AREHS: effects of changing boundary conditions on the development of hydrogeological systems: numerical long-term modelling considering thermal–hydraulic–mechanical (–chemical) coupled effects

. The objective of the AREHS project, funded by BASE (FKZ 4719F10402), is to model the effects of changing external boundary conditions on the hydrogeologically relevant parameters and effects (e.g. hydraulic permeability, porosity, migration pathways, ﬂuid availability, hydraulic gradients) of a generic geological repository in Germany in all three potential host rocks (clay, salt, and crystalline rocks) in its hydrogeological setting. Special attention is paid to the cyclic mechanical loading due to glaciation events. This results in stress changes ( M – mechanical processes) as well as induced temperature effects ( T – thermal processes) due to per-mafrost and warm periods. Since the thermal, hydraulic ( H – hydraulic processes), and mechanical processes are strongly coupled, they have to be covered by state-of-the-art coupled thermal–hydraulic–mechanical (THM) ( C ) modelling. Thepresentation consists of a (a) presentation of important ﬁndings for all three host rock types and a (b) presentation of the overall workﬂow. Complex, time-varying boundary conditions have been formulated for modelling glacial cycles. Investigating the effects of these boundary conditions with THM simulation reveals a variety of coupling effects. To conduct the complex modelling workﬂow for claystone and salt rock effectively, an automated workﬂow was developed and tested. It handles the transformation from a simulator-independent geological model to a numerical model speciﬁc


Funding
The research project AREHS was funded by Bundesamt für die Sicherheit der nuklearen Entsorgung (BASE) ID: FKZ 4719F10402 The presentation reflects the opinion of the AREAS research team but does not necessarily represent the opinion of BASE

Objectives of AREHS
• Future cold and warm periods and associated glaciation events will change the (petro-) physical properties as well as the natural hydrogeological properties of the overall system and especially in the host rock • This has to be well understood quantitatively to be able to conduct long term safety analysis • The objective of AREHS is the development of a THM(C)modeling toolbox to simulate the impacts of the changing boundary conditions on all three types of host rocks (salt, clay and crystalline rocks) • To increase the confidence in the numerical codes, this modeling toolbox has to be verified 1 , C. B. Silbermann 2 , F. Tiedtke 2 , F. Zill 3 , T. Meisel 3 , A. Carl 4 , A. Gabriel 1 , M. Schlegel 1 , H. Konietzky 2 , T. Nagel 2 , H. Weiß 3 , A. Jockel 4 1 G.E.O.S., 2 TU Freiberg, 3 UFZ Leipzig, 4 Ercosplan safeND, 12.11.2021

