Fault Rock Properties page

This is the Next >> page accessed from WizGen in Basic project mode from the Miscellaneous Options page or by selecting the Goto... button for Determine the fault rocks properties from the Contents page.

The Fault Rock Properties page of WizGen in Basic project mode offers options to:-

calculate Fault permeability as a function of combinations of two Fault Seal Potential measures, i.e. Clay Smear Potential (CSP) and/or Shale Gouge Ratio (SGR)

define the constants (a and b) in the equation for calculating fault rock Thickness from fault Displacement.

select the Effective Vshale computation method dependant on the input data (i.e. the imported Net-to-Gross and/or Vshale data defining the shale content of the cellular grid).

define the form of the Clay Smear Potential algorithm.

define the relationship between SGR, CSP and fault rock permeability (CSP cutoff and SGR constants a, b, c, d)

In order for the calculations in TransGen version 3 to run, plugins to compute thickness and permeability must be supplied. Using WizGen in Basic project mode automatically generates and rewrites these plugins whenever new Fault Rock Properties page settings are saved. The two plugins are stored in the following specially named files in the project's `_INPUT/.plugins' directory as:-

<current_directory>/<project_title>_INPUT/.plugins/_AUTO_THICK_PLUGIN.cpp
<current_directory>/<project_title>_INPUT/.plugins/_AUTO_PERM_PLUGIN.cpp

and are added as strings referenced to the TGPLUGIN keyword in the TGDATA run file.

For further details refer to the section on Plugins generated by WizGen in Basic project mode.

The selected CSP and/or SGR settings are saved to the TGDATA file under the new TGFSP keyword.

The relationships between the calculated Fault Seal Potential(s) of the fault-rock and the permeabilities are shown in the Fault property relationships text, top right on the Fault Rock Properties page (as shown below).

HINT:- See the Technical Description for details on how TransGen calculates fault-rock permeabilities and transmissibilities from the Fault Seal Potential(s) using the Fault Rock Properties settings.

Fault permeability is a function of

TransGen in "Basic project" mode offers three options for calculating fault-rock permeability based on combinations of two different Fault Seal Potential measures, i.e. Shale Gouge Ratio (SGR) and/or Clay Smear Potential (CSP).

NOTE:- This and other Fault Property relationships settings, defined on the Fault Rock Properties page, determine the format of the automatically generated PERM plugin used to calculate fault permeabilities and ultimately transmissibility multipliers (see Technical Description of what TransGen does for details).

SGR (Shale Gouge Ratio)

SGR provides a measure of the proportion (ratio) of shale in the fault zone assuming shale material is incorporated into the fault gouge in the same proportions as it occurs in the wall rocks of the slipped interval.  A high SGR is expected to correspond to more phyllosilicates in the fault zone and therefore greater seal potential.

The shale gouge ratio is the distance-weighted average of the shale content of the rocks that have moved past a point on a fault.  For each faulted cell, TransGen calculates the SGR from the effective vshale content of the cells that have moved past it along the direction of the COORD lines. SGR is calculated at each vertex of the intersection polygon, on both sides of the fault, so there are two SGR values for each vertex. An average SGR is calculated for each faulted cell-to-cell connection, i.e. the SGR of the fault-rock is taken as the average of the footwall and hangingwall SGR values for connecting cells:-

See section on the Calculation of Shale Gouge Ratio in the Technical Description of what TransGen does for further details.

CSP (Clay Smear Potential)

CSP provides a measure of clay smear (in terms of thickness) incorporated into the fault zone based on the shale source bed Thickness and displacement Distance. CSP increases with shale source bed thickness (there is more material to incorporate into the fault zone) and decreases with increasing distance from the source bed (as displacement at the fault increases, the shale incorporated into the fault zone becomes thinner). The thicker the Clay Smear, the greater the fault seal potential. The shale source beds are identified as Named layers, via the Evshale cutoff or as Defined shale cells.

See section on the Calculation of Clay Smear Potential in the Technical Description of what TransGen does for further details.

NOTE:- As the calculation of Clay Smear Potential is strictly only valid where reservoir (i.e. non-shales) are juxtaposed across a fault, inactive CSP values of -1 are automatically assigned where shale layers are juxtaposed across a fault, i.e. incalculable CSP values that tend to infinity. For further details, see Inactive FSP values in the section on Issues associated with FSP calculations. The calculated permeability, faulted transmissibilities and transmissibility multiplier values will be set to 0.0 where the CSP values are -1.0.

Only SGR

Using the Only SGR (default) setting, the Fault permeabilities are calculated from the SGR values using the following expression, where a, b, c and d are set by default to 0.4, 4.0, 0.25 and 5.0 in the SGR options.

