QC of data before & after velocity modeling (when using well TDR)
1 – Defining velocity layers : Cross plotting interval velocity (of the check-shots) with depth gives an idea of the trend of velocity field. But the definition of velocity interval is more accurately defined by plotting the well tops in the same function window. Procedure:
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Open calculator by using MB1/LMB from Attributes of well tops folder Use equation X_Axis = Ran (1, 30). Here you are basically creating an imaginary X Axis for the well tops. Use the general template. Open a function window >> Plot interval velocity versus Depth (Z) of chec k shots In the same function window, plot X_Axis versus Depth (Z) of well tops
NOTE – This is a good process to do a quick QC for all wells at one go. You can use ‘well filter’ under check shots folder to go through well by well.
2 – Using Saved Search: Many a times check shots or TDR are available only for a few wells. You can either use a shared check shot or use a user-defined no. of wells (especially when the wells are far apart). Procedure:
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Under ‘Wells’>> Saved searches Use MB3/RMB and select ‘New Search’ You have different options to filter the wells and select the wells of interest Re-name the new saved search.
When setting up the velocity model, you can restrict the wells to be used by selecting a saved search.
3 – Residual Error: Also called as ‘Depth Error’. It is the difference between the measured depth and calculated depth of all samples and all velocity locations. How to get it – You have to chose the option “Add residual on well tops” under ‘Output’ in ‘Make velocity model’ process. After running the velocity model, the residuals are stored under ‘well-tops’ folder in the ‘Input’ tab. How to display it – Use a function window and cross-plot X_Axis and Residual under well-tops >> Attributes
Residual Error Correction – These are some of the common corrections that can be used to reduce residual error.
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Re-gridding TWT input surfaces using a smaller grid spacing Using different gridding algorithms in the above case. Do a QC by displaying the gridded surface and the corresponding well top in a 3D window Setting up a depth correction or depth scaling model as shown below.
