Introduction
The Define simulation case process allows the engineer to pull together the 3D grid with the fluid and rock physics functions to initialize the model, that is, to compute initial saturations, pressure and transmissibilities. The process is also used to select which simulator to run, and which development strategy to use.
Define simulation case
Grid properties
Porosity and Permeability
is set on the Grid tab of the Define simulation case
process.
Porosity and Permeability is automatically inserted on the
Grid tab when you select a
3D grid.
Additional Grid Properties
For example, insert a water saturation to apply end point scaling.
• Add a row by clicking • Select template • Drop in the 3D property
In addition to permeability and porosity (required input), you can drop in other properties that are defined in the 3D grid. For example, you can select to use a local grid set, fault transmissibility multiplier, or endpoint scaling for saturation functions.
Three ways of Initializing
Enumeration (Black oil)
The user inserts initial saturation and pressure as 3D grid properties on the
Grid tab. Equilibration
The simulator computes initial saturation and pressure using the Fluid model and the Saturation function that is inserted on the Functions tab.
Restart
Initial saturation and pressure is read from a restart file.
Initial gas, oil, and water pressure distribution and initial saturation distributions must be defined in the reservoir model. Pressure data is usually given with reference to a datum depth. In Petrel, the datum depth is mean sea level by default.
There are three initialization options:
Equilibration – Initial phase pressures and saturation are
computed by the simulator, using the fluid model. The equilibration facility is a means of calculating the initial conditions on the basis of hydrostatic equilibrium. If necessary, the reservoir can be divided into separate “equilibration regions” in which hydrostatic equilibrium exists independently of the other regions. The number of equilibration regions is specified in the Make fluid model process. Within each
equilibration region, all grid blocks must use the same pressure table for their PVT properties, but they can use different rock physics functions tables as specified in the Make rock
physics functions process.
Enumeration – The initial value of pressure, saturation, and
bubble point pressure is set explicitly in each grid block by the user. The 3D grid property must be dropped into the Grid tab of the Define simulation case process.
Restart – The initial conditions are read from a restart file of a
previous run.
Functions
Black oil (PVT)
Left-click the Black oil fluid model (PVT).
Drop in the initial condition of the fluid model.
Deselect Initialize by equilibration if you use
Functions
Black oil regions
Different fluid models can be assigned to different regions.
Any discrete property (such as segments) can be used as a region index property.
Fluid model: Spatial variations
Initially, the fluid model in reservoirs may vary vertically and aerially. This can be modeled by assigning separate fluid models to separate regions of the model. It is important to understand how the fluid model is used in a simulator with fluid movement. When fluid moves from one region to another, the simulator will change the fluid model used, such as the viscosity and formation volume factor, as the fluids cross into the separate regions. Unless mixing between different oils has occurred or the pressure has changed, the fluid properties will remain the same as they move through the 3D grid.
Functions
Rock physics
Drop in:
• Relative permeability curves • Rock compaction function Select Region index property
to assign different functions to different rock types.
Initialize by equilibration
Given the fluid densities, the equilibration procedure sets up saturation against depth curves so that in the transition zone, when two phases are mobile, the hydrostatic pressure variation is balanced by the capillary pressure between the phases.
Separate rock physics functions should be used for each significant rock type.
Equilibration
Compute phase pressures
The contacts in the Fluid model
are used.
Phase pressure is computed using the fluid density as input.
Within each equilibration region, the calculation is performed in two stages. In the first stage, an internal table of phase pressures and Rs and Rv against depth is set up. In the second stage, the fluid conditions in each grid block in the region is computed using interpolated values from the table.
Equilibration
Saturation in the water, oil, and gas zone
Oil zone: So = 1-Swmin Gas zone: Sg = 1-Swmin
In the second stage of the equilibration calculation, the local fluid conditions are determined in each grid block in the equilibration region. The internal table is interpolated to obtain the values of Po, Pw, Pg, Rs and Rv at the grid block center depth. The water saturation is
Equilibration
Saturation in the transition zones
• Calculate Pcog and Pcow in the transition zone. • Find Sg and Sw by
inverse lookup from the capillary pressure curves.
Strategies
TheStrategies tab can be left
blank when initializing the model. Development strategies and Well segmentation will be described later in the course.
