The drillstring is a long slender metallic body, which can locally disturb the Earth’s magnetic field. Rotation of the string and its shape causes the magnetisation to be aligned along the drillstring axis.
The magnetised drillstring locally corrupts the horizontal component of the Earth’s magnetic field and hence accurate measurement of magnetic azimuth is difficult. For sensible magnetic azimuth measurement, the magnetic effect of the drillstring has to be reduced and this is done by the insertion of non-magnetic drill collars (NMDC) into the drillstring.
Non-magnetic drill collars only reduce the effect of magnetic interference from the drillstring – they do not remove it completely. An acceptable azimuth error of 0.25° was chosen based on Wolff and de Wardt (References CUR 443 and CUR 86) as this was the limit for ‘Good Magnetic’ surveys in their systematic error model. It should be noted that more recent work has suggested that magnetic interference azimuth error is likely to be of the order of 0.25 + 0.6 x sin (Inc) x sin(azimuth) so these values can be exaggerated at high angle heading east west.
By making assumptions about the magnetic poles in the steel above and below the NMDC, the expected optimum compass spacing to minimize azimuth error and the magnitude of the expected azimuth error can be calculated.
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