Introduction
Inclination-Only, undefined azimuth, survey data are common in both legacy and modern data sets. This document describes good practice for uncertainty calculations, clearance scanning, target sizing and data management purposes. The recommendations are the consensus of the ISCWSA error modeling and collision avoidance sub-committees.
Method
If the tie-on is nominally vertical (e.g. a vertical wellhead or another Inclination-Only survey log):
- The well path is considered vertical, independent of the measured inclination.
- The actual deviation of the well path from vertical is treated as an inclination dependent uncertainty and is combined with the actual measurement uncertainties.
- For planned well paths, special treatment of uncertainty is required to model the anticipated inclination (described in section 3).
- TVD is calculated using minimum curvature calculations from the recorded MD and Inclination, assuming a constant azimuth.
If it is possible to assign an estimated azimuth to the Inclination-Only data:
- An estimated azimuth (e.g. derived from the previous survey log) is assigned to the Inclination-Only survey stations. (The error model’s estimate of azimuth uncertainty must reflect the accuracy of the tie-on azimuth and likelihood of subsequent azimuth wander.)
- The well path is calculated in the normal way from the resulting MD, Inclination, Azimuth survey.
- No special treatment of planned well paths is required.
- Uncertainty based calculations such as clearance scanning and target sizing require no special treatment.
Error models
If the tie-on is nominally vertical:
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Recommendations for Management of Inclination-Only Survey Data