Torgeir Torkildsen

High integrity wellbore surveys: methods for eliminating gross errors

□ Although a second independent survey cannot separately identify all potential errors, it is still the most powerful external data QC check. □ Statistical tests are used to ensure that directional survey measurements have an accuracy that is compatible with the applied error model. Internal QC checks, which are based only on single-source downhole data, are not sufficient to fully validate a survey against its error model (see SPE 103734). – John Weston, SPE – Roger Ekseth, SPE – Kazimir Kovalenko, SPE

Directional Surveying: Rotating and Sliding Operations Give Different Wellbore Position Accuracy

We present a novel method for analysing the position uncertainty of directional surveys. By this method, the uncertainty can be accurately calculated by conceptually simple analytical formulae. The theory covers any correlation in measurement error and any correlation in toolface, for both stationary and continuous surveys. The results demonstrate the importance of handling the toolface dependence properly. A direct benefit is that too optimistic or overly pessimistic accuracy estimates are avoided. Furthermore, the difference between rotating and sliding operation is significant for many error terms. For example, a misalignment error typically causes 10 times higher position uncertainty in sliding mode than in rotating mode, for a 3000 m long survey. These findings should have implications for the future specification of survey instrument operation procedures, and thus the design of directional survey programs.