You can use a global permeability log to compute the KH term of the connection transmissibility factor. For every well, Petrel will check if it has a permeability log that will be used for computing KH. Otherwise, the grid properties will be used as usual.’
Results
3D initial properties
Request to write out initial saturation and pressure. Non standard 3D properties can be added using Additional Properties (e.g., bubble point
for each cell).
Advanced
Select grid type
The Editor gives access to all
simulator keywords including those not supported from Petrel.
Select:
• Grid export type (OPF default) • Simulator version
A choice of formats for exporting grid data from the Models pane has always been available in Petrel. However, you can also select these formats from the Advanced tab in the Define simulation case process.
The considerations in choosing which format to use are:
Pillar geometry: Petrel supports vertical, straight, listric and
curved pillar geometry. Several of the export formats only support vertical and straight pillars. If your grid contains curved or listric pillars and you use a format that does not support it, the grid will be distorted on export. Check this in the
Statistics tab of the grid settings.
ASCII or Binary: This is important if you are planning to edit the
data outside Petrel. In that case, you need to export the data in ASCII format.
Properties: Some of the formats do not include the properties on
export. In that case, you need to export them separately.
Compatibility: Not all formats are supported by all simulators or
third party applications. Check the available formats in the other application before exporting.
File size: Some of the formats provide a higher efficiency
compared to others, resulting in much smaller files. This will minimize both disc space requirements and the time needed to move files between systems (if you are running remotely). By default, the OPF (Open Petrel Format) format is used for all simulation runs. It supports curved and listric pillars, is binary and includes the properties. Note that ECLIPSE has to be 2005a version or later in order to read this format.
Advanced
Transmissibilities tab
Transmissibility and pore volume multipliers are written to the simulator deck.
Use the Transmissibilities tab to:
• Set a minimum pore volume • Compute transmissibility in Petrel
and export to the simulator
Typically, the individual simulators have calculated the pore volume of cells, fluid transmissibility and thermal conductivity between them. These are straightforward calculations, dependent upon the grid geometry and properties and need only be calculated once prior to the first simulation step. The advantage of sending these to the simulator is that the set of connections and their strengths will be the same
whichever simulator is being used. The transmissibility and pore volume values are sent from Petrel to the simulator as keywords or inside the GSG file when the option Calculate and export transm. and pore
Exercises – Initialization of the model
The next exercises show you how to initialize the model.
Exercise Workflow
• Initialize the model
Exercise Data
In the following exercises, we will continue with the project we have made earlier. Alternatively, you may load the project Mod_3_
Completed.pet from Backup Projects folder.
Initialize model
In this exercise, we will initialize the 3D model with fluid and rock physics functions. We will leave the development strategy field empty.
Exercise steps
1. In the Processes pane, expand the Simulation folder and open the Define simulation case process.
2. Select Create new case and name the new case Simple. 3. Make sure the Simulator is set to ECLIPSE 100, and the Grid
is set to Simple.
4. In the Grid tab, deselect the check boxes in front of
Permeability and Porosity.
5. Enter a permeability of 1000 in the I- and the J- direction and of 50 in the K-direction.
7. Go to the Functions tab and select the Drainage relative
permeabilities in the left panel by left-clicking it.
8. Drop in the saturation function Saturation function 1 that you imported in the Input pane (by selecting it) and clicking the blue arrow in the dialog.
9. Still in the Functions tab, select the Black oil fluid model from the list in the left panel.
10. Ensure that the Initialize by equilibration option is selected. 11. Drop in the Initial condition 1 of the black oil model, Light
oil + gas, that you created earlier. As you only have one
region, there is no reason to use a region index property. 12. Select the Rock compaction function from the list in the left
panel. Drop in the Rock compaction function 1 from the
Rock physics functions folder in the Input pane.
13. The Strategies tab should be left empty as you are only initializing. If there is an empty row, delete it by selecting the row and clicking the Delete selected row(s) in table button 14. Click Apply to save the case. The case is saved to the Cases
pane.
15. Click Run. ECLIPSE 100 is launched. Wait for the initialization of the case to finish.
Once the run is finished, go to the Cases pane, right-click on the simulation case (ECLIPSE 100) and select Show print file. This will open the print file in your default text editor.
At the bottom of this file, you will find the initial fluid in place report. You can view the initial properties that are stored in the Simulation
grid results folder on the Results pane. The next lesson explains how