If you have chosen a continuous property for which to compute a variogram, you must first decide if you are going to condition those property values to a specific facies distribution. If you have no facies model, you will not have this choice. [Q 1]
Conditioning petrophysical properties to a facies model
If you have gone to the trouble of making a facies model, and you have at least a reasonable number of wells, then “conditioning to facies”, makes sense as an option during the modeling step. Selection of this option causes the petrophysical data falling in each facies to be treated independently during modeling. For example, if porosity were the property being conditioned, and if the cell being evaluated had been classified as a fine sandstone in the facies model, then only those porosity values associated with fine sandstone would be used in the computation of porosity for that cell. This is called segregation of data values by facies. If the modeling algorithm we choose requires a variogram, then we must address the creation of variograms with respect to the facies distribution as discussed below.
A property variogram for each facies represented in the data
Because properties behave differently in different faces, if you choose the “condition to facies” option, you will need to make a separate variogram for the continuous property in each 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 in the variogram, and during the
modeling. [Q 3] While making a conditioned variogram for one facies, you have an option to copy it and associate it with another. That is, you can copy variogram definitions from one facies to another. [Q 2]
Below, you can see the mechanics of preparing to create variograms for continuous properties in the Petrel Data Analysis dialogs, with or without “conditioning”.
Schlumberger Private The mechanics for preparing for variogramming of continuous
properties in the Petrel Data Analysis dialog
1 – 6. To prepare for a continuous property, perform the same steps 1 through 6 as for discrete, but pick a property in step 3. After selecting the Zone and the unlocking the parameters, you can proceed to the Variogram tab. If you do not wish to condition the variogram(s) of the property to the facies, then do not click on the Facies button, and simply proceed to make the vertical and horizontal components of a single variogram as described in the generic section earlier or in the section below called
“Modeling the Variogram in Data Analysis”. Otherwise, to condition to facies, continue to the next step.
7. 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.
[Q 4]
8. Pick the facies classification whose variogram you want to compute.
9. Follow the generic variogram outline in the previous section or the one below.
Schlumberger Private Questions for review:
1. An option available when making variograms in Data Analysis for petrophysical properties is the ability to ___________ (condition) the variogram of petrophysical properties to a ___________ (facies) model.
2. When creating variograms for discrete data classes, is it possible to “copy” a variogram from one facies classification to another? – Y or N (Y).
3. When conditioning a property variogram to a particular facies in a model, the data used for the variogram must reside only within the selected _________
(facies).
4. If, when making variograms for a petrophysical property and you want to condition the variogram to a facies model, you must click on the
________(facies) button.
Schlumberger Private Chapter: Petrel Interactive Tools and Icons for Making Variograms
Pages: - Lag, Azimuth, and Search Angle Icon - Variogram Display
Interactive tools and icons for variogramming in Petrel
Below, you see the interactive tools which are provided in Petrel for the manipulation of the experimental variogram and the model variogram.
Lag, Azimuth, and Search Angle Icon in Petrel
Petrel variogram display, showing cloud histogram [Q 2,3]
- As yet undefined variogram model, to be dragged by user (blue line) - Histogram showing how many variogram cloud pairs were averaged to compute each experimental variogram point
- Experimental variogram points - Default variogram model
Click/drag here to change LAG Click/drag here to change AZIMUTH
Click/drag here to change SEARCH ANGLE
[Q 1]
Schlumberger Private Questions for review:
1. By using the 3-dot icon in the Data Analysis variogram tool, you can modify which three parameters during variogramming? _____________,
______________, ______________ (Lag Distance, Search Angle, Search Azimuth)
2. The histogram superimposed on the Petrel variogramming tool shows how many variogram cloud __________ (pairs) were averaged to compute the experimental variogram point.
3. The blue line on the Petrel variogramming tool can be _______________
(dragged) by the user to model the variogram.
Schlumberger Private Chapter: Modeling the Variogram in the Data Analysis dialog in Petrel
Page: Simple Petrel procedure
In the previous two sections, you learned how to prepare for variogramming in the Petrel Data Analysis dialogs for two cases – discrete and continuous data. Now, we’ll show you how to create the variogram so it can be used. For both types of data, the creation of the variogram proceeds in the same manner.
Previously, the variogramming mechanics were described generically. Here, we show how it works specifically in Petrel, using Petrel terminology and dialogs. Most of the procedure is quite similar to the generic mechanics, to which we will refer..
