Schlumberger

Technical Paper: A Comparison of Collision Avoidance Calculations

Society: IADC
Paper Number: 140183
Presentation Date: 2011
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Abstract

This paper will describe procedures commonly used in the industry for avoiding unwanted wellbore collisions.  Unwanted collisions may occur if the actual position of a wellbore differs from its presumed position.  Such differences may arise from errors in positioning the wellhead, errors in directional survey measurements, or incorrect assumptions regarding the survey errors or the trajectory between survey stations.  

Normal practice is to assign an error model to each survey.  The presumed position of the well then consists of a wellpath centerline and an associated envelope of uncertainty which is derived from the error model.  Collision avoidance practices may be based on specifying minimum clearance between the centerlines or between the envelopes of uncertainty, or they may be based on estimates of collision probability.

Exercises have been conducted to compare uncertainty envelopes and estimates of collision probability, calculated by different service companies using different software packages.  The exercises showed substantial variation among the results obtained, given the same input data.  These results highlight the need for detailed specification of the problem, oversight of the process by suitably qualified personnel, and for clear annotation of all results to indicate the methods employed and assumptions made.

In spite of the complicated error models which are now available, it must be remembered that the values and error distributions assigned to several critical input parameters are little more than educated guesses.  Under these circumstances, there are limits to how far it is worth proceeding with refining the details of error modeling, and the use of ample safety factors should be encouraged.

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