••AP2
Compilation of input data and development of a data base, • Climate models for 1 mill.yrs • Geological conditions in Germany • State of the art of coupled THM(C)-modelling in geoscience WP 2: Development of a THM-(C) model approach: • Physical characterization of the problem • Mathematical equations incl.coupling • Analytical solutions and 1D-THM-(C)-models for all three host rocks • Benchmark published field data on THM (C) processes from at least one case study of a large-scale geoscience application three host rocks • Benchmark for reproducing field data on past icing events three host rocks • Benchmark with published modelling results three host rocks WP 3: Modeling the effects of future icing cycles on THM-(C) processes • Definition of the model area and creation of the 3D geological models as a basis for the dynamic simulations • Formulation of the initial and boundary conditions incl.modelling approach • THM-(C)modelling for clay rock • THM-(C)modelling for salt rock • THM-(C)modelling for crystalline rock • Comparison of the model approaches and the results Compilation of input data and development of a data base, • Climate models for 1 mill.yrs • Geological conditions in Germany • State of the art of coupled THM(C)-modelling in geoscience WP 2: Development of a THM-(C) model approach: • Physical characterization of the problem • Mathematical equations incl.coupling • Analytical solutions and 1D-THM-(C)-models for all three host rocks • Benchmark published field data on THM (C) processes from at least one case study of a large-scale geoscience application three host rocks • Benchmark for reproducing field data on past icing events three host rocks • Benchmark with published modelling results three host rocks WP 3: Modeling the effects of future icing cycles on THM-(C) processes • Definition of the model area and creation of the 3D geological models as a basis for the dynamic simulations • Formulation of the initial and boundary conditions incl.modelling approach • THM-(C)modelling for clay rock • THM-(C)modelling for salt rock • THM-(C)modelling for crystalline rock • Comparison of the model approaches and the results Development of a workflow for 2D vertical sections and 3D OGS models from GOCAD/GIS projects • Automation of the workflow concept with container technologies (contains all data/code for the complete application) • Simulation of cold-warm times (glaciation) Comparison with simulations from literature -OGS-Simulation • Model description according to Bruns et al. (2012).• Loading of model under self-weight for 1 million years equilibrium state for plastic creeping in salt.• Result provides initial stress state for glacier crossing AP2: Salt dome -Halokinesis: Model setup Stress magnitude after 1 mill.yr.(THM coupling) Bruns, J, L Boetticher, H Doose, M Cottrell, P Wolff, R.-M.Günther, D Naumann, T Popp und K Salzer (Aug.2012).Glazigene Beeinflussung von Wirtsgesteinstypen Ton und Salz und deren Einflüsse auf die Eignung zur Aufnahme eines HAW-Endlagers.Techn.Ber.Celle: Golder Associates GmbH in Kooperation mit IfG Institut für Gebirgsmechanik GmbH, S. 292 Model geometry (glacier als load boundary conditions) AP2: Salt dome -Halokinesis: Results AP2: Salt domeglacier propagation Comparison Bruns -OGS-Simulation • Glacier crossing results in uplift of salt dome and subsidence of side rock • Qualitatively similar results • The subsidence modelled in OGS is smaller than modelled by Bruns (different material model, Bruns uses interface elements between salt and hanging wall, OGS without) Comparison of the vertical displacement at two measurement points during a glacier crossing between the study (left, Bruns et al.,2012) and the OGS simulation (right).AP2: Salt dome -Halokinesis: Process coupling • Investigation of the influence of the couplings of the T, H and M processes on the model behavior.• Hydraulics seem to have a greater influence than thermal coupling • Mechanics alone are not sufficient to simulate the system adequately -process coupling has a clear influence • Vertical displacement after complete glacier propagation cycle field data on past icing events and Benchmark with published modelling results for clay rock Glacier height evolution from Bense and Person, 2008; Bea et al., 2018 Sedimentary basin with glacier advance from Bense and Person, 2008; Bea et al., 2018 • Glaciers captured by transient BCs (load, temperature, hydraulic head) • Consideration of freezing and thawing processes incl.development of the ground frost body • Permafrost: several couplings (TH, TM), for example: temperature dependent soil properties, reduction of ground water flow and changed thermal gradients, … Frost body evolution during glacier advance (vertical exaggeration 10) • blue polygons show frozen regions • delayed thawing in the sediment layers: emergence of ice lense fracture opening calculated in 3DEC and 3DEC+DFN.Lab with the analytical solution of (Sneddon, 1946; Green and Sneddon, 1950) Model Description: 1. Injection into single fracture (DFN.Lab Calculation) 2. Import of the hydraulic pressure field into 3DECblock model 3. Calculating fracture opening/mechanical response due to increased hydraulic pressure (model of the single fracture.There are 3 different fracture contours indicated.The fracture contour is determined by the convex hull of the failed subcontacts in 3DEC Performing thermo-mechanical calculations with the DEM-Software "3DEC" (Itasca, 2020)  Performing hydraulic calculations with the DFN-Software "DFN.Lab" (Le Goc et al., 2020) (Assumption: matrix flow in comparison to joint flow negligible) Generic model of the crystalline host rock (left: 3EDC, right: DFN-lab) including DFN of different scales indication the mechanical and hydraulic pressure field

AP3: Selection of processes based on FEP catalogues Selection of relevant FEPs: AP3: Selection of processes based on FEP catalogues
FT FEP-Table of Energy Agency: NEA FEP Database https://www.oecd-nea.org/fepdb/login/

Selection of processes based on FEP catalogues (example) FEPs covered by scenarios AP3: Selection of processes based on FEP catalogues NEA -Nr. FEP Description Salt Clay Crystalline 1. External Factors 1.2 Geological Factors
AP3: