Summary
Magnetic contamination of the drilling fluid may contribute significantly to
errors in directional surveying of wellbores. Such contaminations shield the
field measured by the magnetic sensors in measurement-while-drilling (MWD)
directional tools. The resulting error often exceeds typical sensor
uncertainties, and so, the accuracy of the inferred wellbore position is
severely degraded. Although the magnetic interference can be reduced by careful
attention to procedures for transport, pumping, and handling of drilling fluids
and their associated additives, the problem has not yet been eliminated.
To understand the magnetic shielding effect in a well-defined setting, we
have initiated a series of laboratory measurements where magnetic material of
known properties are added to a well-defined nonmagnetic drilling fluid, and
the resulting magnetic shielding is measured with a fluxgate magnetometer
immersed in the fluid.
In our experiments, powdered magnetite was added to a mixture of xanthan gum
in water to obtain a mixture of nominal magnetic susceptibility typical of
drilling fluids containing metal from pipe wear and erosion. Immediately after
stirring, the vertical component of the Earth's magnetic field inside the
liquid was measured and found to be significantly damped. Without touching the
system, the field was subsequently logged. It was found to continue to decay
for the next hour or so, reaching a fairly sharp minimum. The magnetic field
then started to increase slowly and irregularly again over the next few
days.
This observed time dependency of the magnetic shielding is so far unknown to
the surveying industry. The effect, combined with analysis of directional data,
can explain errors encountered in directional surveying. Recommendations for
how to minimize such errors are presented.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
31 January 2008
- Meeting paper published:
4 March 2008
- Revised manuscript received:
22 October 2008
- Manuscript approved:
25 November 2008
- Published online:
16 July 2009
- Version of record:
14 June 2010
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