We present results from an international model comparison study of the Full-Scale Emplacement (FE) experiment in Opalinus Clay at the Mont Terri Laboratory, Switzerland. Based on a provided parameter set the teams decided which parameters they adopted for their models, whether they considered the excavation and the ventilation phase in addition to the heating phase and if they included technical features like the shotcrete or the EDZ. The teams were able to reproduce the measured parameters temperature, relative humidity and pore pressure. The modelled results for temperature agree very closely between the teams especially in the sensors in Opalinus Clay. All teams were able to reproduce the redistribution of water in the bentonite backfill due to heating. The evolution of the relative humidity showed similar trends with differences in the intensity of the dry out effect. To model the pore pressure evolution is more complex because it comprises the full interaction of the coupled THM processes. The spread between the pore pressure modelled by the teams was larger, with some teams overestimating the pressure increase due to heating and some teams overestimating the extent of drainage. The agreement of modelled results with measurements improves with larger distance to the heater. We conclude that the EDZ and the shotcrete potentially influence the behaviour of the rock causing higher differences closer to the heater. Further research is needed to better implement those influences into the models.Based on the calibrated models, the future evolution of temperature, relative humidity and pore pressure was predicted over the next 10 years following a change of the heat power applied in 2023 and 2024. Again, the predicted temperatures agree very closely between the teams. Most teams do not expect an increase in relative humidity during the next 10 years after the initial dry-out.

This study addresses the thermal hydrofracturing behavior in claystone within the context of the high-level and intermediate-level long-lived radioactive waste disposal. The heat generated by the waste packages will lead to a temperature increment within the host formation, inducing a pore pressure build-up essentially due to the difference between the thermal expansion coefficient of the pore water and that of the solid skeleton. If the induced pore pressure build-up is too high, the host formation will experience tensile stresses, potentially exceeding its tensile strength and resulting in fracturing. Understanding of these processes and improving numerical models to reproduce them will help the design, optimization, and safety of the repository. Additionally, it will contribute to demonstrating robustness by showing that such processes are not expected to occur at the repository scale.This study was conducted as part of the DECOVALEX-2023 project and synthesizes the efforts of six research teams modelling laboratory thermal extension tests conducted on Callovo-Oxfordian claystone (COx) samples, as well as an in-situ thermal hydrofracturing experiment conducted at the Meuse/Haute-Marne Underground Research Laboratory in France. The teams used different numerical codes with different approaches, including continuum and discrete approaches, to model these two tests. The laboratory tests were used to calibrate the teams’ models, such as the fracturing criterion. The teams considered a thermo-hydromechanical formulation under saturated conditions. One of the key features of their models was the incorporation of changes in the hydraulic properties of the COx through hydromechanical coupling.The approaches developed by the teams demonstrated their capability to analyze and reproduce fracture initiation in the COx in terms of time of occurrence and location based on their respective stress analyses. However, attempts to reproduce fracture aperture or fracture propagation were less accurate and remain areas for future research, which were beyond the scope of this study.

Under all global emissions scenarios compatible with a 2°C warming, which was adopted as a goal by 196 Parties at COP 21 in Paris in 2015, Carbon Capture and Storage (CCS) is necessary to allow a phased transition from fossil fuels. In the UK, CCS has been recognised as a critical technology in achieving net zero emissions. The North Sea Transition Authority (NSTA) regulates offshore carbon dioxide storage in the UK and works closely with government, industry and other stakeholders to support the drive to achieve net zero carbon by 2050. In anticipation of increased interest in CCS projects on the UK Continental Shelf (UKCS), a risk register has been developed to assist the NSTA in carbon storage licence stewardship. In this paper, the process of building the register and the description of how it might be used are provided. The register incorporates the full spectrum of technical, political, economic, commercial, environmental and organisational sources of risk, and how they relate to transportation of carbon dioxide (CO2) offshore and storage in the subsurface. The risk register is based on the current understanding of offshore subsurface storage of CO2 from various state-of-the-art projects that have been carried out to date. Some of these projects are fully commercial ventures, whereas others are first of a kind demonstration projects or designed to facilitate research. Risks apply at different stages in the storage process and are relevant over time periods of different length, extending to thousands of years. Overall, the risk register consists of the main checklist, which covers risk categories central to NSTA’s regulatory role; the wider factors checklist, which covers risk outside the NSTA’s regulatory remit, but which are pertinent to ensuring the success of a CCS project; and individual risk review forms, which include detailed project-specific risks used in the risk collation process and formation of the risk register. As experience grows from the implementation of CCS projects on the UKCS, it is likely that improved information will become available about risks that might occur and the preventative measures or mitigations that satisfy UK’s regulatory regime, which can then be directly incorporated into the risk register.

