•two thermo-hydro-mechanical models are derived with simplified representations of thermo-mechanical effects•exact agreement between both complexity-reduced models and the fully coupled ...thermo-hydro-mechanical model if basic simplifying assumptions hold•good agreement can still be reached if basic simplifying assumptions are violated•complexity-reduced models allow for scoping evaluation with computational efficiency of a hydro-thermal model
Coupled thermo-hydro-mechanical models are commonly used to model the evolution of temperature, pore pressure, and stress in a wide range of geotechnologies such as geothermal applications or around canisters of high-level radioactive waste in deep underground storage facilities. Their numerical modeling is often computationally highly demanding, especially if parameter identification, sensitivity analyses or uncertainty quantification require many model evaluations. Often, the thermally driven pore pressure evolution and the subsequently altered flow processes are the primary targets of an analysis. To benefit from the computational efficiency of hydro-thermal (HT) models while maintaining the accuracy of the thermo-hydro-mechanical (THM) model, we derived two cases of a simplified representation of mechanical deformations in a coupled hydro-thermal model. Deformations induced by pressure as well as temperature changes are consistently incorporated into the mass balance storage terms. We demonstrate the exact coincidence of THM and modified TH formulations in isotropic and orthotropic materials as long as the basic assumptions like constant hydrostatic stress conditions or uniaxial strain hold. By modeling of a point heat source in isotropic or anisotropic porous media it is shown that a good agreement between TH and THM models can be maintained even though the assumptions underlying the simplification are no longer valid exactly. On our test-machine, a significant speed-up could be achieved by the reduction of the problem size when transitioning from a THM to a TH model. The highest speed-ups were achieved when Taylor-Hood elements were employed in order to avoid the problem of spurious pressure oscillations in the fully coupled THM model.
In this paper we describe the OpenGeoSys (OGS) project, which is a scientific open-source initiative for numerical simulation of thermo-hydro-mechanical-chemical processes in porous media. The basic ...concept is to provide a flexible numerical framework (using primarily the Finite Element Method (FEM)) for solving multifield problems in porous and fractured media for applications in geoscience and hydrology. To this purpose OGS is based on an object-oriented FEM concept including a broad spectrum of interfaces for pre- and postprocessing. The OGS idea has been in development since the mid-eighties. We provide a short historical note about the continuous process of concept and software development having evolved through Fortran, C, and C++ implementations. The idea behind OGS is to provide an open platform to the community, outfitted with professional software-engineering tools such as platform-independent compiling and automated benchmarking. A comprehensive benchmarking book has been prepared for publication. Benchmarking has been proven to be a valuable tool for cooperation between different developer teams, for example, for code comparison and validation purposes (DEVOVALEX and CO
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BENCH projects). On one hand, object-orientation (OO) provides a suitable framework for distributed code development; however, the parallelization of OO codes still lacks efficiency. High-performance-computing efficiency of OO codes is subject to future research.
A general description of the mathematical and numerical formulations used in modern numerical reactive transport codes relevant for subsurface environmental simulations is presented. The formulations ...are followed by short descriptions of commonly used and available subsurface simulators that consider continuum representations of flow, transport, and reactions in porous media. These formulations are applicable to most of the subsurface environmental benchmark problems included in this special issue. The list of codes described briefly here includes PHREEQC, HPx, PHT3D, OpenGeoSys (OGS), HYTEC, ORCHESTRA, TOUGHREACT, eSTOMP, HYDROGEOCHEM, CrunchFlow, MIN3P, and PFLOTRAN. The descriptions include a high-level list of capabilities for each of the codes, along with a selective list of applications that highlight their capabilities and historical development.
