Combinations Tool for Cross Validation
Many depth conversion methods are reliant on well velocities, either in the form of sonic, check shot or apparent interval velocities. The Combinations tools, driven from the Wizard module, allow you to systematically leave out the value derived from a well from the velocity modelling process, derive a new depth conversion function, depth convert and then calculate the mistie at the well to see how well the model fits.
Combinations, or cross-validation, is the automated version of blind well testing, which can quickly be performed using the Wizard and is encouraged in order to see the effects of wells on the model. Manual depth conversion or workflows can also be used if a method isn’t supported by the Wizard.
The general principle is to understand how sensitive the model is to both the lateral and spatial information provided by well data. The depth conversion method is selected, a depth conversion performed, and the residual errors computed and posted on the map. A well is then deselected and the depth conversion re-performed, and the new residual errors may be compared with the previous error.
Such information can increase confidence in a selected depth conversion technique or indicate where further analysis might be required, reducing depth uncertainty.