• Determine if anisotropy exists
• Determine anisotropy by using the “Settings” method for computing a variogram map as we do here, or alternatively use the previously described Trial and Error method. [Q 1]
- Right click “Settings” on the upscaled property you want to variogram [Q 2]
- Stretch the window open so you can see and click the Variogram tab - Choose Variogram Map and Execute.
- Open a map window and display the variogram to see if anisotropy is revealed. In the left example, there does not seem to be any. If there were, you would see a symmetrical display with the axis of symmetry being the major azimuth of anisotropy, as in the map on the right.
Another problem with the left map are the holes which are due to the search range being specified too small.[Q 3] If anisotropy is revealed as on the right by the oval shape of the contours, make a note of the azimuth, in this case about 25 degrees. [Q 4,5]
Schlumberger Private
• Optimize experimental major variogram shape– click the “Major” tab - Regardless of the method used to determine anisotropy, be sure to set
the correct azimuth for the Major direction before computing its experimental variogram. Verify your choice of azimuth by using the 3-button icon to alter it in small increments, making sure that the shape of the variogram is optimized.
- Further optimize the shape of the experimental variogram point distribution by interactive movement of the lag icon and search icon.
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. The higher the column for a point, the more observations contributed to its presence.
- In some cases, the points cannot be seen clearly until you drag the blue curve all the way to the bottom.
• Model the variogram for the major direction
Once you have the best experimental shape you can get, then - Decide on the model Type (Exponential, Spherical, ...)
- Interactively drag the Nugget Point where you think it should be - Interactively drag the Range Point where you think it should be - Click Apply to save the major component.
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• Compute experimental variogram for the minor direction and model it, if anisotropic
- This step is unnecessary if the major azimuth is 0.0; that is, when the variogram is to be omnidirectional, or isotropic. Otherwise, click on the Minor tab, then follow the same shape-optimization 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.
- Click Apply to save the minor component.
• Create the Vertical Component of the Variogram
• After you click on the Vertical tab, the procedure is basically the same for vertical variograms as for the major component above, except that you should start with a small lag distance which relates to the sample spacing along the borehole (1ft. – 2ft.). [q 1]
• Click Apply to save the vertical component.
The Power of Variogram Analysis
As we mentioned before, there are many relationships, analysis techniques, and interpretive procedures which can be brought to bear during data analysis using variograms. Important facts about the geometry, size, and even composition of a
reservoir can be inferred and indeed discovered by simply making a thorough inspection of the available data and grids with variograms. This kind of information is invaluable during modeling and uncertainty studies, but such topics and techniques is not in the scope of this training course. Instead, we recommend perusal of the resources of the Internet, or advanced courses offered by Petrel and others.
Schlumberger Private Questions for review:
1. Before the horizontal component can be modeled, it must be established if _____________ (anisotropy) exists in the data set.
2. To create a variogram map, go to the ________________ (Variogram) tab of an objects ____________ (Settings).
3. If a variogram map exhibits holes, then the _______________ (Search Range) is probably too small.
4. In a variogram map, circular contours centered around the middle of the map suggest that the data set is _______________ (isotropic).
5. The major axis of anisotropy can be measured by the ____________(azimuth) of the long side of the oval shapes defined by the contours on a variogram map.
6. When creating a vertical variogram, the lag should be initially set to a _________ (smaller) number than you would set for horizontal variograms
Schlumberger Private Page: Using the variogram
Using the Variogram
Once the variogram is saved, you can always return to Data Analysis and modify it if you like. Variograms made in this way are available for use during property population. As you begin to model your facies or property, make sure that you click on the icon in the modeling dialog named
“Use Variograms Created in Data Analysis”. [Q 1]
as we discussed earlier. Recall 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. [Q 2]
The variograms you see in the Property and Facies Modeling dialogs are the default variograms for modeling. When you click on the icon mentioned above, they have no relation to the variogram which will be used in the modeling. [Q 3]
Questions for review:
1. To use a variogram created in Data Analysis, you should click on the “Use Variograms in Data Analysis” icon in the _____________ (Modeling) dialogs.
2. When modeling, the variograms created in Data Analysis can be edited directly. – T or F (F).
3. When the “Use Variograms in Data Analysis” icon is turned on, the variogram shown in the modeling dialog takes on the values of the one created in Data Analysis – T or F (F).