This paper is devoted to the study of the Thermo-Hydro-Mechanical (THM) responses of a porous rock with low permeability under thermal loading in the context of deep geological disposal of ...radioactive waste. To this aim, numerical simulations of a benchmark exercise of a hypothetical high-level radioactive waste (HLW) repository were performed. This benchmark exercise considered as a host formation the Callovo-Oxfordian claystone (COx), which has been selected for a deep geological disposal in France. Within the framework of the DECOVALEX-2019 project, five modelling teams (Andra, LBNL, NWMO, Quintessa, UFZ/BGR) adopted a thermo-poro-elastic approach and proposed different 3D representations of the HLW repository. The differences between the teams consisted mostly in the simplification of the geometrical model and the interpretation of the boundary conditions. Numerical results for temperature, pore pressure, and effective stress evolution in the far field (i.e., at the mid-distance of two HLW cells) were compared between the teams, to quantify the impact of modelling simplifications/assumptions for the assessment of the HLW repository. The THM behaviour of the COx formation in the near field (i.e., excavation damaged zone around the HLW cells) is not the objective of this study. Moreover, plane strain conditions were considered and evaluated in comparison to 3D modelling. Key parameters influencing the THM responses of the HLW repository were assessed by both mono- and multi-parametric analyses. Spatial variability analyses of THM parameters were also carried out to study the influence of the spatial correlation length on the Terzaghi effective stress and to estimate its probability distribution. The conclusions of this study provide reliable numerical techniques for modelling large-scale deep geological disposals and deduce the main behavior of the HLW repository.
The coupled Thermo-Hydro-Mechanical (THM) behavior of the Callovo-Oxfordian claystone (COx) is of great importance for the design and safety calculations of the high-level radioactive waste disposal ...project in this potential host rock in France. The heat emitted by the waste causes a pore pressure increase within the surrounding rock essentially due to the differential thermal expansion of the pore water and the solid skeleton. The low permeability of the COx and its relative rigidity inhibits the discharge of the induced pressure build-up. Moreover, thermal loading may provoke thermo-mechanical stresses within the formation due to mechanical confinement by the rigidity of the surrounding host rock. An important research program has been conducted by the French National Radioactive Waste Management Agency (Andra) since 2003 in order to investigate the THM response of the COx under thermal loading, through laboratory tests, in-situ experiments, model development and numerical modeling. Within Task E of the DECOVALEX-2019 project, five research teams investigated upscaling THM modeling from a small-scale in-situ experiment (TED) to a full-scale in-situ experiment (ALC). The upscaling modeling started with a verification test to validate the numerical codes. Then, an interpretative modeling of the TED experiment was performed to calibrate the THM parameters of the COx. Finally, the calibrated THM parameters were used for a blind prediction of the ALC experiment. The modeling teams each adopted a thermo-poro-elastic approach which yielded satisfactory results. The blind prediction of the temperature field showed an overestimation of less than 2 °C which was considered acceptable. On the other hand, pore pressure was well predicted only in the direction parallel to the bedding whereas the slow dissipation of the pore pressure in the direction perpendicular to the bedding was not captured by any of the modeling teams – which remains an open question of the present study.
In the context of geotechnical and geological barriers, a thorough analysis of uncertainty and sensitivity is a crucial aspect of any physics-based performance assessment. While experimental data are ...scarce in actual waste repositories, large-scale experiments in underground research laboratories (URLs) provide such data that can be used to not only qualify THMC process models but also uncertainty assessment methodologies. In this paper, we adopt a Design of Experiments (DoE)-based history matching workflow – an approach popular in the oil and gas industry – and scrutinize its applicability for multiphysical analyses of nuclear waste disposal-related processes using synthetic experimental data. Based on an analytical solution of a coupled thermo-hydro-mechanical (THM) problem of a heat source embedded in a fluid-saturated porous medium mimicking a disposal cell in an argillaceous host formation, we discuss the adaptability of the workflow as a way to address parameter and model uncertainties for barrier integrity assessment. We thereby put particular focus on the relative importance of providing defined input parameter distributions for quantities generally afflicted with epistemic uncertainty and the constraints imposed by experimental (URL) or monitoring (repository) data. We found that once constraining data is available, the particular a priori distribution plays only a minor role for the outcome, such that we can conclude that the often unknown distributions can be substituted by uniform priors under such conditions. However, detailed knowledge of parameter distributions can increase the efficiency of the workflow significantly. We conclude that the presented workflow is particularly suitable for performing uncertainty quantification and sensitivity analysis for geotechnical applications where monitoring or other experimental data are available, as it allows us to deal with models of great complexity, epistemic uncertainty and it incorporates canonically to use of measured data in order to reduce uncertainty.