NOTE:- Only SGR is the default function to determine fault permeability using TransGen version 3.1 in Basic project mode, performing fault permeability calculations in a similar way to the default calculations in TransGen version 2.

Only CSP

Using the Only CSP setting, the Fault permeability is set to zero when the calculated CSP value is greater than the user-specified CSP cutoff (i.e. clay smear thicknesses greater than the cutoff value are assumed to be totally sealing). Otherwise, the fault permeabilities are set to the average of the unfaulted Permeabilities on either side of the fault.

CSP then SGR

When CSP then SGR is used, the Permeability is set to zero when the CSP value is greater than the user-specified CSP cutoff (i.e. clay smear thicknesses at or greater than the cutoff value are assumed to be totally sealing). Otherwise, the Fault permeabilities are calculated from the SGR values using the above expression, where a, b, c and d are set by default to 0.4, 4.0, 0.25 and 5.0 in the SGR options.

Displacement to Thickness options

These constants define fault rock thickness as a fraction of fault displacement.

The equation used to calculate fault thickness as a function of fault displacement is:-

where THICKf and Df are the thickness and displacement at a particular connection vertex.  The constants a and b default to 0.005882 (1/170) and 1 respectively (following Manzocchi et al. 1999).

To change either Constants, double click on the current value and type in required new constant.

NOTE:- These constants are substituted in the automatically generated THICK plugin used to calculate fault-rock thickness and ultimately the transmissibility multipliers (see Technical Description of what TransGen does for details).  

Effective vshale computation method

The fault-rock composition is controlled by the Effective Vshale (Evshale) values of the grid blocks. The Effective vshale can be calculated in one of the 3 following methods dependant on the availability of Net-to-Gross and vshale data. i.e. what data has been imported via the Included data page.

Net-to-gross only - TransGen considers the non-net region to be shale and takes the shale content to equal 1 minus the Net-to-Gross value (where Net-to-Gross is the ratio of the net thickness of good reservoir, i.e. sand to gross interval thickness).

Vshale only - TransGen takes Vshale as the shale content.

NGT and VShale - TransGen assumes the non-net region to be pure shale, and takes the Effective Vshale content of a grid block i to be:-

The Net-to-gross data has to be input into the current TransGen project as data associated with the Eclipse format keyword NTG from the flow simulation model.
To use Vshale data in the calculations, it must be defined and loaded using the TGVS keyword.
The file(s) containing either the NTG and/or TGVS keyword(s) plus the associated data for all the grid cells in the current model must be added to the current TransGen run via the Included Data page of WizGen.

SGR options

If either the Only SGR or CSP then SGR fault permeability calculation option has been selected, the SGR options displays editable default values for the SGR equation constants (a, b, c and d) in the following equation.

To change any constant, double click on the current value and type in the required new constant.

CSP options

If you have chosen to calculate Fault permeability and Transmissibility multipliers as a function of either Only CSP or CSP then SGR, you will need to set the CSP options.

Shale definition & Shale data settings

Evshale cutoff

With this option selected, the cells treated as shales can be defined by a single Shale data cutoff value.
Click the Evshale cutoff option "on" and input a single cutoff value in the Shale data box, i.e. a floating point number between 0.0 and 1.0, such as 0.5. Cells with eVs below the specified cutoff are designated shale cells and are used in the current CSP calculation.

Named layers

With this option selected, named layers define the cells treated as shales.
Click the Named layers option "on" and input the layers as a list separated by commas in the Shale data box. Multiple adjacent layers can be defined by a hyphen separating the first and last of these layers, e.g. `1,2,3,5,7,8,9,10' can be input as `1-3,5,7-10'. The list can be up to 256 characters long. Spaces are ignored (indeed spaces are removed by WizGen to shorten the length of the string).

Defined shale cells

With this option selected, the cells with values of 1.0 associated with the TGSHALE keyword are treated as shales.
With an appropriate TGSHALE data file loaded into TransGen (via the Included Data page), click the Defined shale cells option "on". Cells with the TGSHALE value at 1.0 are designated shale cells and are used in the current CSP calculation.

CSP cutoff (m)

The CSP cutoff value is used when calculating fault permeability as a function of either Only CSP or CSP then SGR. If the calculated Clay Smear Potential exceeds the cutoff value, the permeability is automatically set to zero, i.e. clay smear assumed to be totally sealing.

When the Fault Rock Properties page is set as required, click on Save to save the current settings to the TGDATA run file, then click on the Next>> button to view/edit the Output - simulator input page.

Alternatively, click on the << Back button to return to the Miscellaneous Options page to view/edit the Lower limits for Multiplier and/or Cell volume cutoff and whether to perform calculation or not via ViewGen.

Or click on the Contents button to access the Contents page to view/edit any of the current TGDATA file settings, inspect the project's TGDATA file and/or inspect the log generated by last ViewGen calculation.