TECHNICAL DESCRIPTION

(WHAT TRANSGEN DOES)

Introduction

TransGen generates and allows visualisation of fault transmissibility information for inclusion in reservoir simulation models.  Input (model geometry and grid-block properties) and output (transmissibility multiplier information) are Eclipse-format  data files; the output can be included directly in the EDIT section of the Eclipse DATA file.    

Typically, when using TransGen in "Basic project" mode, fault permeability and hence transmissibility are calculated as functions of the Fault Seal Potential (FSP) of the fault-rock (as estimated via the calculated Shale Gouge Ratio and/or Clay Smear Potential) and the fault-rock thickness.  This methodology is described below (see also Manzocchi et al (1999), Petroleum Geoscience, 5, 53-63).

However, using in TransGen in "Flexible project" mode offers (as the mode implies) vast flexibility in modelling fault-rock properties for inclusion in flow simulation models, through the introduction of a generalised method for calculating fault seal potential measures, and of "plugins" with which the expert user can interact directly to define user-specific algorithms for determining fault-rock thickness and permeability.

Also having upgraded a TransGen project from "Basic" to "Flexible" there is an optional separately licensed Fault drag and hierarchical zone effects functionality, which allows the user to include sub-resolution fault-zone structure in the model (for details see section on TransGen version 3.2 and Using WizGen in Flexible project mode).

Alternatively, another separately licensed facility allows the user to include Two-phase fault rock calculations for oil and water in the flow simulation model (for details see Using 2PhaseGen and Using WizGen in Flexible project mode).

Fault Displacement Calculation

Fault displacement is calculated and averaged for each faulted grid-block to grid-block connection.  Displacement is calculated at each vertex of the intersection polygon, on both sides of the fault.  (For any particular connection vertex, the displacements on each side of the fault are identical).

NOTE:- Fault displacement in TransGen is always measured parallel to the COORD lines bounding the faulted connection faces. TransGen therefore implicitly assumes the displacement vector of the fault is parallel to the COORD lines used to construct the 3D model geometry.

The calculated Displacement can be selected via the Fault Properties menu in ViewGen for display on the faults. The Displacement values can either be displayed as a single average value for each connection (Average) or with the values at each connection interpolated across the connection surface (Smoothed).

Fault Thickness Calculation

Fault thickness is calculated (at each connection vertex) as a function of displacement (D(f)) using the relationship defined in the THICK plugin:

where a and b are constants, normally set to 0.005882 and 1.0 respectively.

When using TransGen in "Basic project" mode, the THICK plugin is automatically generated using the Displacement to Thickness constants as defined on the Fault Rock Properties page of WizGen. The plugin is stored, when the current WizGen settings are saved, as _AUTO_THICK_PLUGIN.cpp in the <project_name>_INPUT/.plugins directory (see Plugins generated by WizGen in Basic project mode).

When using TransGen in "Flexible project" mode, the code to define this relationship has to be input by the user via the User-defined plugins page of WizGen.

An area-weighted harmonic average fault thickness is taken for each connection based on the thickness values at each vertex in the connection.

Effective Vshale

Fault-rock composition is controlled by the Effective Vshale (VshaleEff) values of the faulted grid-blocks.

When using TransGen in "Basic project" mode, the Effective Vshale can be calculated using one of the 3 following methods dependant on the availability of Net-to-Gross and/or Vshale data in the current project, i.e. what data have been imported via the Included data page of WizGen:-

Net-to-Gross only - TransGen uses only the input data associated with the Eclipse NTG keyword to compute Effective Vshale values taking the non-net grid-block values as pure shale and the shale content as equal to 1 minus the Net-to-Gross value (where Net-to-Gross is the ratio of net thickness of good reservoir, i.e. sand to gross interval thickness).

Vshale only - TransGen uses only the input data associated with the TransGen TGVS keyword as the effective Vshale values.

NTG and Vshale - TransGen uses input data associated with both the NTG and TGVS keywords, assuming the non-net grid-block values to be pure shale and calculating the Effective Vshale content of a faulted grid block to be:-

When using TransGen in "Flexible project" mode, the Effective Vshale can be calculated by any of the methods described above or Based on a User-defined property.

Fault Seal Potential (FSP) calculations

New to TransGen version 3, use of a generalised fault seal potential (FSP) algorithm for defining the fault-rock property at each faulted connection allows up to 5 FSP measures (e.g. SGR, CSP, SSF) to be calculated per WizGen run in "Flexible project" mode. When using WizGen in "Basic project" mode, the choice is restricted to Only SGR, Only CSP or CSP then SGR, but the same generalised FSP algorithm is used.

