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Making and Using Variograms in PetrelMaking and Using Variograms in Petrel1. Creating a variogram from an objects Settings panel2. Creating a variogram in the Modeling dialogs.3. Making a variogram in the Data Analysis dialogs

3.1 Preparing to make variograms for facies models3.2 Preparing to make variograms for continuous properties4. Modeling the variogram in Data Analysis5. 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 DiagramThe most interactive and robust methodology resides in the Data Analysis menus, and the leastinteractive method involves simply filling in the pertinent parameters, such as Range, Sill, andNugget in the modeling dialogs themselves. The Settings panel provides a semi-interactivemethod 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 objects Settings panel With this method, you create a graphic display for a Computation window by predefining thevariogram parameters as we will see below. You have the choice of creating a variogram mapwhich will automatically reveal the direction and degree of anisotropy, or a horizontal or verticalexperimental variogram which you can then edit and turn into a variogarm model for use inproperty modeling. In Petrel, experimental variograms are called sample variograms.1. Highlight the object and right-click on it, selecting Settings2. In Settings , stretch the window to the right until you can see the tabVariogram, 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 searchradius before the computation begins.4. After the sample variogram has been created, it will show up under the Variogram folderin 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 ownvariogram 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, aswell 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 prettymuch 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 dialogsHere, we have the best control over our variogram, although there does not seem to be a way toapply this tool to arbitrary data such as scatter points or grids. It appears only to work with thewell logs.

Preparing to Make Variograms for Facies Models1. Click on Data Analysis in the Property Modeling section of the Process Diagram

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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 visible6. Click on Use the Raw Logs7. 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, theresulting variogram will provide valuable spatial information about its spatial distribution in3 dimensions for algorithms such as Sequential Indicator Simulation. These variogramswhich are computed for discrete data are actually different than those for continuous data.

Preparing to Make Variograms for Continuous PropertiesIf you have chosen a continuous property for which to compute a variogram, first decide if youare going to condition your property values to certain facies distributions. Sometimes you have nofacies model and you will not have this choice. If you have gone to the trouble of making a faciesmodel, and you have at least a reasonable number of wells, then it would probably make sense todo 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 thechannel, and others as it exists within, say, the levee and the plain. For each facies class, only thedata which is located there will be used. There is also an option of making a variogram for onefacies class and using (or copying) it in another.16. 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. Selectthe facies model you wish to use. It may contain more than one facies classification. Ifyou 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 AnalysisHere, well go through the mechanics of creating a variogram model. All these steps are relevantwhether we are designing a variogram for the entire upscaled property, or for only part of itassociated with a particular facies.4.1 Create the Horizontal Variogram4.1.1 Determine anisotropy by using the Settings method for computing avariogram map- Right click Settings on the upscaled propertyyou 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 isrevealed. 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 anisotropy4.1.3 Compute experimental major variogramclick the major tab4.1.3.1 Optimize the shape of the variogram point distribution by interactivemovement of the lag/azimuth icon:Click/drag here for LAGClick/drag here for AZIMUTHClick/drag here for SEARCH ANGLE- As yet undefined variogram model- Histogram showing how many variogram cloud pairs were averaged

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to compute the variogram point- Experimental variogram points- Default variogram modelOptimization of experimental variogram shape, continued...+ By varying the azimuth, verify the direction of anisotropy, if any, as revealed bythe 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 classicvariogram shape+ note the histogram in the background of the variogram. Use it to decide if aparticular 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 entireData Analysis window downwards.4.1.4 Model major variogram4.1.4.1 Decide on the model Type (Exponential, Spherical, ...)4.1.4.2 Interactively drag the Nugget Point where you think it should be4.1.4.3 Interactively drag the Range Point where you think it should be4.1.4.4 Click Apply to save the major variogram

Petrel Variogram Components4.1.5 Compute experimental minor variogram and model it, if anisotropic4.1.5.1 This step is unnecessary if the major azimuth is 0.0. Otherwise, click on theMinor tab, then follow the same shape -optiomization and modeling stepsas for the major variogram, but note that youll 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 variogram4.2 Create the Vertical Variogram4.2.1 After you click on the Vertical tab, the procedure is exactly the same for thisvariogram as for the major one above.4.2.2 Click Apply to save the vertical variogram

5.0 SummaryWe 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 dataresiding in the seed property grids, or based on all of the original data in the logs used for theupscaling, or in the case of a fully defined grid, we can compute a variogram based on all of thevalues in the grid.We also have the ability to isolate only part of the property data, based on where it is locatedrelative to a particular facies in a multi-facies model (3D grid), and to create a variogram basedon 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 surethat you click on the icon in the modeling dialog which tells the system to Use VariogramsCreated 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 tothe 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 forthe modeling unless you click on the icon mentioned above. It has no relation to the variogramscomputed in Data Analysis.Posted byI Made Sutha at21.06Labels:variogram

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