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 degrees 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.

Radioactive waste disposal facilities for heat generating waste are designed to maintain a compressive stress regime in the host rock. Field-scale tests have been undertaken to study how heating of the host rock can reduce the effective stress on the rock. However, it is not possible to carry out experiments at the scale of a whole repository (km-scale), so variation of host rock properties that might be encountered at this scale is not captured by the experiments. Previous numerical modelling of field scale experiments has demonstrated that the physical processes leading to changes in the stress regime are well understood. Here those models are applied at the km-scale, considering approaches to representing a whole repository and the effect of variability in the properties of the host rock. A dataset from the Callovo-Oxfordian Claystone at the Meuse/Heute-Marne Underground Rock Laboratory is used to characterise the natural variability of a potential host rock at the scale of a disposal facility. The modelling demonstrates that for understanding the generation of thermal stresses, considerable learning can be gained from models that employ symmetry conditions and represent a small part of the repository, hence reducing the size of the computational problem. The host rock at the mid-point between waste cells remained in vertical compression for all the parameter combinations used, which builds confidence that horizontal fracturing of the host rock between waste cells due to thermal pressurisation is unlikely for the disposal concept discussed here. Ground surface uplift was also considered and is in the range of 9-13 cm for disposal system studied. Spatial variability in the properties of the host rock has the potential to lead to significant variability in temperature, pressure, effective stress and displacement around the facility. In particular, thermal conductivity, permeability, Young's modulus and coefficient of thermal expansion need to be well characterised.

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.

Andra performs a wide range of in-situ experiments at its Meuse/Haute-Marne Underground Research Laboratory (MHM URL). The purpose of these experiments is to study the feasibility of a radioactive waste repository in the Callovo-Oxfordian claystone formation (COx). An important research program has been conducted by Andra since 2005 to investigate THM response of the COx to a thermal load through laboratory and in situ experimentations. A step-by-step approach is followed, which starts with small scale heating boreholes (TED experiment) and extends to full-scale (ALC experiment). Modelling and interpretation of the TED and ALC experiments are conducted in the context of the Task E within the DECOVALEX-2019 framework. DECOVALEX-2019 is a multidisciplinary, co-operative international research effort focused on modelling coupled Thermal-Hydraulic-Mechanical-Chemical (THMC) processes. Based on the TED experiment measurements, the THM parameters of the COx are calibrated through a fully-coupled THM model using COMSOL. Then, the ALC experiment is successfully blind predicted and interpretatively modelled using the calibrated THM parameters of the COx and the proposed coupled THM model.

Low permeability rocks provide an attractive option for disposal of radioactive waste because transport of radionuclides will be diffusion dominated and hence slow. However, heat generated by the waste will cause an increase in the pore pressure which may become high enough to damage the rock creating higher permeability areas and faster radionuclide transport paths. It is therefore important to understand the magnitude of pore pressure changes caused by the heating effect of the waste, to develop confidence that the rock will maintain its low permeability.This paper considers coupled Thermal-Hydraulic-Mechanical (THM) modelling of two experiments at the Meuse/Haute-Marne URL: the TED experiment at 1/5th scale and the ALC experiment at full repository scale. Modelling these experiments has enabled testing of understanding of key processes and parameterisation through a blind prediction of the results of the ALC experiment based on modelling of the TED experiment. Calibration of models to both experimental data sets has improved understanding of the importance of processes and parameters in determining pore pressures.The modelling captures the key features of the experiment, demonstrating that the physical processes are well represented in the model whilst spatial variability of physical properties can lead to significant uncertainties in pore pressure estimates. The work demonstrates the benefits of conducting experiments at full-scale, ideally located close to the repository site, and of building a good understanding of the spatial variability of the host rock properties.

