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5.2 MWD Magnetic Spacing
Clearly, if we are to make use of magnetic sensors in an MWD tool, we need to ensure that there is sufficient magnetic isolation to avoid significant magnetic influences from the other drilling equipment.
5.2.1 Drill String Magnetic Interference The drillstring is a long slender metallic body, which can locally disturb the E h’s m gn c f ld. o on of h s ng and its shape causes the magnetisation to be aligned along the drillstring axis. The magnetised drillstring locally corrupts the horizontal component of the E h’s m gn c f ld nd h nc ccu 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 86) s hsw s h lm fo ‘GoodM gn c’su ys n h sys m c o mod l. should b no d h mo 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. The following explanation is included courtesy of Dr Steve Grindrod of Copsegrove Developments Ltd. NON-MAGNETIC DRILL COLLAR LENGTH REQUIREMENTS This section describes the theoretical background to drillstring magnetic interference, explains the origin of NMDC charts and makes recommendations on NMDC usage and inspection. This is based on [Reference CUR 252] (SPE 11382 by S.J. Grindrod and C.J.M. Wolff on Calculating NMDC length).
By making assumptions about the magnetic poles in the steel above and below the NMDC, the expected optimum compass spacing to minimise azimuth error and the magnitude of the expected azimuth error can be calculated.
5.2.2 Pole Strength Values Field measurements by Shell (Reference CUR 252) have been made of pole strengths for typical Bottom Hole Assemblies. These values are for North Sea area in Northern Hemisphere; these should be reversed for Southern Hemisphere. However, it should be noted that the polarity and intensity of magnetic interference is not easily predictable. In many cases the interference is mainly caused by the use of magnetic NDT techniques which of course have nothing to do with geographic location. The numbers suggested here are merely a guide and certainly not an upper limit.
Upper Pole
Lower Pole
ll coll s up o + 900 μWb.
Stabilisers and bit up to -90 μWb. 10m drill collar below NMDC up to -300 μWb Turbines up to -1000, μWb
5.2.3 Azimuth Error Drillstring magnetisation affects the observed horizontal component of the local magnetic field. A magnetic compass detects the horizontal component of the E h's m gn c f ld. h d lls ng nduc d o,Δ z cs long h drillstring axis and this affects the east/west component of the observed field in proportion to (Sine Inclination x Sine Azimuth). This means that the compass error increases with inclination and with increased easterly or westerly azimuth of the wellbore. 5.2.4 NMDC Length Selection Charts Using the formulae from SPE 11382 by S.J. Grindrod and C.J.M. Wolff, NMDC charts can be constructed for various well inclinations and azimuths and for a maximum acceptable azimuth error. The latter is taken as 0.25 degrees as the limit for good magnetic surveying practice. By varying the DIP and B for local conditions, charts can be prepared for various areas of the world.
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