HydroGeoSphere/Discrete Fractures (Saturated)

Default fractured media saturated flow properties

Unless you modify the default values, all discrete fracture zones (and 2-D fracture elements) in the domain will be assigned the default saturated properties which are listed in Table 5.6. These values are representative of a 100 micron fracture:

Table 5.6: Default Values for Fractured Media Saturated Flow Properties

Parameter	Value	Unit
Name	Default 100 micron fracture	-
Aperture $w_{f}$	100 × 10−6	m
Conductivity (computed) $K_{f}$	$({\rho }g~w_{f}^{2})/(12{\mu })$	m s−1
Specific storage (computed) $S_{sf}$	${\rho }g{\alpha }_{w}$	m−1
Unsaturated flow relation type	Pseudo-fracture	-

You can use the general methods and instructions outlined in Section 5.8.1 to modify the default distribution of saturated fractured media properties.

As was the case for the instructions which modify porous medium properties, these instructions also have a scope of operation, the only difference being that they would appear in the .fprops file instead of the .mprops file.

Aperture

Scope: .grok .fprops

1. val Fracture aperture [L], $w_{f}$ in Equation 2.11.

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High-k plane

Scope: .fprops

1. valx Thickness [L] for the high-conductivity plane.
2. valx Hydraulic conductivity [L T⁻¹] for the high-conductivity plane.

Treat the fracture material as a high-conductivity plane where the hydraulic conductivity and thickness are given by the user.

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Specific storage

Scope: .grok .fprops

1. val Specific storage [L⁻¹], $S_{sf}$ in Equation 2.14.

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Coupling length

Scope: .grok .fprops

1. val Coupling length [L].

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Coupling hydraulic conductivity

Scope: .grok .fprops

1. val Coupling hydraulic conductivity [L/T].

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Impermeable matrix

Scope: .grok

This command causes the matrix to be considered impermeable and so flow and transport will only be computed for the fractures. This overrides any values defined in the prefix.grok or .fprops file so you do not have to alter them.

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Make fractures from fgen

Scope:.grok

1. mat_name Name of the fracture material whose properties are to be read and assigned.
2. fname Name of the FGEN file which contains the random aperture information.

Chosen faces are assigned to a new fracture zone and given the properties of material mat_name, which is read from the .fprops file fname. Any aperture information contained therein will be overwritten by the information that is then read from the FGEN file. The FGEN file aperture values are distributed on a rectangular grid with uniform spacings in the three principal axis directions. Element aperture values are then interpolated to the element face centroids by bilinear (if the FGEN data is on a 2-D plane) or trilinear (if the FGEN data is fully 3-D) interpolation.

grok assumes that the FGEN file contains log aperture values and so the following conversion is done:

$w_{f}=exp(w_{f_{FGEN}})$

where $w_{f_{FGEN}}$ is the fracture aperture value read from the FGEN file.

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Make fractures from tecplot file

Scope: .grok

1. tecplot_frac_file Name of the Tecplot file which contains the fracture triangle information.
2. label Name of the fracture material whose properties are to be read and assigned.

A new fracture zone is created based on triangle data given in the tecplot file tecplot_frac_file. Fracture zone properties are assigned from the material label.

The tecplot file must contain one or more tecplot ZONE statements. Each tecplot ZONE statement must contain, or be followed by a statement containing the string N= followed by the number of vertices and then by the string E= followed by the number of triangles in the tecplot ZONE.

Comments beginning with the character # are allowed.

The tecplot variables represent the xyz-coordinates of the vertices and must be given in three-column, point format.

The fracture elements are defined so that they conform closely to the triangulated surface given in the tecplot file, and can include diagonal (i.e. inclined) faces.

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The following Tecplot file produced the fracture elements shown in Figure 5.11:

    VARIABLES="X","Y","Z"
    ZONE T="Fractures_primary"
    N= 5,E= 3,DATAPACKING=POINT, ZONETYPE=FETRIANGLE
    110. -1. -5.
    51. 102. -3.
    -3. 49. 99.
    0. 100. 100
    0. 0. 0.
    1 3 5
    1 2 3
    2 3 4

Figure 5.11: Example of fracture generation showing 3D porous medium domain (grey cube), tecplot triangles (yellow triangles) and the resulting fracture elements (blue triangles)

Make recharge spreading layer

Scope: .grok

1. frack Hydraulic conductivity [L T⁻¹] of the recharge spreading layer.
2. aperture Thickness [L] of the recharge spreading layer.
3. remove_fixed_head_conn A logical switch which determines whether the recharge spreading layer is connected to an existing specified head boundary.

Chosen faces which are on the top of the domain are assigned to a new fracture zone which has the properties of a recharge spreading layer. A recharge spreading layer is a zone of relatively high hydraulic conductivity which allows recharge water to infiltrate preferentially into zones with high hydraulic conductivity (e.g. fractures).

A recharge spreading layer may allow water to bypass the rest of the system if it is connected to or in close proximity to a constant head boundary which can act as a discharge point for the system. In such cases you can set the variable remove_fixed_head_conn to be .TRUE, to prevent such direct connections. If your intent is to distribute water preferentially between fractures and low K matrix, experience has shown that recharge spreading layer transmissivities two orders of magnitude higher than the matrix are usually sufficient.

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