In addition to the capability to create distinct Continuous Porous Medium (CPM) and Discrete Fracture Network (DFN) models, ConnectFlow offers the option to construct embedded models that integrate sub-models of different types. That is, the model can be split into two different domains: one that uses the CPM concept, and one that uses the DFN concept. However, DFN and CPM domains have to be exclusive, i.e. the two approaches cannot be used simultaneously in any part of the model.
Internal boundary conditions are defined at the interface between the domains to ensure continuity of pressure and conservation of mass. On the DFN side of the interface, the boundary conditions are defined at nodes that lie along the lines (traces) that make up the intersections between fractures and the interface surface. On the CPM side, the boundary conditions are applied to nodes in finite-elements that abut the interface surface. Thus, extra equations are added to the system to link nodes in the DFN model to nodes in the finite-element CPM model. By using equations to ensure both continuity of pressure and continuity of mass, a more rigorous approach to embedding is obtained than by simply interpolating pressures between separate DFN and CPM models.
In order to construct embedded models of the same fractured rock (mixing DFN and CPM sub-models), the data used for the DFN and CPM models should be self-consistent. For example, if a repository-scale DFN model is combined within a CPM model, then flow statistics on an appropriate scale (the size of the elements in the CPM model) need to be consistent. This is achieved by fracture upscaling techniques.
The advantage of being able to employ combined DFN/CPM models is that this provides greater flexibility in the construction of groundwater flow models. For example, combined DFN/CPM models can be constructed in ConnectFlow to model:
- Detailed flow in fractures around tunnels, shafts, canisters or boreholes using local DFN models nested within a CPM model that extends the model beyond the boundaries of the DFN model to a wider regional-scale boundary;
- Interaction between flow in a fractured media and backfilled tunnels by using detailed CPM models of tunnels, shafts and canisters within a DFN model;
- Continuous representation of deterministic faults/fracture zones through DFN and CPM sub-models using consistent data formats and a combination of explicit fracture planes in DFN regions and an implicit fracture zone (IFZ) method in the CPM region.