Making Rock Physics Functions
There are a number of functions of saturation or pressure used in simulation that
represent the physics of the fluids, the rock, or the interaction between rock and fluids.
The Make fluid model process creates the functions that represent the physics of the
fluids. The Make rock physics functions process is used to create functions that represent
the physics of the rock and the interaction between rock and fluids, allowing users to
make saturation functions and rock compaction functions. In future releases of Petrel the
range of rock physics functions available within this process will be extended.
The Make rock physics functions process incorporates and replaces the Make saturation
functions process from earlier versions of Petrel.
Making rock physics functions
Rock physics functions are shown in the Input pane in the Rock physics functions
folder. The different types of functions have different icons.
Saturation functions are tables showing relative permeability and capillary pressure
versus saturation. These tables are used to calculate:
the initial saturation for each phase in each cell;
the initial transition zone saturation of each phase;
fluid mobility to solve the flow equations.
Creating saturation functions using the Make rock physics functions process will also
generate curves for gas-oil and water-oil capillary pressure versus saturation. These are
set to zero by default. The relative permeability and capillary pressure curves are grouped
together under a saturation function icon in the Rock physics functions folder.
Rock compaction functions are tables showing pore volume multipliers versus pressure,
or a single rock compressibility value used by the simulator to calculate the pore volume
change. Their icon in the Rock physics functions folder is. Creating rock compaction
functions using the Make rock physics functions process will also create a
transmissibility multiplier versus pressure curves. These are set to zero by default.
Make rock physics function process steps
In Petrel you can add new rock physics functions, and update or delete existing rock
physics functions. You can also manipulate the settings for a rock physics function,
together with details for their component tables; for example, capillary pressure curves
within a saturation function. Once created the Rock physics function is placed in the
Rock physics functions folder on the Input pane. The object settings are described on
their settings pages. The contents of these will depend on what type of function you are
examining.
How to make a saturation function from Corey correlations
To create a new saturation function by using Corey correlations follow the steps below:
1.
In the Processes pane, open Simulation and select Make rock physics function.
This opens the Make rock physics functions dialog.
2. Select Saturation Function in the Select function drop-down menu.
3.
Select Create new function and enter the name for the new saturation function into
the adjacent field.
4. Select the phases for the function by checking the appropriate boxes.
5.
In the Table entries field enter the total number of points that will be used to create
the relative permeability and capillary pressure saturation function tables.
6.
If you know the values for relevant saturation end points and Corey coefficients,
enter them into the appropriate fields.
7.
If you do not have parameter values, click on Use defaults and choose the rock
characteristics. Default values will be written into the parameter fields.
8. Click on OK or Apply to create the new saturation function.
How to make a saturation and/or rock compaction function from ECLIPSE
keywords
You can create a new saturation function or rock compaction function from a file
containing the relevant ECLIPSE keywords:
1. Petrel assumes that the units in the keywords file are the same as the simulation unit
system (as specified in the Project | Project settings… | Units and coordinates menu).
Make sure that the units are set correctly (you can change the simulation units system
back again afterwards, if necessary).
2. If you do not already have a Rock physics functions folder in the Input pane, create
one by selecting Insert | New rock physics folder.
3. Right-click on the Rock physics functions folder and select Import (on selection).
4. Select your ECLIPSE keywords file from the Import file dialog and click on Open.
5. A number of saturation functions and rock compaction functions are created in the
Rock physics folder that corresponds to the keywords in the file (if present). A report on
the conversion of the ECLIPSE keywords is displayed at the end of the message log.
How to make a rock compaction function from correlations
To create a new rock compaction function by using a correlation follow the steps below:
1.
In the Processes pane, open Simulation and select Make rock physics functions.
This opens the Make rock physics functions dialog.
2. Select Rock Compaction Function in the Select function drop-down menu.
3.
Select Create new function and enter the name for the new rock compaction
function into the adjacent field.
4.
In the Table entries field enter the total number of points that will be used to create
the pore volume multiplier pressure function tables.
5. Select a correlation in the Correlation drop-down menu, for example Hall correlation.
6.
Select the rock type in the Rock type drop-down menu, for example Consolidated
sandstone.
7.
Type in the average reservoir porosity into the Porosity field. Or check the check
box beside it and drop a porosity property for your grid into the Porosity field.
8.
Then enter the values for the remaining parameters into the appropriate fields: for
the Hall correlation these are the reference, minimum and maximum pressure and
overburden gradient.
9.
If you do not have parameter values, click on Use defaults and select the rock type.
Default values are written into the parameter fields.
10. Click on OK or Apply to create the new rock compaction function.
How to update an existing rock physics function
If you have a rock physics function that was created in the Make rock physics function
process, follow the steps below:
1.
In the Processes, open Simulation and select Make rock physics functions. This
will open the Make rock physics functions dialog.
2. Select the type of function in the Select function drop-down menu.
3. Select Edit existing function data and choose a function from the drop-down list.
4. Change the function parameters as required.
5. Click on OK or Apply to update the saturation function.
How to manipulate function details
You can view and manipulate the details of the individual tables within a rock physics
function, that is of relative permeability and capillary pressure functions within saturation
functions and of pore volume multiplier and transmissibility multiplier functions within
rock compaction curves
1.
