Issues associated with FSP calculations

Shale face bunching

A geocellular model may contain shale layers that are more than one grid-block thick. In this case, the general FSP expression will give different answers if the shale is treated as two separate shale faces rather than as a single shale.

Shale bunching refers to the operation of combining multiple, vertically adjacent shale faces into a single face for use in the FSP calculation. This is illustrated in the figure below, showing the equations used to define the thickness ('bunched t') and effective vshale (`bunched eVs`) values used. Distance (`bunched d`) and throw (`bunched D`) are also calculated as a function of the bunched face.

Shale bunching is important for correctly calculating the values of FSP measures that are based on the concept of discrete shale layers (e.g. CSP). However, for FSP measures based on the concept of a continuous distribution of effective Vshale (e.g. SGR), all cells are designated as shale cells (i.e. the Shale definition is Based on all cells) and bunching is inappropriate. Hence, shale bunching is ALWAYS applied, EXCEPT when the Shale definition is Based on all cells.

Inactive FSP values

The concept of undefined vertices and connections has been introduced as a consequence treating shale faces and non-shale faces in different ways. Any FSP measure which does not use all faces as shale faces (i.e. all models in which the Shale definition is NOT set to Based on all cells) will contain some undefined (i.e. incalculable) values.

The FSP value of a vertex or connection is made "undefined" if the cell on either side of the connection being processed is a designated shale cell. The reason for inactivating the FSP measures in this case is to avoid calculating a FSP value of infinity.

Vertices with a FSP measure inferred as "undefined" have the FSP value assigned to -1. If one vertex within a connection evaluates the FSP to be "undefined", all other vertices within that connection automatically have the value for the FSP set to -1. Since all valid FSPs have a range equal to or greater than 0.0, any vertices or connections with negative FSP should be considered undefined.

Care must be taken in the permeability plugin (See User-defined plugins page) to assign appropriate values for fault permeability to these "undefined" FSP values (either by using an FSP which does have defined connection, by using the cell properties adjacent to these connection, or by assigning a constant permeability to them).

Invalid FSP options

Certain FSP strings have the potential to introduce ambiguity into the calculation, in which case TransGen will issue an error message and stop. Only two such cases have been identified to date:-

With the Combination option set to Maximum from layers then sum and with the Shale definition set to Effective Vshale cutoff

With the Combination option set to Maximum from layers then sum and with the Shale definition set to Based on user-property

The reason for disallowing these combinations is owing to shale bunching, resulting in, for example one shale layer existing on the footwall side of the fault, while two are present on the hangingwall side (as shown in the Figure above). The particular Combination setting demands that the maxima of particular pairs of layers are submitted to the summation, but even in this simple illustration there is no unambiguous way in which pairs of layers can be compared.

Displaying Effective Vshale and designated Shale layers

In TransGen Version 2, the item Effective Vshale appeared in the cell properties pulldown menu in the graphical interface. In Version 3 there is no longer necessarily a unique value of Effective Vshale in each grid-block, since each FSP measure (up to 5 of which can be included in a single TransGen run) can be based on a different definition of Effective Vshale. Therefore, if graphical display of an Effective Vshale associated with a particular FSP is required, it must be defined as a user-defined cell property. Up to ten user-defined cell properties are allowed and, if defined, new properties are dynamically added to the cell properties menu.

Similarly, the cells used as designated shale layers may differ from one FSP to another. Therefore, to be available for visualisation, the shale descriptions must be defined as user-defined cell properties and the FSPs constructed in such a way as to use shale definition based on a user-defined cell property.

For more information, see TGNEWKEY and the CELLPROP plugin and changes to the graphical interface.

FSP value calculation and storage

FSP values are calculated and stored at each vertex of each connection. In addition, the area-weighted arithmetic average FSP value is computed from the FSP values at the vertices and stored in the connection. In the graphics interface, the average FSP value (i.e. connection value) is displayed when the average sub-option is selected, and an interpolation of FSP vertex values when the smooth sub-option is selected in the fault properties pulldown menu.

Both vertex and connection FSP values can be accessed in the permeability and thickness plugins. We recommend using the vertex FSP wherever possible, since the value of the FSP may change rapidly over the area of a connection.

Dimensions of FSPs.

The dimensions of an FSP are Length^(Exponent L + Exponent M + Exponent N). If the keyword TGMETRIC is included in the TGDATA file (i.e. with the FSPs in metric option selected on the Miscellaneous Options page of WizGen in Flexible project mode), TransGen will calculate and report all FSP values (and fault displacement/thickness) using the units of Length set to Metric (i.e. metres).

IF the keyword TGMETRIC is omitted (i.e. with the FSPs in native units option selected on the Miscellaneous Options page of WizGen in Flexible project mode), the unit of Length corresponds to the Units specified for the project (i.e. feet for FIELD and centimetres for LAB as set on the Coordinate System page of WizGen). Use of the TGMETRIC keyword does not influence the dimensions in which the transmissibility calculations are made - these are scaled to the input UNITS.

If the TGDATA file is created by WizGen in Basic project mode, TGMETRIC is inserted into the file by default. This is required for `basic' plugin compatibility, since cutoff values of non-dimensionless FSP measures (e.g. CSP) used in the automatically generated plugins are in metric units.