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.

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