The introduction of shale faces in the FSP calculations is a significant conceptual difference between the Shale Gouge Ratio (SGR) in TransGen Version 2 and the FSP calculation in Version 3. In Version 2, SGR was calculated as a function of the faulted faces of all the cells in the model that have slipped past the vertex of the connection being processed. In Version 3, a subset of these faces (which can include none or all of them) are designated shale faces (i.e. EffectiveVshale) and only these shale faces are used in the FSP calculation.

The Figure below shows a cartoon of a simple 2D model with dipping COORD lines. Cells that have been designated shales are shown in brown, while non-shales which are not therefore included in the FSP calculation are shown in yellow. For each of the four shale faces (1 to 4), Figure (a) shows the definition of Throw (D), Thickness (t) and Effective Vshale (eVs) included in the FSP equation. Figure (b) shows the the three possible definitions of the Distance term for a calculation on the vertex highlighted with a red dot.  

NOTE:-The Throw term is not necessarily the same for the 4 shale faces - for example if there is stratigraphic growth across the fault and these throws can differ from the fault displacement of the vertex, calculated from the Throw in the hangingwall and the the Throw in the footwall. These displacements are functions of the vertex being processed, rather than of the shale layers that have passed this vertex.

By default, TransGen reports the FSP measures in 3D, by measuring all distances, thicknesses and throws parallel to the COORD lines bounding the faulted connection faces as shown in Figures (a) and (b) above. TransGen assumes implicitly, therefore, that the displacement vector of the fault is parallel to the COORD lines used to construct the Three dimensional model geometry.

By changing the Plunge correction option to Strike projection, the FSP measures can be reported on a vertical projection of the fault. The definition of throw (D), thickness (t) and distance (d) terms in the FSP calculation using this option are shown in Figure (c) above. However this option is NOT available for Fault displacement which is always reported in 3D. If desired, the vertical component of fault displacement can be calculated in a thickness plugin.

NOTE:- By default, all measures of fault seal potential (FSP) are made parallel to the COORD lines bounding the faulted connection faces. Optionally, by changing the Plunge correction setting to Strike Projection on the Fault Seal Potential Variables page of WizGen in "Flexible project" mode, the FSP measures can be reported on the vertical projection(s) of fault(s).

Calculation of Shale Gouge Ratio (SGR)

Shale Gouge Ratio (SGR) provides a measure of the proportion of shaly material in the fault zone, which in sand-shale sequences is the first-order control on fault-zone permeability.  SGR is calculated from the net shale content of the grid blocks that have slipped past that point on the fault (see Yielding et al, 1997, AAPG Bull., 81, 897-917).A high SGR is expected to correspond to more phyllosilicates in the fault zone and therefore to a greater seal potential.

An average SGR is calculated for each faulted cell to cell connection.  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.

If the displacement of one or more of the connection vertices is zero, then the effective Vshale values of the blocks are used as the SGR values of the vertex.

The following diagram shows the SGR calculation.  Vcl is the Effective Vshale in each layer.

NOTE:- The Effective Vshale is calculated in TransGen as described above.

When using TransGen in "Basic project" mode, the user can choose to calculate fault permeability as a function of the following Fault Seal Potential measures:- Only SGR, Only CSP or CSP then SGR using effective vshales of the faulted grid-blocks calculated from the input data associated with either NTG and/or TGVS keyword(s). The TGFSP keyword with the FSP definitions to calculate SGR and/or CSP are automatically added to the TGDATA run file when the WizGen settings are saved.
For example, when the CSP then SGR option is selected via the Fault Rock Properties page of WizGen, the following TGFSP settings are added to the TGDATA run file.

TGFSP
'csp' 0 2 -1 0 2 '1.0' 2 2 1 '' 1 /
'sgr' -1 1 0 1 4 '' 1 6 3 '' 1 /
/

The equation constants in the generalised FSP equation (as described above) are automatically set to calculate the SGR and/or CSP, with SGR calculated as shown above from throw, fault-rock thickness and effective Vshale. The calculation includes all currently active grid-cells with the SGR calculated at the centre of the beds in both the footwall and hangingwall and then averaged.

When using TransGen in "Flexible project" mode, the user can choose to calculate fault permeability as a function of up to 5 Fault Seal Potential (FSP) measures calculated in a single TransGen run, with far greater flexibility in how the individual FSPs are calculated (see setting the Fault Seal Potential Variables page in Using WizGen in Flexible project mode for details).

Calculation of Clay Smear Potential (CSP)

Clay Smear Potential (CSP) provides a measure of clay smear (non-lithified ductile shear-type smears) from shale beds in the fault zone and its variation with source bed thickness and fault displacement. CSP increases with shale source bed thickness, but decreases with increasing distance from the source bed (as throw increases, the shale incorported into the fault zone becomes thinner). The thicker the Clay Smear, the greater the fault seal potential. This measure of fault seal potential can then be used on its own or with other FSPs to calculate the fault-zone permeability.  CSP is calculated from the net shale content of the grid blocks that have slipped past that point on the fault (see Yielding et al, 1997, AAPG Bull., 81, 897-917).

