Fracture
Use the Fracture node to model flow along a boundaries representing fractures within a porous medium. By default, it adds the tangential form of the continuity equation and of Darcy’s Law.
(4-17)
(4-18)
The Fracture feature has two default subfeatures — the Fluid (Fracture) subfeature where the fluid density ρ (SI unit: kg/m3) and dynamic viscosity μ (SI unit: Pa·s) are defined and the Fracture Material subfeature where the permeability κ (SI unit: m2) and porosity εp (dimensionless) are specified.
Coordinate System Selection
Select a coordinate system from the Coordinate system list for the interpretation of directions in anisotropic material properties. The default is the Global coordinate system, and the list contains any additional orthonormal coordinate system (except boundary coordinate systems) added under the Definitions node.
The subnodes inherit these coordinate system settings. In particular, the Permeability or Hydraulic conductivity (in the Fracture Material subnode) should be set according to the coordinate system selected in this section.
Aperture
Enter a value for the Fracture thickness df.
Flow Model
Define if the flow follows a linear or nonlinear pressure-velocity relationship.
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Darcian flow (default) defines a linear relationship using Equation 4-17 and Equation 4-18.
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Non-Darcian flow defines a nonlinear relationship using Equation 4-17 and the pressure-velocity relationship
where the nonlinear parameter β (SI unit: 1/m) is further specified in the Fracture Material subfeature.
Storage Model
Equation 4-17 can also be formulated with respect to a storage term.
The storage coefficient S can be interpreted as the weighted compressibility of the porous material and the fluid. Define the time dependent storage term by selecting one of the following options from the drop-down menu:
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From density and porosity (default), uses the formulation of Equation 4-17
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Linearized storage, where the following linear equation is used to define the storage:
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User defined to directly specify the storage coefficient S
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About Darcian and Non-Darcian Flow in the Porous Media Flow Module User’s Guide
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Storage Model in the Subsurface Flow Module User’s Guide
Fracture Model
Choose between two options to model fractures that have different characteristics.
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Highly conductive fracture represents a fracture filled with a highly permeable material.
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Thin conducting barrier represents a fracture that is filled with a low permeable material and therefore acts as a barrier for the flow which adds a jump in the pressure across the boundary:
The barrier thickness db and permeability κb are defined in the Fracture Material subnode.
In addition, either on the upside, downside, or on both sides a highly conductive fracture material is present and Equation 4-17 and Equation 4-18 are solved for the highly conductive side. Further specifications are done in the Fracture Material subnode.
Note that the option to specify the Fracture Model is only available for the Fracture feature within the Darcy’s Law interface.
For the Fracture boundary node several subnodes are available from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu. As they are almost identical to the boundary conditions of the Fracture Flow interface, see Domain, Boundary, Edge, Point, and Pair Nodes for the Fracture Flow Interface for further description. Note that Precipitation is only available for the Fracture Flow interface.
This feature requires a specific license. For a detailed overview of the functionality available in each product, visit https://www.comsol.com/products/specifications/.
Flow in a Fractured Reservoir: Application Library path Subsurface_Flow_Module/Fluid_Flow/fractured_reservoir_flow