PDF
Discrete Fracture Network — 2D
Introduction
Modeling discrete fracture networks (DFN) helps to understand the characteristics of flow, transport, and mechanics in fractured rocks and reservoirs. Therefore, it is often used for geotechnical and hydrological applications.
With this add-in you can create a DFN based on various distribution functions for size, orientation, and aperture. You can either use the add-in for adding a DFN to your 2D model or start with the add-in to create a completely new model.
When creating a DFN, a new 2D part is created. The fractures are created as lines and their size, position, and orientation are determined by the selected settings. This part is imported into the geometry of the selected component or a new component and the geometry is created.
The add-in also creates a Fracture Flow feature following a cubic law in a new or existing Darcy’s Law interface. The values for the aperture are defined as variables under a variable group node.
The following distributions and distribution functions are used:
Uniform Random
The probability density function for the variable x is given by
(1)
Power Law
The probability density function for the variable x is
(2)
Fisher (Orientation Only)
The probability density function for the Fisher distribution describes the angular deviation θ (SI unit: rad) from the mean angle of the orientation of the ellipse:
(3)
Here, K is the dispersion coefficient or Fisher constant.
Size Proportional (Aperture Only)
The aperture a (SI unit: m) is proportional to the length l of the fracture
(4)
with the proportionality factor c.
After adding a DFN you can modify the settings. The more complex the DFN becomes, the more likely it is that manual modifications of the geometry and/or the mesh are necessary, because the statistical distribution can lead to small details in the geometry. You can address them by either modifying the geometry or creating a manual mesh that first meshes the fracture sets with a triangular mesh before meshing the remaining geometry.
Reference
1. B. Berkowitz, “Characterizing flow and transport in fractured geological media: A review,” Advances in Water Resources, vol. 25, pp. 861–884, 2002.
Add-in Library path: Subsurface_Flow_Module/discrete_fracture_network_2d
Discrete Fracture Network — 2D
When you load the add-in into your model it automatically detects 2D components and Darcy’s Law interfaces to which a DFN could be added.
Toolbar
Use the toolbar at the top of for different actions.
Reset resets all values to factory settings. With Add a DFN with the current settings is added to the selected component and interface. Update allows to update the last created DFN with the new settings. Delete All deletes all DFN in the model and related features but does not delete any components or interfaces.
Data
You can select between creating a new component and Darcy’s Law interface or select between existing ones in your model.
General
This determines the overall size of the DFN. Enter the Number of Fractures. The total number is limited to 200 for a single fracture set. If you want to create a DFN with more than 200 fractures you can add multiple fracture sets instead.
When a geometry is present you can switch between From geometry and Manual in the Bounding box drop down menu. If From geometry is selected, a Selection drop down menu appears. The list will be populated with all available domain selections that are defined under the geometry node and the default is All domains. Depending on these settings the bounding box is determined either by the size and position of the whole geometry or of the chosen selection. If you want to define the bounding box differently or if no geometry is present enter the values for the size and lower left corner of the bounding box.
Size
Select between different distribution functions for the size of the fractures. The position of the fractures always follows a uniform random distribution function (Equation 1).
From the Distribution list, select Constant to specify a constant Length for all fractures. Select Uniform random and specify the Minimum length and Maximum length. The length is randomly distributed within the given range (Equation 1). Select Power law (default) and specify the Minimum length, Maximum length and the Power law exponent. The length distribution then follows a power-law distribution function (Equation 2).
To be able to create reproducible DFN select the Use random seed check box and enter an integer.
Orientation
Select between different distribution functions for the orientation of the fractures.
From the Distribution list, select Constant to specify a constant angle for all fractures. The angle defines the inclination to the horizontal (x-axis). Select Uniform random to get a random orientation distribution for all fractures (Equation 1). Select Fisher to specify a mean Angle and the Dispersion coefficient. The orientation distribution then follows a fisher distribution (Equation 3).
To be able to create reproducible DFN select the Use random seed check box and enter an integer.
Properties
In this section you specify the hydraulic properties of the fracture network. Specify Porosity and Roughness factor and select between different distribution functions for the aperture.
Select Constant to assign a constant Aperture to all fractures. Select Uniform random and specify a Minimum aperture and Maximum aperture. The aperture is then randomly distributed to each fracture (Equation 1). Select Size proportional (default) and specify a Proportionality factor. The aperture is then proportional to the size of each fracture (Equation 4). Select Power law and specify a Minimum aperture and Maximum aperture and a Power law exponent. The aperture distribution then follows a power law distribution function (Equation 2).
To be able to create reproducible DFN select the Use random seed check box and enter an integer.
Information
This section provides status information.