Using the Clay Smear Potential (see Yielding et al, 1997, AAPG Bull., 81, 897-917) calculation method for each clay smear, the distance is measured to the mid-point of the clay bed on both sides of the fault.  The thickness of the footwall (upthrown) and hangingwall (downthrown) clay is also measured.  The value of (thickness squared) / distance is then calculated for the footwall and hangingwall portions of the smear: the larger value is used (usually this corresponds to using the smear from the nearest half of the clay bed).

The following diagram shows the CSP calculation.

When using TransGen in "Basic project" mode, the user can choose to calculate fault permeability as a function of Only SGR, Only CSP or CSP then SGR using effective vshales of the faulted grid-blocks calculated from the input data associated with either NTG and/or TGVS keyword(s). The TGFSP keyword with the FSP definitions to calculate SGR and/or CSP are automatically added to the TGDATA run file when the WizGen settings are saved.
For example, when the CSP then SGR option is selected via the Fault Rock Properties page of WizGen, the following TGFSP settings are added to the TGDATA run file.

TGFSP
'csp' 0 2 -1 0 2 '1.0' 2 2 1 '' 1 /
'sgr' -1 1 0 1 4 '' 1 6 3 '' 1 /
/

In "Basic project" mode, the equation constants in the generalised FSP equation (as described above) are automatically set to calculate the SGR and/or CSP, with CSP calculated as shown above from fault-rock thickness and distance, including data as set by the CSP options on the Fault Rock Properties page of WizGen.

When using TransGen in "Flexible project" mode, the user can choose to calculate fault permeability as a function of up to 5 Fault Seal Potential (FSP) measures calculated in a single TransGen run with far greater flexibility in how the individual FSPs are calculated (see setting the Fault Seal Potential Variables page in Using WizGen in Flexible project mode for details).

Fault Permeability Calculation

Fault permeability is calculated (at each connection vertex) from one or more Fault Seal Potential measures (e.g. SGR and/or CSP in a "Basic" TransGen project) using the code as set in the PERM plugin:-

When using TransGen in "Basic project" mode, the PERM plugin is automatically generated using the settings on the Fault Rock Properties page of WizGen The plugin is stored, when the current WizGen settings are saved, as _AUTO_PERM_PLUGIN.cpp in the <project_name>_INPUT/.plugins directory - see Plugins generated by WizGen in Basic project mode).

When using TransGen in "Flexible project" mode, the code to define this relationship has to be input by the user via the User-defined plugins page of WizGen.

For each connection, the area-weighted arithmetic average permeability is calculated from the permeability values at each connection vertex.

For example, calculating the permeability from Only SGR values uses the relationship:

where D(f) and SGR are the displacement and shale gouge ratio of the vertex.  The parameters a, b, c, d e and f are constants specified in the PERM plugin.  This relationship is based on a compilation of data by Manzocchi et al (1999), but with the addition of parameter d, which allows the sensitivity of permeability to displacement to be varied at low shale fractions:

Unfaulted Transmissibility Calculations

The transmissibility calculations are based on the NEWTRAN Eclipse calculation. The X direction unfaulted transmissibility (taken to mean the transmissibility including the fault juxtaposition, but NOT any fault-rock thickness) between two connecting blocks i and j is given by the expression:

where

A(x), A(y) and A(z) are the projections normal to X, Y and Z of the intersection area between the two cells, and D(ix), D(iy) and D(iz) are the X, Y and Z components of the distance between the centre of cell i and the centre of the face of cell i which forms a connection with cell j.  PERMX(i), NTG(i) and MULTX(i) are the permeability, net/gross ratio of cell i, and grid-block transmissibility multiplier of cell i,and CDARCY is Darcy's constant (0.008527 in metric units, 0.001127 in field units and 3.6 in lab units).

The expression for  T(j) is analogous to T(i).

CDARCY is included in the calculation for compatibility with the Eclipse simulator, and results output in the transmissibility files TRANX and TRANY are dimensioned according to Eclipse conventions. See the section on Transmissibility Units for discussion about units and dimensions used for input, output and calculations in TransGen.

NOTE:- New to TransGen version 3, the Eclipse grid-block based multipliers MULTX and MULTY can be included in TransGen in the same way as any other grid-block property (e.g. PERMX and PERMY). If they are not included, the Unfaulted Transmissibilities are calculated as in TransGen version 2, i.e.:-

See section on Including Eclipse Transmissibility Multipliers in TransGen for a discussion on the influence of these on faulted connections.