Thermal energy storage technologies can facilitate the transition to an energy system based largely on renewable sources and enable efficiency gains for industrial processes in general. Due to their ...specific advantages, various concepts of thermo-chemical storage systems are being developed. They share characteristic features of mass and heat transport that are strongly coupled through a variety of physical and chemical phenomena. To facilitate the understanding of the coupled multi-physics processes inside such systems, a versatile conceptual model for directly permeated reactive beds was developed in part 1 of this work. It was based on thermodynamic principles and the Theory of Porous Media. The model was then implemented into OpenGeoSys, a scientific finite element simulation software. In this article, the model is specified to the well-studied calcium hydroxide reaction system to illustrate its practical applicability. Sensitivity analyses reveal the influence of particle diameter, porosity, permeability, mass flux, and reaction rate. Two distinct “reaction waves” are identified to migrate through the reactor. The power required to pump the gas stream was decomposed into parts related to the classical mechanical pressure drop and to the chemical reaction. The results can be used for the optimization of thermochemical heat storage systems.
•Detailed investigation of coupled multiphysics in thermochemical heat storage.•Thermodynamically consistent model for thermochemical heat storage systems.•Analysis of thermal power depending on material and process parameters.•Two reaction waves are identified that traverse the reactor.•Mechanical pumping power splits into mechanically and chemically induced parts.
Use of the subsurface for energy resources (enhanced geothermal systems, conventional and unconventional hydrocarbons), or for storage of waste (CO
, radioactive), requires the prediction of how ...fluids and the fractured porous rock mass interact. The GREAT cell (Geo-Reservoir Experimental Analogue Technology) is designed to recreate subsurface conditions in the laboratory to a depth of 3.5 km on 200 mm diameter rock samples containing fracture networks, thereby enabling these predictions to be validated. The cell represents an important new development in experimental technology, uniquely creating a truly polyaxial rotatable stress field, facilitating fluid flow through samples, and employing state of the art fibre optic strain sensing, capable of thousands of detailed measurements per hour. The cell's mechanical and hydraulic operation is demonstrated by applying multiple continuous orientations of principal stress to a homogeneous benchmark sample, and to a fractured sample with a dipole borehole fluid fracture flow experiment, with backpressure. Sample strain for multiple stress orientations is compared to numerical simulations validating the operation of the cell. Fracture permeability as a function of the direction and magnitude of the stress field is presented. Such experiments were not possible to date using current state of the art geotechnical equipment.
In a repository for radioactive waste hosted in a clay formation, hydrogen and other gases may be generated due to the corrosion of metallic materials under anoxic conditions, the radioactive decay ...of waste and the radiolysis of water. If the gas production rate exceeds the gas diffusion rate within the pores of the clay, a discrete gas phase will form and accumulate until its pressure becomes large enough to exceed the entry pressure of the surrounding material, at which point dilatant, advective flow of gas is expected to occur.
The purpose of Task An under DECOVALEX-2019 is to better represent the processes governing the advective movement of gas in both low-permeability argillaceous repository host rocks and clay-based engineered barriers within numerical codes. In this paper special attention is given to the mechanisms controlling gas entry, flow and pathway sealing and their impact on the performance of the engineered clay barrier. Previous work suggests gas flow is accompanied by the creation of dilatant pathways whose properties change temporally and spatially within the medium. Thus, four new types of approaches have been developed: (i) standard two-phase flow models (continuous techniques) incorporating a range of different mechanical deformation behaviours, (ii) enhanced two-phase flow models in which fractures are embedded within a plastic material (continuous techniques) or incorporated into the model using a rigid-body-spring network (discrete approaches), (iii) a single-phase model incorporating a creep damage function in which only gas flow is considered, and (iv) a conceptual approach used to examine the chaotic nature of gas flow. The outputs from these different approaches are compared. This is an essential step as the choice of modelling approach strongly impacts the representation and prediction of gas flow in a future repository. In addition, experience gained through this task is of direct relevance to other clay-based engineering issues where immiscible gas flow is a consideration including hydrocarbon migration, carbon capture and storage, shale gas and landfill design.
This paper summarises the outcomes of work in Task A conducted between May 2016 and May 2019 and provides a brief overview of the experimental data and a synthesis of the work of the participating modelling teams.
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