Between September 2013 and November 2015 Capture Power Limited and National Grid Carbon Limited, undertook a Front End Engineering and Design (FEED) study for the White Rose Carbon Capture and Storage Project. The project planned to construct an oxy-fuel power plant at Drax, Yorkshire, England, and transport high-purity CO2 produced by the plant via a c.135 km long pipeline to a location c.75 km offshore from eastern England. There, it was planned to inject the CO2 into an underground “saline aquifer” storage reservoir, the Endurance CO2 Store. A thorough risk assessment provided confidence that the store would contain CO2 permanently.

A comparative modelling exercise involving several independent teams from the DECOVALEX-2015 project is presented in this paper. The exercise is based on various laboratory experiments that have been carried out in the framework of a French research programme called SEALEX and conducted by the IRSN. The programme focuses on the long-term performance of swelling clay-based sealing systems that provide an important contribution to the safety of underground nuclear waste disposal facilities. A number of materials are being considered in the sealing systems; the current work focuses on a 70/30 MX80 bentonite–sand mixture compacted at dry densities between 1.67 and 1.97 Mg/m 3 . The improved understanding of the full set of hydro-mechanical processes affecting the behaviour of an in situ sealing system requires both experiments ranging from small-scale laboratory tests to full-scale field emplacement studies and coupled hydro-mechanical models that are able to explain the observations in the experiments. The approach was to build models of increasing complexity starting for the simplest laboratory experiments and building towards the full-scale in situ experiments. Following this approach, two sets of small-scale laboratory experiments have been performed and modelled. The first set of experiments involves characterizing the hydro-mechanical behaviour of the bentonite–sand mixture by means of (1) water retention tests under both constant volume and free swell conditions, (2) infiltration test under constant volume condition, and (3) swelling and compression tests under suction control conditions. The second, more complex, experiment is a 1/10th scale mock-up of a larger-scale in situ experiment. Modelling of the full-scale experiment is described in a companion paper. A number of independent teams have worked towards modelling these experiments using different conceptual models, codes, and input parameters. Their results are compared and discussed. This exercise has enabled an improved modelling of the bentonite–sand mixture behaviour, in particular accounting for the dependence of its retention curve on the dry density. Moreover, it has shown the importance of the technological voids on the short-term behaviour of the sealing system.

Bentonite barriers perform safety critical functions in many radioactive waste disposal concepts, but it is challenging to accurately predict bentonite resaturation behaviour in repository settings. Coupled models of the hydro-mechanical response of bentonite are used to demonstrate understanding of bentonite behaviour in experiments and to predict the response of bentonite in a repository environment. Following trials of a range of numerical approaches, a new model is presented, referred to as the Internal Limit Model, which makes use of key observations on limiting stresses supported in bentonite samples in experimental data. This model is based on the Modified Cam Clay model, and uses the observation that for a given dry density of bentonite, there is a limiting stress that the sample can support, be that stress due to swelling, compaction or suction, to explicitly couple the hydraulic and mechanical models. The model is applied to experimental data from the SEALEX experiments, involving a 70/30 by mass mixture of MX80 bentonite and sand. The model is able to reproduce the experimental data using a single set of parameters for all the experiments considered. This builds confidence that the model will be useful in the future for predictive modelling given appropriate data to characterise the bentonite material being used.