Faulted Transmissibility Calculations

The transmissibility between the two blocks separated by a thickness of fault-rock is given by  the expression:

where

D(fx), D(fy) and D(fz) are the X, Y and Z components of the fault thickness, and PERM(f) is the fault permeability.  T(j) is analogous to T(i).

NOTE:- New to TransGen version 3, the Eclipse grid-block based multipliers MULTX and MULTY can be included in TransGen in the same way as any other grid-block property (e.g. PERMX and PERMY). If they are not included, the Faulted Transmissibilities are calculated as in TransGen version 2, i.e.:-

See section on Including Eclipse Transmissibility Multipliers in TransGen for a discussion on the influence of these on faulted connections.

Introduction of the AREA term

Including the AREA plugin allows modification of the Area term used in the equation for faulted transmissibility. The equations for both faulted and unfaulted transmissibility (see above) are calculated in 3D, as a function of X, Y and Z components of the connection area. The 2D calculation of the connection area made in TransGen is therefore not included directly in these calculations.

The basic transmissibility equations in TransGen will continue to be based on the fully 3D calculations of faulted and unfaulted transmissibility as shown above. The faulted transmissibility, however, is then multiplied by the ratio:-

where Area(NF) is the area of the 2D projection of the connection area as calculated at present, and Area(F) is the area of the 2D projection of the connection after it has been modified in the AREA plugin.

The unfaulted transmissibility is unchanged.

Transmissibility Multiplier calculation

Once unfaulted transmissibilities (excluding fault-rock effects) and faulted transmissibilities (including fault-rock effects) have been calculated, the transmissibility multiplier is given simply by the ratio of faulted to unfaulted transmissibilities:-

The calculated Transmissibility Multiplier values will range between 0 and 1 with:-

low values (-> 0) occurring where the fault-zone is thick (high displacement), has low permeability (high % clay) and the adjacent reservoirs have high permeability.

high values (-> 1) occurring where the fault-zone is thin (low displacement), has high permeability (low % clay) and then adjacent reservoirs have low permeability.

The Y directional transmissibility and transmissibility multiplier expressions are analogous to the X directional ones described above.

Output - simulator input

Eclipse discriminates between two types of connection: neighbour connections and non-neighbour connections.  A faulted connection is a neighbour connection if it occurs between two cells which would have been connected if no fault were present, while a non-neighbour connection is formed by across-fault juxtaposition of stratigraphically disconnected grid-blocks.

The calculations of  TRANX(iNF), TRANY(iNF), TRANX(iF), TRANY(iF), TMULTX and TMULTY are identical for both neighbour and non-neighbour connections, but the output from TransGen is different for the two connection types, as they are treated differently by the Eclipse simulator.

The purpose of TransGen is to replace TRANX(iNF) and TRANY(iNF), which are used by Eclipse and used to calculated flow between grid-blocks, with TRANX(iF) and TRANY(iF), which include the fault rock-properties.  The "classic" way of doing this is by including TMULTX and TMULTY as transmissibility multipliers which would act on TRANX(iNF) and TRANY(iNF) to provide TRANX(iF) and TRANY(iF).  

This is not possible through the MULTFLT Eclipse keyword, as this does not allow an independent definition of the multiplier for each neighbour or non-neighbour connection.  The MULTFLT keyword (which is associated with the FAULTS keyword) assigns fault transmissibility multipliers to grid-block fault-faces, rather than to grid-block to grid-block connections.  Hence the same transmissibility multiplier is used on every connection across the grid-block fault face, rather than defined individually for each connection.  (Each grid-block fault-face may connect with more than one grid-block across the fault.)

TransGen output is constrained by Eclipse input, but a full definition of TRANX(iF) and TRANY(iF) is possible by using the EDITNNC  keyword for faulted non-neighbour connections and the TRANX and TRANY keywords for faulted neighbour connections.

The EDITNNC output from TransGen contains TMULTX and TMULTY for all faulted non-neighbour connections.  This file should be included in the EDIT section of the Eclipse DATA run file, where they will be used in conjunction with TRANX(iNF) and TRANY(iNF) to produce TRANX(iF) and TRANY(iF).

The TRANX and TRANY output consists of a file containing TRANX(iF) and TRANY(iF) values for each faulted neighbour connection. These files should be included in the EDIT section of the Eclipse DATA run file, in which case they will replace the TRANX(iNF) and TRANY(iNF) values calculated by the simulator.

Note:- Equivalent output options for inclusion in a Roxar More (Modular Oil Reservoir Evaluation) simulator model or to output a file suitable to update a Shell MoReS simulator model are also available on the Output - simulator input page of WizGen.

See section below for details on the Transmissibility Units.