Making and Using Variograms in Petrel

Making and Using Variograms in Petrel

1. Creating a variogram from an object’s Settings panel

2. Creating a variogram in the Modeling dialogs.

3. Making a variogram in the Data Analysis dialogs

3.1 Preparing to make variograms for facies models

3.2 Preparing to make variograms for continuous properties

4. Modeling the variogram in Data Analysis

5. Summary

There are three locations from which you can create variograms in Petrel:

- In the Settings panel for any data or grid object

- From the Facies Modeling and Petrophysical Modeling dialogs

- From the Data Analysis function under Property Modeling in the Process Diagram

The most interactive and robust methodology resides in the Data Analysis menus, and the least

interactive method involves simply filling in the pertinent parameters, such as Range, Sill, and

Nugget in the modeling dialogs themselves. The Settings panel provides a semi-interactive

method for creating variograms, but does have the advantage that several variograms can be seen

on the same display at once, and they are retained as graphic entities.

1. Creating a variogram from an object’s Settings panel

With this method, you create a graphic display for a Computation window by predefining the

variogram parameters as we will see below. You have the choice of creating a variogram map

which will automatically reveal the direction and degree of anisotropy, or a horizontal or vertical

experimental variogram which you can then edit and turn into a variogarm model for use in

property modeling. In Petrel, experimental variograms are called sample variograms.

1. Highlight the object and right-click on it, selecting “Settings”

2. In Settings , stretch the window to the right until you can see the tab

“Variogram”, then click on it.

3. In this tab, look at the “Hints” tab, then note that you can set the transform type

for the variogram, its orientation parameters, as well as the lag and search

radius before the computation begins.

4. After the sample variogram has been created, it will show up under the Variogram folder

in the Petrel Explorer.

5. If you then create a new Function window, you can display it as below:

6. Use the Make Variogram icon to turn the experimental variogram into your own

variogram model by setting the parameters appropriately in the dialog which appears:

7. Each variogram model you create will appear in the data hierarchy under Variograms, as

well as graphically in the Function window.

2. Creating a Variogram in the modeling dialogs.

Here, we see the dialog for Petrophysical Modeling. The dialog for Facies Modeling looks pretty

much the same. In this mode of creating a variogram, we cannot see the data, the experimental

variogram, nor the variogram model. We can only fill in the parameters we decide upon.

3. Making a variogram in the Data Analysis dialogs

Here, we have the best control over our variogram, although there does not seem to be a way to

apply this tool to arbitrary data such as scatter points or grids. It appears only to work with the

well logs.

Preparing to Make Variograms for Facies Models

1. Click on Data Analysis in the Property Modeling section of the Process Diagram

2. Click on the Variograms tab.

3. Select an upscaled facies grid whose variogram you wish to compute.

4. Select the Zone you wish to work with (data outside of this zone will not be seen).

5. Unlock the parameters so they are visible

6. Click on “Use the Raw Logs”

7. If you have chosen facies as your property, select which classification you want to make

the variogram for.

8. Now model the variogram according the to the instructions in the section “

Note: Even though all the raw data for one discrete facies class will have the same value, the

resulting variogram will provide valuable spatial information about its spatial distribution in

3 dimensions for algorithms such as Sequential Indicator Simulation. These variograms

which are computed for discrete data are actually different than those for continuous data.

Preparing to Make Variograms for Continuous Properties

If you have chosen a continuous property for which to compute a variogram, first decide if you

are going to condition your property values to certain facies distributions. Sometimes you have no

facies model and you will not have this choice. If you have gone to the trouble of making a facies

model, and you have at least a reasonable number of wells, then it would probably make sense to

do this. In this case, what you want to do is make separate variograms of the continuous property

for each of the facies. For example, make a separate variogram of porosity as it exists within the

channel, and others as it exists within, say, the levee and the plain. For each facies class, only the

data which is located there will be used. There is also an option of making a variogram for one

facies class and using (or copying) it in another.

1 – 6. So, to continue in the case of a continuous property, perform steps 1 through 6 above,

but pick a property in step 3.

