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Points to Ponder
For some reason, Sag correction is still rarely applied in land operations. The reduction in uncertainty of inclination is 60%, so why not? The same software that allows us to model the performance of a BHA provides the natural deflected shape of the BHA as a byproduct. The models are quite forgiving and do not need every minor component of the BHA to populate them, but only the main structural components.
Make sure the BHA you run in the well is the same as the one you analysed!
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13.1 Drilling magnetisation
It is well known that the MWD sensors in themselves are extremely accurate but the weak link in the system is the accuracy of the magnetic field in which the azimuth readings are taken. We discussed earlier the effects of local crustal anomalies on the accuracy of our background reference vector but in this section we will look at the effects of the drillstring magnetisation which is always present to some degree. It should be noted
Figure 59: Drillstring magnetisation.
however that since the sensor pack is always set inside non-magnetic material, the major interference component is likely to be either from below or above the sensor in the drillstring and therefore the z axis interference is usually the major influence on azimuth accuracy.
In the 1990s, Dr Robin Hartman of Shell International developed SUCOP (Survey Correction Program) to implement a technique for measuring and removing the magnetic influence of the drillstring.
If a magnetometer is placed in a magnetic field it will measure the component of the field along its own axis. A good analogy is if a tube with a small flow meter was inserted in stream, the flow through the tube will be the component of flow along the axis of the tube. Clearly if the tube is held perpendicular to the flow there will be no flow in the tube and if it is in line with the flow it will experience the full flow rate of the stream. At any other angle it will experience the flow vector x cosine of the angle of incidence. This value is often referred to as the vector dot product. This can be defined as the product of two vector lengths x cosine of the angle between them.
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