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Hydraulic-mechanical coupling in claystone is of great importance for repository research, as claystone is considered a possible host rock and hydraulic-mechanical coupled processes can affect the integrity of the barrier. Of particular importance in this context are excavation damage zones and hydraulically-induced microcracks and fractures. In this work, two constitutive equations developed for these hydraulic-mechanical coupled processes are combined in a novel way. The approach exploits the different dominant stress states that prevail in the respective processes. This approach is applied in several numerical simulations for validation and compared with experimental data. The measured data were obtained during gas-injection tests in the scope of the PGZ experiment in the underground laboratory of Bure, France, where the hydraulic-mechanical coupling of the is the Nabla operator claystone was investigated. Five numerical models were calculated, each with a different model configuration, to simulate the excavation damage zone and dilatancy-controlled flow during gas-injection tests. The numerical results show that the developed approach can first satisfactorily predict the extent and behaviour of the excavation damage zone under different stress conditions and the subsequent dilatancy-dependent permeability due to the increased gas pressure.

This manuscript presents a comprehensive study on the numerical simulation of gas transport in clay rock using the finite-element method, with a specific focus on the transition of the transport regime from single-phase to two-phase conditions. Our code demonstrates the capability to cover this transition seamlessly, without relying on common approaches such as the use of persistent primary variables or the switching of primary variables. In our simulations, the primary variables are gas pressure, capillary pressure, temperature, and displacement of the solid phase. To validate our approach, two benchmark tests were conducted. The first benchmark replicates a well-known scenario in the field of radioactive waste disposal, where gas injection induces a transition from single-phase to two-phase flow. The second benchmark simulates a core drilling experiment, where the mechanical unloading of a fully saturated domain results in the appearance of a gas phase. In addition to analyzing primary quantities, a comprehensive set of secondary variables was introduced to gain deeper insights into the model’s operation and enhance understanding of the underlying processes. By plotting these secondary variables alongside the primary quantities, a comprehensive understanding of the system’s behavior during the transition of flow regimes was obtained. The primary objective of this work is to improve our understanding and confidence in the model used for simulating large repository systems, particularly in the context of nuclear waste disposal and CO2 storage. By successfully capturing the transition from single-phase to two-phase gas transport, our study provides valuable insights into the behavior of gas in clay rock. This enhanced understanding lays the groundwork for utilizing the model effectively in large-scale repository simulations, contributing to the advancement of the field of gas transport in clay rock.

In this paper, the governing equations of non-isothermal two-phase flow in unsaturated, deformable porous media are presented based on different representations of the gaseous phase along with their implementations in the open-source FEM code OpenGeoSys 6. As one implementation utilises the Richards equation to represent the unsaturated system whereas the second implementation relies on a two-component two-phase flow formulation, the impact of a free gaseous phase on model predictions is illustrated by a series of tests of increasing complexity, motivated by similar investigations in Task C of the DECOVALEX 2023 project. We demonstrate that the governing equations of both implementations collapse and become identical in fully saturated regions and that the numerical implementations remain stable in the special cases. Furthermore, the implicit assumptions made in the Richards equation are explicitly applied to the physically more complex and comprehensive TH2M model. Thus, the impact of a constant residual gas pressure on the model predictions is investigated. Finally, the examples chosen for this work feature thermal consolidation and pore fluid pressurisation effects in the Opalinus Clay due to heat release from a nuclear waste canister as well as dry-out with subsequent resaturation of a bentonite buffer around the heat source. All test cases are evaluated on the level of the governing equations of each model as well as on the level of results obtained by each model for a discussion of conceptual model uncertainty.

Advective gas transport in bentonite, a possible buffer material in repositories for radioactive materials, is difficult to simulate in numerical continuum models, partly due to the complicated microstructure of bentonite. To generate reliable models of repositories nevertheless, spatially distributed heterogeneous material properties can be used to allow localization of gas flow. In this study, a pore-size-dependent stochastic approach of the gas entry pressure is derived from Mercury Intrusion Porosimetry, which is used to replicate measurements from the LASGIT experiment. In addition, three benchmark tests are simulated to investigate the dependence of heterogeneous distributions of material properties on the mesh discretization, the temporal dependence, and the coupling between the processes influenced by the heterogeneous parameters. The numerical modeling results of the LASGIT experiment show that the onset of gas flow into the system and the subsequent increase in pressure and stress can be well reproduced using heterogeneous distributions. Compared to a model with homogeneous material properties, heterogeneous distributions may allow the generation of dilatancy-controlled microfractures—an important feature with regard to the advective gas flow in bentonites. However, it can be observed that the heterogeneous distributions in LASGIT are less significant, as technical gaps or differences in material types could have a greater impact.

In this paper, thermo-hydro-mechanically (THM) coupled processes triggered during the construction, operation and closure of a deep geological repository for heat generating, high level radioactive waste are discussed based on a generic disposal concept. For this purpose, we are using the numerical non-isothermal two-phase-two-component flow in deformable porous media ((THM)-M-2) implementation (Grunwald et al. in Geomech Geophys Geo-energy Geo-resour, 2022) in the open-source software OpenGeoSys (Bilke et al. in Transport Porous Media 130(1):337-361, 2019, ). THM coupled effects covered in this work focus on single and two-phase-flow phenomena, gas and heat generation as well as poro-elastic medium deformation. A suitable set of benchmarks covering aforementioned THM-effects, devised in the scope of the BenVaSim benchmarking project (Lux et al. in Synthesis report. BenVaSim-International Benchmarking for Verification and Validation of TH2M Simulators with Special Consideration of Fluid Dynamical Processes in Radioactive Waste Repository Systems. Tech. rep., 2021, ) is chosen and one additional benchmark is presented, allowing for the demonstration and comparison of the OGS-6 (THM)-M-2 implementation against results obtained by other well-established codes used in the field. Apart from the code comparison, the benchmarks also serve as means to analyze THM coupled processes in a repository based on very simplified geometries. Therefore, they can help to improve the process understanding, but any quantitative results should not be interpreted as predictions of the behaviour of a real repository. The results obtained in this work agree well with the results presented by the project partners in BenVaSim-both in single phasic, fully liquid saturated cases and in partially saturated two phase regions. Hence, the suitability of the OGS-6 (THM)-M-2 implementation for the application in the field of radioactive waste management, supporting the safety case and analyzing the i