Discrete Fracture Networks (DFN)
The Discrete Fracture Network (DFN) concept is a computational approach in which rock fractures are explicitly represented in the model, with individually-assigned geometric and hydraulic properties. The technique is well suited to modelling geological regimes in which flow and transport are predominantly confined to the void-space of an interconnected network of fractures. ConnectFlow's DFN module has been developed over a period of more than 25 years and has been extensively verified in international comparison exercises. The software employs a highly efficient implementation of the finite-element method, allowing it to handle models comprising tens of millions of fractures.
The DFN module incorporates options for:
- Deterministic and stochastic fracture generation.
- Steady-state and transient groundwater flow simulation.
- Generation of regular and irregular meshes (e.g., ZMap, VIP, FEMGEN and CAD formats).
- Multi-component solute transport through fracture networks, with support for rock-matrix diffusion.
- Reactive transport, via an interface with the USGS geochemical modelling software PHREEQC.
- Upscaling of DFN models to equivalent CPM (ECPM) models.
Continuous Porous Media (CPM)
Continuous Porous Medium (CPM) models are appropriate for certain types of rock and porous materials, in which flow is transmitted through the connected pore-space between the solid grains of the rock matrix. Examples include sandstones, soils, clays and unconsolidated deposits. ConnectFlow's CPM module supports a range of facilities for specifying the geometry of the model domain, the properties of rocks, fluids and solutes and the physics to be included in the system.
The CPM module incorporates options for:
- Steady-state and transient flow in saturated and unsaturated conditions.
- Saline groundwater flow with variable density.
- Coupled salt and heat transport.
- Multi-component solute transport with chemical reactions, via interaction with PHREEQC.
- Contaminant transport, including the effects of advection, dispersion, sorption, rock matrix diffusion and solubility limitation.
- Radioactive decay chains, including interacting chains linked by solubility limitation of a common radionuclide.
- Sensitivity to model parameters, using the adjoint method.
Combined CPM-DFN Models
Uniquely among subsurface flow modelling software, ConnectFlow also offers the option to construct embedded models that integrate one or more sub-models of different types. That is, the model can be divided into separate domains that use either the CPM or DFN concepts, nested within one another. Internal boundary conditions are created at the domain interfaces to ensure continuity of pressure and conservation of mass.
Combined CPM-DFN models afford the user far greater flexibility in the construction of subsurface models, allowing them to, for example:
- Model detailed flow-fields in fractures around tunnels, shafts or boreholes, using localised DFN models embedded within a CPM rock volume.
- Apply the CPM concept to a backfilled tunnel or shaft, nested within a larger, site-scale DFN model of a fractured rock mass.
- Include continuous representations of deterministic faults and fractured zones through DFN and CPM sub-models.
Powered by the state-of-the-art VTK graphics library, ConnectFlow's graphical user interface (GUI) provides users with advanced visualisation tools that can be applied to a wide range of three-dimensional subsurface data formats. Examples include visualisation of fracture networks, particle tracking pathlines or contaminant distributions.
In addition to visualisation, the GUI can also be used to construct and run ConnectFlow models from within the graphical environment. Support for interacting with other widely-used modelling packages is also included, for example GOCAD® file formats, ECLIPSE grids, ESRI (TM) surfaces, and many others.
ConnectFlow uses the finite-element approach for spatial discretisation of the modelled region. This is a powerful approach that is well suited to modelling complex geometric domains, such as those that occur in geological environments. ConnectFlow uses a highly efficient finite-element implementation, allowing it to handle large site- and regional-scale models consisting of many millions of grid cells or fractures. Temporal discretisation is achieved using either the Crank-Nicolson method or Gear's method. Non-linearities are treated using the Newton-Raphson iterative method, which converges rapidly given suitable initial conditions. Parameter stepping is implemented for highly non-linear problems, to overcome potential convergence difficulties.
A range of solvers are implemented in ConnectFlow, including an efficient Frontal direct method, the Generalised Minimum Residual (GMRES) iterative method and the algebraic multi-grid (AMG) iterative method. A range of preconditioners can also be applied to help with stability and convergence.
ConnectFlow supports inclusion of hydrogeochemical reactions in multi-component solute transport calculations in both the CPM and DFN modules. This is achieved by linking ConnectFlow to the USGS geochemical software PHREEQC, using the iPhreeqC library. ConnectFlow can be used to model the following types of reactions in transport calculations:
- Equilibrium reactions
- Ion-exchange reactions
- Surface complexation
- Kinetic reactions
Upscaling is the process in which a DFN model is converted to an equivalent CPM (ECPM) model. This can be helpful if the ECPM representation allows faster calculation times or a functionality not available for DFN (such as heat transport). Upscaling is achieved by dividing the DFN model region into a three-dimensional grid of cells. Effective properties (i.e., porosity, permeability tensor, and flow-wetted surface) are then calculated for each cell using a least-squares method, such that the characteristics of the fracture network are replicated (on average) in the ECPM model. Algorithms for geometric, flow-based and transport-based upscaling are available in ConnectFlow. The process can be subdivided across multiple calculations to reduce calculation run-times.
ConnectFlow can read, write and visualise many open-source and third-party file formats:
- AutoCAD DXF format (.dxf)
- GoCAD Tsurf (.tfc, .ts), polyline (.gpl) and vset (.vs) files
- ESRI raster (.asc) and shape (.shp) files
- FracMan FAB files (.fab)
- Ordnance Survey Landform Profile files in National Transfer Format (.ntf)
- XYZ surface and polyline files (.xyz)
- Eclipse GRDECL grids (.dat)
- Reservoir fracture set intensity files (.rfi)
- Seismic SEG-Y format (.segy, .sgy)
- Stanford polygon files (.ply)
- SURPAC String (.str) and DTM (.dtm) files
We can often produce bespoke readers and writers for proprietary file formats.