Shale and argillite geological formations have been considered as potential host rocks for geological disposal of high-level radioactive waste (HLW) throughout the world because of their low permeability, low diffusion coefficient, high retention capacity for radionuclides, and capability to self-seal fractures. Other favorable characteristics of argillite/shale are the strong sorptive behavior for many radionuclides, reducing conditions because of the lack of oxygen transport from the surface, and chemical buffering the effects of materials introduced during repository construction, operation, and emplaced materials.
LBNL’s work on argillite disposal started in 2010 by leveraging on previous experience on coupled THM processes modeling within domestic and international nuclear waste programs. Investigations aimed at the development and validation of coupled THM simulators for modeling of near-field coupled processes. From a safety assessment perspective, near-field coupled processes are relatively short-lived, but could give rise to permanent changes, such as the formation of a thermally altered or a damaged zone around excavations, and which could provide a pathway for transport of radionuclides if released from a waste package. For a repository hosted in clay-rock, the mechanical evolution and swelling of the protective buffer surrounding the waste package (often bentonite) are imperative to its functions, such as to provide long-term mechanical support to seal the EDZ. At the same time, the mechanical evolution of the buffer is governed by complex coupled interactions of temperature and hydraulics, micro- and macro-structures of bentonite, as well as the host rock. Currently, more advanced constitutive mechanical models are being applied, but those require a large number of input parameters for describing processes at the different field scales. It is important to test and validate the models at a relevant field scale, in addition to verification and validation against independent analytical and numerical solutions and laboratory experiments.
LBNL is developing two complementary coupled simulation approaches to model THM processes TOUGH-FLAC, based on linking LBNL’s TOUGH2 multiphase fluid flow simulator with the FLAC3D geomechanics code, provides an efficient continuum modeling approach with state-of-the-art constitutive models for bentonite and host rock. The complementary TOUGH-RBSN simulator, based on linking the TOUGH2 simulator with the Rigid-Body-Spring Network (RBSN) model, enables for explicit modeling of discrete fractures and a fracturing process. The TOUGH-RBSN is most suitable for detailed analysis of fracturing in laboratory samples, as well as within the EDZ. The TOUGH-FLAC enables modeling of the evolution of the EBS, EDZ, and surrounding host rock at a larger scale. TOUGH-FLAC with appropriate constitutive models is also used to calculate the evolution of permeability and transport properties in the EDZ, which can then be used as input to future safety assessment models and the Geologic Disposal Safety Analysis (GDSA). In addition, work is focused on modeling of the fault activation. This includes, for example, development of capabilities for modeling of fault slip, induced seismicity, and associated changes in fault permeability.
Coupled processes model development and GDSA integration
Research highlight: The GDSA and PA modeling is conducted with the PFLOTRAN code by Sandia National Laboratories. PFLOTRAN has the capability of modeling coupled TH processes and geochemistry, including radionuclide transport in a high-performance computing environment. The idea is to conduct the GDSA modeling of the entire repository, including all emplacement tunnels. In such a large-scale repository model, it will not be possible to model the detailed thermally driven coupled THM processes occurring in the EBS and near-field during the relatively early repository time period of elevated repository temperature. As an alternative idea, here we develop a modeling strategy that would enable inclusion of coupled THM processes into the PFLOTRAN GDSA and PA modeling. This will involve development of an approach to calculate the EDZ permeability evolution as a function of temperature, pressure, and a buffer saturation. It is based on observations made from full coupled modeling and the use of analytical solutions to calculate stress changes that then feed into the EDZ permeability evolution.
Sasaki T. and Rutqvist J. Estimation of stress and stress-induced permeability change in clay/shale formation in a thermo-hydrologically coupled modelling of a geological nuclear waste repository. (In revision, July 2020).
Contact: Jonny Rutqvist
Pore-scale modeling of two-phase flow and fracture evolution
Research highlight: Pore-scale processes play an important role in controlling gas bubble migration and fracture evolution, which are important for understanding and predicting long-term evolution and security of nuclear waste disposal systems. For instance, the gas-water interfaces serve as an important vehicle for the transport of radionuclides and micro-organisms due to preferential sorption, whereas trapped gas bubbles can result in immobilization of radionuclides. Using direct numerical simulations, this study examines microscopic processes/properties, including the interfacial properties and fracture geometries, and their impacts on macroscopic flow and pressure behaviors. Micro-continuum reactive transport model is also used to examine pore structure change due to geochemical reactions (e.g. mineral dissolution and precipitation).
Selected publications: technical reports (2018-2020)
Contact: Hang Deng
International heater tests modeling and analysis
Research highlight: Underground heater experiments in Argillite are being conducted at several underground research laboratories in Europe for research and development related to thermally-driven coupled processes. LBNL is currently involved in coupled processes modeling of experiments at the Mont Terri Laboratory in Switzerland and at the Meuse/Haute-Marne (MHM) underground research laboratory in France. The heater experiments modeled are the Mont Terri FE (Full-scale Emplacement) Experiment, conducted as part of the Mont Terri Project, and the TED and ALC experiments conducted in Callovo-Oxfordian claystone (COx) at the MHM URL. Modeling of the TED and ALC heater experiments have been conducted as part of a modeling task (Task E) of the international DECOVALEX-2019 project. These experiments and DECOVALEX-2019 Task E focused on modeling pore-pressure evolution as a result of thermal pressurization. A follow up to the will be part of the newly launched DECOVALEX-2023 where fracturing as a result of thermal pressurization will be investigated by new experiments and by modeling. The Mont Terri FE experiment will also be part of DECOVALEX-2023 focusing on large scale pore pressure evolution in Opalinus clay.
Xu H., Rutqvist J., Plúa C., Armand G., Birkholzer J. Modeling of Thermal Pressurization in Tight Claystone using Sequential THM Coupling: Benchmarking and Validation against In-situ Heating Experiments in COx Claystone. Tunnelling and Underground Space Technology. 103, 103428. (2020) https://doi.org/10.1016/j.tust.2020.103428.
Contact: Jonny Rutqvist
Modeling of fracture development and gas migration in bentonite/clay
Research highlight: This work involves studies of fracture development and gas migration in bentonite and clay host rocks. TOUGH-RBSN is a discrete fracture model that has been developed to model damage evolution and fracturing in clay host rocks such as in the excavation distrubed zone of tunnels at the Mont Terri Laboratory, but have also been applied for modeling gas migration in bentonite as a process of dilatant flow paths. Modeling of gas migration in bentonite laboratory samples was part of DECOVALEX-2019 with experimental data from the British Geological Survey. A follow up will be conducted within the newly launched DECOVALEX-2023 using data from the large scale LasGit experiment at the Äspö Hard Rock Laboratory in Sweden. We plan to investigate different modeling approaches, including discrete flow path models and continuum dual-structure models.
Kim K., Rutqvist J. and Birkholzer J. Lattice modeling of excavation damage in argillaceous clay formations: Influence of deformation and strength anisotropy. Tunneling and Underground Space Technology, 98, 2020. https://doi.org/10.1016/j.tust.2019.103196.
Tamayo-Mas E, Harrington J.F., Brüning T., Shao H., Dagher E.E., Lee J., Kim K., Rutqvist J., Kolditz O., Lai S.H., Chittenden N., Wang Y., Damians I. and Olivella S. Modelling advective gas flow in compact bentonite: lessons learnt from different numerical approaches. International Journal of Rock Mechanics and Mining Sciences (In Review, May 2020).
Contact: Mengsu Hu
Fault slip and EDZ testing with HPPP
Repository-induced effects such as creation of an Excavation Damage Zone (EDZ), gas generation and thermally-induced pore pressure perturbations may result in the reactivation of pre-existing features (fractures, faults or bedding planes) within the host rock and consequent permeability increase. Understanding such reactivation due to pressure and stress changes, the possible formation of permeable pathways, and their potential long-term sealing is critical in assessing the performance of radioactive waste repositories in shale formations. In 2015, a fault activation experiment was conducted by injecting high-pressure synthetic pore water in a fault zone intersecting the Opalinus Clay formation at 300 m depth in the Mont Terri URL (Switzerland). In 2018, a new experiment started with the permanent installation of a new sensor to monitor long term coupled fault pore pressure and three-dimensional displacements. Key concluding points from these experiments are:
- Complex opening and slip was measured on the fault,
- High transmissivity flow paths developed at least temporarily and “local” (~meters) to the injection under high pressure injection,
- When injection pressures dropped, the feature apparently closed, but, since 2015, the pressure in the ruptured patch did not recover, remaining ~0.45 MPa below its initial value.
Guglielmi, Y., Nussbaum, C., Jeanne, P., Rutqvist, J., Cappa, F., & Birkholzer, J. (2020), Complexity of fault rupture and fluid leakage in shale: Insights from a controlled fault activation experiment. Journal of Geophysical Research: Solid Earth, 125, e2019JB017781. https://doi.org/ 10.1029/2019JB017781.
Guglielmi, Y., Nussbaum, C., Rutqvist, J., Cappa, F., Jeanne, P. and Birkholzer, J. (2020), Estimating perturbed stress from 3-D borehole displacements induced by fluid injection in fractured or faulted shales. Geophys. J. Int. (2020) 221, 1684–1695.
Contact: Yves Guglielmi