Workflows for generation of high quality meshes for simulating THMC processes in porous and fractured media

Many nations are electing to deal with high-level radioactive waste by developing and licensing deep geological repositories (DGRs). A key objective of DGR safety cases is to demonstrate that the engineered and natural barrier systems will remain resilient and functional far into the future, ensuring that no harmful contaminant levels ever reach the biosphere. DGR safety cases are typically supported by numerical modelling of the repository system and its environment, simulating the thermal, hydraulic, mechanical and chemical (THMC) coupled processes that influence the barrier system evolution over such timescales.

Numerical modelling of DGRs often calls for representation of complex geologies and closure engineering with varying scales of structures and physical processes, requiring unstructured meshes to balance accuracy and efficiency. Crystalline host rocks present particular challenges because simulation of THMC processes often involves both 3D volumetric meshes to model porous media such as engineered barriers, and discrete fracture network (DFN) meshes to represent the fractured host rock. Furthermore, the required mesh type may differ depending on the physics to be simulated. For example, thermal and mechanical processes, which are primarily transmitted via the bulk rock mass, are best simulated on volumetric meshes. Modelling of hydraulic processes and their associated chemical interactions, by contrast, may be undertaken on either a DFN or volumetric mesh, depending on the medium and the level of geometric detail required.

Here, we present examples of near-field THMC simulations enabled by new meshing workflows in the FracMan® modelling software. The workflows facilitate creation of a flexible set of medium representations, including so-called ‘hybrid’ meshes consisting of both volumetric and DFN representations in different regions of a single model.

In the case studies, illustrative DGR components are meshed using a coupling to the LaGriT gridding library, which supports the creation of unstructured meshes with deformable tetrahedral elements that accurately capture curved and complex geometries. Fractured rock sections of the model are represented either by a DFN mesh of 2D triangular elements or by a volumetric mesh, depending on the physics. In both cases, targeted mesh refinement is applied to regions of greater geometric complexity and/or steep numerical gradients, facilitating simulation of variable-scale processes. Iterative smoothing algorithms are employed to create a high-quality Delaunay mesh, with quality metrics included to assess the likely performance in simulations. In the final step, the mesh is converted to its Voronoi dual; a discretisation that is optimal for finite volume simulators that employ a two-point flux approximation. It also provides a flexible mesh connectivity topology, which allows for high-fidelity representation of material property contrasts and heterogeneity.

The generated meshes are applied in example THMC simulations, undertaken in the subsurface flow simulator PFLOTRAN together with FracMan geomechanics codes. FracMan post-processing calculations are used to simulate particle flow paths, providing input to performance assessments. The results demonstrate how the developed workflows are an innovative combination of methods that provide accurate and efficient solutions to some specific challenges of DGR systems – namely, strong contrasts in the dimensions and properties of individual barrier system components.