper (see Part II, this issue).

Modeling coupled THM processes of geological porous media with multiphase flow: Theory and validation against laboratory and field scale experiments

A FEM model for analysis of fully coupled multiphase flow, thermal transport and stress/deformation in geological porous media was developed based on the momentum, mass and energy conservation laws of the continuum mechanics and the averaging approach of the mixture theory over a three phase (solid–liquid–gas) system. Six processes (i.e. stress–strain, water flow, gas flow, vapor flow, heat transport and porosity evolution processes) and their coupling effects are considered, which not only makes the problem well-defined, but renders the governing PDEs closed, complete, compact and compatible. Displacements, pore water pressure, pore gas pressure, pore vapor pressure, temperature and porosity are selected as basic unknowns. The physical phenomena such as phase transition, gas solubility in liquid, thermo-osmosis, moisture transfer and moisture swelling are modeled. As a result, the relative humidity and other related variables in porous media can be evaluated on a sounder physical basis. A three dimensional computer code, THYME3D, was developed, with eight degrees of freedom at each node. The laboratory CEA Mock-up test and the field scale FEBEX benchmark test on bentonite performance assessment for underground nuclear waste repositories were used to validate the numerical model and the software. The coupled THM behaviors of the bentonite barriers were satisfactorily simulated, and the effects and impacts of the governing equations, constitutive relations and property parameters on the coupled THM processes were understood in terms of more straightforward interpretation of physical processes at microscopic scale of the porous media. The work developed enables further in-depth research on fully coupled THM or THMC processes in porous media.

Numerical simulation of thermal-hydrologic-mechanical-chemical processes in deformable, fractured porous media

A method is introduced to couple the thermal (T), hydrologic (H), and chemical precipitation/dissolution (C) capabilities of TOUGHREACT with the mechanical (M) framework of FLAC 3D to examine THMC processes in deformable, fractured porous media. The combined influence of stress-driven asperity dissolution, thermal-hydro-mechanical asperity compaction/dilation, and mineral precipitation/dissolution alter the permeability of fractures during thermal, hydraulic, and chemical stimulation. Fracture and matrix are mechanically linked through linear, dual-porosity poroelasticity. Stress-dissolution effects are driven by augmented effective stresses incrementally defined at steady state with feedbacks to the transport system as a mass source, and to the mechanical system as an equivalent chemical strain. Porosity, permeability, stiffness, and chemical composition may be spatially heterogeneous and evolve with local temperature, effective stress and chemical potential. Changes in total stress generate undrained fluid pressure increments which are passed from the mechanical analysis to the transport logic with a correction to enforce conservation of fluid mass. Analytical comparisons confirm the ability of the model to represent the rapid, undrained response of the fluid-mechanical system to mechanical loading. We then focus on a full thermal loading/unloading cycle of a constrained fractured mass and follow irreversible alteration in in-situ stress and permeability resulting from both mechanical and chemical effects. A subsequent paper [Taron J, Elsworth D. Thermal-hydrologic-mechanical-chemical processes in the evolution of engineered geothermal reservoirs. Int J Rock Mech Min Sci 2009; this issue, doi:10.1016/j.ijrmms.2009.01.007] follows the evolution of mechanical and transport properties in an EGS reservoir, and outlines in greater detail the strength of coupling between THMC mechanisms.