FORGAS Integration
Previously, if any well had any change (e.g. ALTER), even a non-FORGAS well, then all FORGAS wells were reset for the following timestep to the current IMEX specification, over-writing the FORGAS values. Now, even if a FORGAS well is ALTER’d, except for the time where it is initially defined, the *ALTER will be ignored and the value from FORGAS will be used.
The IMEX calculated values for a and b for the IPR calculation for FORGAS are now based, for wells perf’d in more than one grid block, on the surface mobility weighted average delta P’s (or delta P squared for quadratic pressure) rather than the value at the reference layer.
Also, the reservoir pressure passed to FORGAS was formerly the reservoir pressure in the reference layer. Now it is the surface mobility weighted average of (reservoir pressure - head). This new approach, in some cases, significantly reduces oscillations in rates.
NEW KEYWORDS AND OPTIONS ADDED TO IMEX 2000.10 Input/Output Control
Sectors are now viewable in RESULTS. When probing a block in RESULTS, the sectors the block is in will be displayed. In addition, It is possible to select viewable regions by sector in both 2D and 3D views.
Command Line Options:
Command Line Options:
IMEX has the new command line option
-log Directs screen input to a log file which has the root file name of the data set and the extension ".log". Screen input is read from a file which has the root name of the data set and the extension ".in". If a file name has not been defined using the –f command line option and the –log option is used, it can be entered in the ".in" file. In addition, the user *INTERRUPT *INTERACTIVE choice is also entered in the ".in" file when the –log option is used. A carriage return in the ".in" file where the simulator is expecting to read the interrupt option defaults to *RESTART-STOP.
*NDBETCR, *WSRFTN, *GSRFTN, *CRSETN, *SWCON, *SWCRIT, *SOIRW,
*SORW, *SGCON, *SGCRIT, *SLCON, *SOIRG, *SORG
IMEX has added the following variables to the res_list for both *OUTPRN
*RES and *OUTSRF *RES. *NDBETCR displays the non-Darcy correction factor, *WSRFTN and *GSRFTN display the water-oil and gas-liquid surface tension arrays respectively. *CRSETN displays the compaction rock type of each block. The remaining keywords display water-oil and gas-liquid relative permeability table end point arrays.
User's Guide IMEX Introduction • 33
*PCOW, *PCOG, *RFG, *SWCON, *SWCRIT, *SOIRW, *SORW, *SGCON, *SGCRIT,
*SLCON, *SOIRG, *SORG
IMEX has added the following variables to the grid_list for both *OUTPRN
*GRID and OUTSRF *GRID. *PCOW and PCOG display the capillary pressures of each block, *RFG displays the non-Darcy flow reduction factor.
The remaining keywords display water-oil and gas-liquid relative permeability table end point arrays.
*WELL-SECTOR
This keyword specifies that well in sector information will be printed out.
Well in sector information consists of two tables. The first is ordered by sectors (sector table) and lists all active wells within a sector (an active well has at least one perforation open). The second table is ordered by well (well table) and lists all sectors associated with each active perforation of every well.
Wells which have all perforations closed or have not had any perforations defined will not be listed in either table. Individual perforations that are no longer active will not be listed in the well table. Perforations which have had their production/injection zeroed due to use of the *XFLOW-MODEL
*ZERO-FLOW keyword will not be listed in the well table for as long as their production/injection is zeroed. Active perforations of wells that are
“shut in” will be listed in the well table, but “shut in” wells are not listed in the sector table.
The table information is printed out every timestep to a separate file with extension “sct”, the file will be created if when scanning the data IMEX finds the *OUTPRN *WELL-SECTOR keywords. The amount of printout can be controlled by using the *NONE sub-keyword in recurrent data.
Reservoir Description
Sector calculations in some instances were taking excessive amount of run-time. This was due to an unfortunate choice of algorithm for sector lookup during sector calculations. This algorithm was revised, resulting in remarkable improvements in run-time. For example, on a field problem with 159 sectors, total run-time was reduced by a factor of 2.56 with an increase in storage allocated of only 0.4%.
The compaction option has been revised to remove several limitations. Compaction can be used with dual porosity and dual permeability models. In addition the following changes were made:
1. When values for the third column in a rock table are blank or negative, horizontal permeability multipliers are defaulted to one.
2. When values of the fourth column in a rock table are blank or negative, vertical permeability multipliers are defaulted to be the same as the corresponding horizontal permeability multipliers.
34 • Introduction User's Guide IMEX
3. When a value of a permeability multiplier is greater than or equal to zero and less than 1.0E-04, it is replaced by a value of 1.0E-04.
4. When a value of a porosity multiplier is less than 1.0E-02, it will be replaced by a value of 1.0E-02.Small values of porosity may result in poor performance of the simulator.
5. If there is only one rock type and there is no keyword *CTYPE used, all the grid blocks are assigned to have that rock type.
6. There is no longer a limit to the number of compaction rock types which may be used in a data set.
7. There is no longer a limit to the number of rows in a table permitted under the
*CROCKTAB or *CROCKTABH keywords.
