Hierarchical fault zone definition page

This is the Next >> page accessed in WizGen (in "Flexible project" mode) from the Drag applied to fault traces page or by selecting the Goto... button for Define sub-gridblock hierarchical fault zone structure option on the Contents page when the Include fault drag and hierarchical zone effects option has been toggled "on" via the Title page of WizGen.

The WizGen Hierarchical Fault zone definition page offers three Fzone options for attaching fault zones to fault traces:-

• Do not apply fault zone structure - to omit fault zones from the model, or to place them manually using a file containing TGFZONE keyword data added to the current run via the Included Data page.


• Assign hierarchical zones - to place fault zones stochastically using the WizGen tool to assign hierarchical zones.


• Use a plugin - to place fault zones according to user-defined criteria using the FZONE plugin.

A fault zone is a new kind of TransGen object associated with the fault traces allowing inclusion in the simulation model of transmissibilities associated with locally paired slip surfaces which may have very different juxtapositions to the single slip surface present on the trace in the parent model (see section on Fault zones for further details). The principal behind the hierarchical construction of fault zones is that a fault, represented as a continuous discontinuity in the reservoir simulation model, might be segmented at a sub-resolution scale with, for example, a low frequency of relatively large unbreached relays, a higher frequency of intermediate sized relays with single breaches and a higher-still frequency of very small, doubly-breached relays. This situation can be modelled using a hierarchy comprising three levels.

Large deterministic fault zones visible seismically, but too small for explicit representation in the flow model can be included using the TGFZONE keyword. Smaller, sub-seismic fault zone structure, however, is better modelled stochastically using the WizGen Assign hierarchical zones tool which will never overwrite those included deterministically.

If Do not apply fault zone structure is selected, only fault zones included using the TGFZONE keyword are applied.
If Use a plugin is selected, the FZONE plugin to use should be selected from the library  or written from scratch in the User-defined Plugins WizGen page.
Alternatively select the Assign Hierarchical zones option to model the fault zones stochastically as described below.

Assign hierarchical zones

A fault zone hierarchy can consist of one or more levels, each level characterised by 8 constants. Hierarchies can be defined via the Hierarchical fault zone definition page in WizGen by selecting the Assign hierarchical zones option (as shown below).

The first time the Assign hierarchical zones option is selected for a TransGen project, the Library selection is blank and a single Hierarchical level (level 1) is assigned with default values for the eight constants (as shown below).  These values can be modified by selecting the current value in the text box and typing in the required new value plus return.

With the first Hierarchical level (1) selected, set the Constants for this level.

Setting the Constants for an hierarchical level

A fault zone hierarchy is defined by the number of levels present and the values of the eight constants defined for each level.

NOTE:- When setting Constants for several hierarchical levels, Level 1 should contain the rarer, larger ramps (i.e. lower values of the Frequency constant and higher values of Width/Throw) because the plugin written by WizGen processes each hierarchy sequentially starting from level 1. Ramps from the particular level being processed are not placed on traces which have already had ramps placed at a higher level.

These constants are:-

The width to throw ratio of the ramp (Width/throw)

The length to width ratio of the ramp. (Length/width)

Two breaching indices (Breaching index1 and Breaching index 2)

A Frequency constant

A multiplier on the ramp-parallel permeability (Parallel damage)

A multiplier on the ramp-perpendicular permeability (Perpendicular damage)

A minimum throw cut-off (Minumum throw).

If a ramp from a particular level is placed on a trace, its geometry is governed by the width/throw, length/width and the two Breaching index values specified for this level and the throw on the trace.
The default values shown above are those for an unbreached relay ramp. To generate a breached relay ramp, decrease the Width/throw ratio setting and either decrease the Breaching index 1 for a single-breached relay or decrease both Breaching indices for a double-breached relay.

The construction of the ramp from these values is discussed in the section on Ramp geometry below.

The decision of whether or not to place a ramp from a particular level on each trace is governed by the Frequency constant and the Minimum throw cutoff, and is discussed in the section on Stochastic ramp placement below.

The petrophysical properties of the ramp are derived using the values of the Parallel damage and Perpendicular damage settings for each level to modify the ramp permeabilities. The permeability in the ramp perpendicular to the trace (KperpA) is given by the harmonic average of the two cell permeabilities (Kperp1 and Kperp2), multiplied by the fault zone variable Perpendicular damage, while the permeability in the ramp parallel to the trace (KparaA) is given by the arithmetic average of the two cell permeabilities (Kpara1 and Kpara2), multiplied by the fault zone variable Parallel damage (see Figure 7(d) in the section on Fault zone dimensions). These two variables should therefore be reset to include effects on permeability of minor faults in the ramp.

Creating more levels in the Hierarchy

A new level can be added using the Add level button. The new level (level n) takes the values of the constants from the highest level prior to its addition (i.e. from level n-1), and the constants for each level can be reviewed and modified by selecting an existing level from the hierarchical level listing.

The Remove level button removes the level currently selected (level m), and moves up all lower levels (i.e. what was level m+1 becomes the new level m). It is impossible to remove level 1 of a hierarchy if it only has 1 level.

Managing the Fault zone hierarchy libraries

Once the Constants (as described above) for all the required level(s) in a hierarchy have been input, the Hierarchy can be saved to the user's fault zone library.
Select Save to library from the File options to access the Name prompt window. Input an appropriate name and click on OK to save the hierarchy, which can then be loaded into a subsequent TransGen run.

A previously saved hierarchy can be loaded into a TransGen run by choosing the Load from library, File option. This lists the names, and number of levels, in each library item in the user's saved hierarchies. Click on the required fault zone Hierarchy followed by the Load option.

HINT:- Hierarchies can also be deleted from the library using this dialog.

The fault zone library is located in the file: ~/.transgen/fzone.lib

When the Hierarchical fault zone definition page is set as required, click on the Next >> button to view the current settings in the Project (TGDATA) File.

Alternatively, click on the << Back button to return to the Drag applied to fault traces page.

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

Click on Save to save any modifications made to the current TGDATA runfile.
Click on Quit to exit from WizGen with or without saving the any changes to the TGDATA runfile.

Alternatively, having completed both the Drag applied to fault traces and the Hierarchical fault zone definition pages (and all the other pages) in WizGen, Save the settings and click on the ViewGen icon to run the model as set.