Low-permeability clay formations provide good candidate host rocks for geological disposal of radioactive waste, because there is expected to be limited movement of gas or water through the formation. However, when constructing tunnels, the stress state in the formation around the tunnel will change, which can lead to damage to the formation, changing the bulk hydraulic properties of the formation close to the tunnel. There is the potential for this damaged zone to act as a preferential pathway for fluid flow and radionuclide transport. A water injection experiment is ongoing at the Tournemire underground rock laboratory to investigate the hydraulic properties of the Toarcian argillite in which the laboratory is constructed. Water is injected into the formation at the end of a sealed borehole and moves preferentially in the damaged zone along the borehole walls. The rate of water injection into the rock changed over the first year of the experiment and the causes of this change are investigated in this paper by numerical modelling. The study demonstrates that the change in water injection rate into the argillite can be explained by the evolving hydraulic properties of the damaged zone around the borehole. The findings of the modelling study are discussed in the context of long-term radioactive waste disposal.

In many subsurface engineering geoscience applications the impact of thermal, hydraulic, mechanical and chemical (THMC) processes needs to be evaluated. Coupled process models require solution of the partial differential equations describing energy or mass balance. Ignoring the coupling of these processes can lead to a significant oversimplification which may not adequately represent the systems being modelled. Incorporation of coupled processes and associated phenomena inevitably leads to numerical stability issues due to very different scales in terms of spatial distribution, time and parametrical heterogeneity. One approach to simplify the computational demands is to integrate analytical and physical models into standard numerical modelling techniques (in this case finite elements), effectively adding sub-grid scale and sub-time scale information to the model. We present such an approach for the simulation of fluid flow through a fracture validated against experimental data and cross comparison with results of other modelling teams within the DECOVALEX 2015 (development of coupled models and their validation against experiments) project ( http://www.decovalex.org ). By replacing the mechanical behaviour and chemical transport processes with physical models, and by utilising the static nature of the temperature changes, only the hydraulic system required numerical solution in a highly coupled problem. Physical models for fracture closure due to pressure solution, fracture opening due to chemical dissolution, the development of channel flow and a change in the reactive transport characteristics with time were implemented and are described here. The main features of the experimental data could be replicated, although lying outside of the parameter range suggested by the literature. Comparison with other teams using different modelling approaches indicated internal consistency.

AbstractThe generation and migration of gas within and around proposed radioactive waste disposal facilities is potentially a safety critical process. A safety case for a facility that generates significant quantities of gas (e.g. through metal corrosion or radiolysis) will require demonstration that gas migration around and away from the waste is sufficiently understood and will not breach the safety case for the facility. Models can be used to understand the likely hydraulic evolution of such a disposal facility, but the models need to consider processes over a range of scales. A whole repository may extend over kilometres, with individual disposal cells at the scale of tens of metres and features which provide pathways for gas migration on a centimetre scale. All of these features may be significant from a safety perspective and capturing the impact of all of these features in a single model is a significant challenge.This paper presents an approach to tackling this multi-scale problem, which allows the whole repository to be modelled in a computationally efficient manner. The approach involves identifying areas within the modelled domain that show very similar behaviour, and representing these areas with sub-models, so that small-scale features are retained, but computational overhead is decreased by using the results in more than one location in the model domain. The approach allowed a model of a whole repository to be run on a single processor core, whilst maintaining the small-scale features of the system. The model results were compared against more conventional upscaling techniques and show the advantage of a more detailed representation of small-scale features. The model results reflect the conceptual understanding of how gas would migrate in a repository.

The SEALEX experiment, performed in the Tournemire Underground Research Laboratory, run by IRSN in France, aims at identifying conditions that will affect the performance of swelling clay-based sealing systems. Current experiments focus on a 70/30 MX80 bentonite-sand mixture compacted at dry densities between 1.67 Mg/m(3) and 1.97 Mg/m(3). Prior to the in-situ tests, a 1/10th scale mock-up has been tested and proposed as modelling benchmark, in the framework of the international project DECOVALEX. Three independent research teams have worked towards modelling the three phases of this experiment, using different codes and input parameters. In the paper, blind predictions and model calibrations of the three phases are compared to the experimental measurements and discussed.