8. There is no limitation in the number of hysteresis curves for a given rock type.
However, the first row of each sub-table under the keyword *CROCKTABH must be identical to the corresponding row on the main path from which the hysteresis path starts as discussed in the user manual.
9. The *CROCKTYPE region definitions may exist for region numbers not specified for any given block using the *CTYPE keyword. However, if a block is set to a particular region type number using *CTYPE, then there must exist a
corresponding *CROCKTYPE definition.
10. The compaction rock region types (input using *CTYPE) may now be output using the sub keyword *CRSETN with *OUTPRN *RES and/or *OUTSRF *RES.
A dilation option was added, see the Reservoir Description section and the Tutorial section for further information.
A pseudo-capillary pressure option was added to the dual porosity and dual permeability option. See the keyword *TRANSFER for a complete description of the option.
The *PINCHOUTARRAY keyword was added to the reservoir description section. This allows for an alternative method of pinchout definition.
Component Properties
The use of surface tension input on *PVT tables has been extended to work with multiple PVT regions. The surface tension is used to correct Pcog with respect to pressure. A single reference pressure is input for the surface tension correction, but this reference pressure
*REFPST is used to calculate a separate reference surface tension for each table.
The use of oil compressibility (Co) tables for oil above the bubble point at any pressure has been supported in IMEX for some time. However, the conversion of Bo’s above the bubble point to Co’s is not trivial as IMEX replaces the original Co’s with integrated averages. We have added the ability of automatically converting Bo’s to Co’s to ModelBuilder. The user may now use ModelBuilder and enter either Bo’s or Co’s above the bubble point.
User's Guide IMEX Introduction • 35
Rock-Fluid Model
*SWCON, *SWCRIT, *SOIRW, *SORW, *SGCON, *SGCRIT, *SLCON, *SORG The end point scaling option had been completely rewritten and redesigned.
Instead of the four scaling points: 1) connate water (*SWCON previously
*SWC), 2) critical gas (*SGCRIT previously *SGC), 3) residual oil in the oil-water system (*SORW), and 4) residual oil in gas-liquid system (*SORG). We now scale with respect to:
Connate Water Saturation (*SWCON)
Critical Water Saturation (*SWCRIT)
Connate Gas Saturation (*SGCON)
Critical Gas Saturation (*SGCRIT)
Irreducible Oil Saturation (oil-water system) (*SOIRW) Residual Oil Saturation (oil-water system) (*SORW) Connate Liquid Saturation (gas-liquid system) (*SLCON) Residual Oil Saturation (gas-liquid system) (*SORG)
There is a complete description of the new option in the Rock-Fluid section under the keyword *SWCON.
Use of the new scaling produces more consistent scaling of the relative permeability tables and initialization.
The use of the new end point scaling option may alter previous results. For the sake of compatibility, we have allowed the older four point end point scaling method to be used (see *RPT *SCALING-OLD).
A Leverett J Function option has been implemented into IMEX. This option allows the user to replace water-oil and/or gas-liquid capillary pressures with Leverett J functions. Capillary pressures are calculated from the J function tables, input surface tension, and account for the varying permeability and porosity of each block. In addition the effect of pressure on Pcog can be accounted for by entering surface tension as a function of pressure on PVT tables. See the keyword *JFUNC, *SRFTNW, *SRFTNG, *PVT, *REFPST for details on the use of this option.
Non-Darcy gas flow in the reservoir can now be modeled in addition to non-Darcy flow into the well (see *TURB on the *PERF card). The non-non-Darcy reservoir gas flow model uses the Forchheimer equation with the
Forchheimer number calculated using any of 1) Geertsma’s 1974 correlation (*NONDARCY *GEERTSMA), 2) Frederick and Graves first correlation (*NONDARCY *FG1) and 3) Frederick and Graves second correlation (*NONDARCY *FG2). The Forchheimer number for gas can be further modified by the scaling parameter defined on the *NDARCYCOR keyword.
See *NONDARCY and *NDARCYCOR for more details.
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Initialization
The Initialization section has no obvious changes, but the new end point scaling option and the Leverett J Function option both can alter initialization results.
Well and Recurrent Data
IMEX now allows the user to perforate null blocks. Previous versions of IMEX stopped and reported errors when perforations went though null blocks. This new feature allows perforations in null blocks, the perforations are closed automatically (*NULL-PREF). This feature is on by default.
IMEX allows the user to enter the layer X, Y and Z coordinates of the
entrance and exit location of a well perforation running through a grid block.
With this information IMEX will calculate an accurate deviated well productivity/injectivity index (*LAYERXYZ).
The *KRPERF keyword allows the user more flexible control of perforation relative permeability arrays.
The *GCOMP and *GCONM keywords now allow the user to shut all wells in a group when group constraints are violated.
Well reference layers can be defined anywhere in a branching well.
The *PERF keyword now can handle all refined grid situations including hybrid grids. The *PERFRG, *PERFVHY and *PERFHY keywords are valid but no longer required.
The *ON-TIME information of each well is now written to the SR2 file system. This now allows RESULTS to display instantaneous rates as well as average rates.
The *FULLY-MIXED wellbore crossflow model is now the default for all wells.
User's Guide IMEX Introduction • 37