8. Click the Facies button under Zones to condition to the facies, if you have them. Select

the facies model you wish to use. It may contain more than one facies classification. If

you do not select Facies, data for the entire property will be used and you can skip step 9.

9. Pick the facies classification whose variogram you want to compute.

10. Compute the variogram as outlined in “4.

4. Modeling the Variogram in Data Analysis

Here, we’ll go through the mechanics of creating a variogram model. All these steps are relevant

whether we are designing a variogram for the entire upscaled property, or for only part of it

associated with a particular facies.

4.1 Create the Horizontal Variogram

4.1.1 Determine anisotropy by using the “Settings” method for computing a

variogram map

- Right click “Settings” on the upscaled property you want to variogram

- Stretch the window open so you can see and click the Variogram tab

- Choose Variogram Map and Execute.

4.1.2 Open a map window and display the variogram to see if anisotropy is

revealed. In the example below, there does not seem to be any. If there is,

you should see a symmetrical display with the axis of symmetry being the

major azimuth of anisotropy

4.1.3 Compute experimental major variogram – click the “major” tab

4.1.3.1 Optimize the shape of the variogram point distribution by interactive

movement of the lag/azimuth icon:

Click/drag here for LAG

Click/drag here for AZIMUTH

Click/drag here for SEARCH ANGLE

- As yet undefined variogram model

- Histogram showing how many variogram cloud pairs were averaged

to compute the variogram point

- Experimental variogram points

- Default variogram model

Optimization of experimental variogram shape, continued...

+ By varying the azimuth, verify the direction of anisotropy, if any, as revealed by

the variogram map above. Changing the azimuth to the expected major direction

of anisotropy should reveal a clearer classic variogram shape in that direction.

+ with sparse data, increase search range and search angle

+ experiment with lag distance , seeing if one size produces a clearer or more classic

variogram shape

+ note the histogram in the background of the variogram. Use it to decide if a

particular variogram point is relevant and should be included in the model.

+ many times the points cannot be seen clearly until you drag the bottom of the entire

Data Analysis window downwards.

4.1.4 Model major variogram

4.1.4.1 Decide on the model Type (Exponential, Spherical, ...)

4.1.4.2 Interactively drag the Nugget Point where you think it should be

4.1.4.3 Interactively drag the Range Point where you think it should be

4.1.4.4 Click Apply to save the major variogram

Petrel Variogram Components

4.1.5 Compute experimental minor variogram and model it, if anisotropic

4.1.5.1 This step is unnecessary if the major azimuth is 0.0. Otherwise, click on the

Minor tab, then follow the same shape -optiomization and modeling steps

as for the major variogram, but note that you’ll not be able to change the

minor azimuth; it will always be normal to the major azimuth.

4.1.5.2 Click Apply to save the minor variogram

4.2 Create the Vertical Variogram

4.2.1 After you click on the Vertical tab, the procedure is exactly the same for this

variogram as for the major one above.

4.2.2 Click Apply to save the vertical variogram

5.0 Summary

We see that we must specify one property and one zone before we begin creating variograms.

But within those constraints, we can create variograms based on only the upscaled well log data

residing in the “seed” property grids, or based on all of the original data in the logs used for the

upscaling, or in the case of a fully defined grid, we can compute a variogram based on all of the

values in the grid.

We also have the ability to isolate only part of the property data, based on where it is located

relative to a particular facies in a multi-facies model (3D grid), and to create a variogram based

on that subset of data.

Once the variogram is saved, you can always return to Data Analysis and modify it if you like.

Variograms made in this way are also available for use during property population. Make sure

that you click on the icon in the modeling dialog which tells the system to “Use Variograms

Created in Data Analysis”. Note that even though these variograms are available to the modeling

algorithms, neither the variogram, nor its parameters can be seen from there. You must return to

the Data Analysis dialogs to be able to see the variograms and their parameters.

The variogram you see in the property/facies modeling menus is the default variogram used for

the modeling unless you click on the icon mentioned above. It has no relation to the variograms

computed in Data Analysis.