Right-click on the Rock physics functions folder in the Input pane and select
Spreadsheet from the menu. You can select a function from the drop-down list and
then modify the tables from here. You can also right-click on individual saturation
or rock compaction functions or individual relative permeability or capillary
pressure functions within a saturation function to access the tables.
2.
Alternatively, you can open a Function window (select Window | New function
window) and display a rock physics function by checking the visualize box that
appears in front of its name. You can then graphically edit the curves by moving or
adding points to the graph in the Function window.
Make rock physics functions dialog settings
The following options are available:
Select function: Choose the type of function that you wish to create or edit (for example,
Saturation function).
Create new function: Choose this if you want to make a new function of the selected type.
Edit existing function: Choose this if you want to modify a function. You can then choose
the function from the drop-down list, which displays functions already created of the type
chosen.
The lower part of the dialog changes according to the type of function selected, to enable
you to provide the appropriate parameters for your function.
For saturation functions:
Phases: Select the check boxes corresponding to the phases present in the reservoir.
Use defaults: You can select rock characteristics from this drop-down list, and this will
apply typical values for the parameters in the dialog. Alternatively, you can enter your
own values for the parameters.
Table entries: Enter the number of points to be used to create the relative permeability
and capillary pressure saturation function plots. Higher numbers give more accuracy but
will cause simulations to run more slowly.
Phase parameters: Enter any known parameters for the three fluid phases. Note that any
parameters that do not apply to your selected phase combination are shown in gray and
you cannot enter values for these parameters. The parameters are:
For gas
Sgcr: Critical gas saturation.
Corey Gas: For values between Swmin and (1-Sorg):
where Swi is the initial water saturation
Cg is the Corey gas exponent
Krg@Swmin: Relative permeability of gas at Swmin (minimum water saturation) value
Krg@Sorg: Relative permeability of gas at the residual oil saturation value
For oil
Sorw: Residual oil saturation to water. Note that (1-Sorw) > Swcr
Sorg: Residual oil saturation to gas. Note that (1-Sorg) > Swcr
Corey O/W: For values between Swmin and (1-Sorw):
where Swi is the initial water saturation Co is the Corey oil exponent
Corey O/G: For values between Swmin and (1-Sorg):
where Swi is the initial water saturation
Co is the Corey oil exponent
Kro@Somax: Relative permeability of oil at the maximum value of oil saturation
For water
Swmin: Minimum water saturation
Swcr: Critical water saturation. This must be greater than or equal to Swmin (minimum
water saturation)
Corey Water: For values between Swcr and (1-Sorw):
where Cw is the Corey water exponent
Krw@Sorw: Relative permeability of water at the residual oil saturation value
Krw@S=1: Relative permeability of water at a saturation value of unity
For rock compaction functions:
Use defaults: You can select rock types from this drop-down list, and this will apply a
typical choice of correlation and values for the parameters in the dialog. Alternatively,
you can enter your own values for the parameters.
Table entries: Enter the number of points to be used to create the rock compaction table
function plots. Higher numbers give more accuracy but will cause simulations to run
more slowly. At export you will have the option to export the rock compaction behavior
(to some simulators) either as tables or as a single rock compressibility value.
Correlation: Select the correlation you wish to use to generate rock compaction tables
(pore volume multipliers). Imported as table correlation cannot be selected: it is used to
mark those rock compaction functions that have been created by importing the ECLIPSE
ROCKTAB keyword.
Rock type: Select the rock type from the available list; different correlations offer
different selections. The combination of correlation and rock type
determines the equation used to calculate the rock compressibility.
Compressibility: Rock compressibility Cr at the reference pressure. You provide this
value if you have selected User defined from the Correlation drop-down list, otherwise
this is the result calculated from the correlation’s equations and the other parameter
values (see below).
Porosity: Rock porosity. This can be specified as a single numerical value, or a porosity
property from a grid may be dropped in here. The mean value of porosity will be used in
the correlations. If a region property was selected, then the average porosity in each
region will be used for that region’s rock compaction function.
Region property: If there are integer valued properties in the same folder as a porosity
property dropped in above, then they will be listed in this drop-down list, which can be
activated using the check box. All cells with the same property value form a region of the
grid, and a separate rock compaction function is created for each region. Note that sets of
rock compaction functions created using a region property can only be edited together, as
a set of siblings.
Reservoir depth: The reservoir depth must be supplied if the rock porosity is specified as
a numerical value. If a porosity property is dropped in, then the depth will be calculated
as the mean depth of the grid from which the porosity comes. If a region property was
selected, then the average depth of the grid in each region will be used for that region’s
rock compaction function.
Reference pressure: The reference pressure applicable to your compressibility value.
Minimum and maximum pressure: Petrel uses these values as pressure bounds for the
pore volume multiplier versus pressure function (rock compaction function) when the
function is exported as a table or when it is visualized in the Function window.
Initial pressure: Initial reservoir pressure for Knaap correlations.
Over burden gradient: The overburden gradient for use in the Knaap correlations.