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Figure 56: The BHAs used in directional drilling are designed to be flexible enough to drill round curves. Inevitably this leads to deflections in the BHA centreline which is unlikely to remain parallel to the wellbore. As a result the inclination observed may need to be corrected for the misalignment between the
MWD sensor and the wellbore centreline.
Sag Correction Software Since finite element software or mechanical beam theory techniques are used anyway to model the side forces and stresses on a BHA as part of its performance design, the same software calculates the deflected shape of the BHA and can predict in advance the corrections needed to apply to the observed inclination. This is often the most important correction required for high angle drilling accuracy for good TVD placement. In the above example, there is also a mechanical bend in the BHA at the bent housing so the sag correction may not be just inclination dependent but may also be toolface dependent if the assembly has a bend that can be oriented.
Figure 57: Typical sag correction software output.
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