Error Ellipses at Sidetrack Depth – Well Seeker vs Compass Comparison


Well Seeker Pro has a Compass EDM import function. This is commonly used by customers who perform their well planning operations in Compass and then wish to import the plan into Well Seeker. It was observed that on some occasions when running an anticollision scan between a sidetrack and its motherbore, that the Semi Major error values generated in the report did not match between Well Seeker and Compass. This document will explain why in some cases this can happen and will provide a potential solution to the issue.


When sidetracking a well, it is likely that at some point, you will need to run an anticollision scan against the motherbore. In this situation, the portion of the well from surface to the sidetrack depth will be identical in both the sidetrack (reference well) and the motherbore (offset well).

In this scenario, when an anticollision scan is run, Well Seeker zeros the Reference and Offset Semi Major Errors from surface to the sidetrack depth, because everything from surface to the sidetrack depth is exactly the same on both wells.

In some situations, this type of scan is performed against wells which were imported from Compass via an EDM file. When this happens, the output from Well Seeker can be compared against the output from Compass. Sometimes these would match and other times the errors would be out by a significant margin.

The source of this difference lies in the Compass setup. In the Survey Program tab of the Actual Design Properties dialog in Compass, there is an option called “Sidetrack Surveys run back into original hole, therefore enter Sidetrack Depth”. This is where the user can enter the sidetrack depth. The value entered here does not actually affect the point the plan or survey sidetracks the motherbore; however, it does represent the point that Compass starts to calculate the ellipses of uncertainty.

In the below left screen shot (taken from Compass), the Sidetrack depth is surface (0) and in the below right, the sidetrack depth is 11,021usft. Either of these setups could be used for the same well.

With the above right setup, an EDM pulled into Well Seeker would match the Compass output based on the sidetrack depth being 11,021usft; however if the Compass setup was the same as the above left, the outputs would not match because the errors in Compass would be generated from surface and not the actual sidetrack depth. This would make the Compass errors far larger than those in Well Seeker.


Innova has implemented a change in v2.2.0.29 which will handle the different options available in Compass. A new “Errors Start” option has been added to the Plan and Actual Well Properties dialog. This allows the user to choose between Surface and ST Point. This option will initially be set as ST Point by default.

With this version when an EDM file is imported, the Errors Start option is selected based on the Compass setup, which ensures that the errors will be generated from the same starting point in Well seeker.

NOTE: With v2.2.0.29, any new EDM imports will have this option selected based on the Compass setup; however, it will not change any existing wells in the database. These would need to be manually adjusted if required.

This change will only affect anticollision scans run between a sidetrack and its motherbore. For all other scans, the errors will start from surface.

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