The magnetic property of the drilling fluid is one of the substantial error sources for the determination of azimuth while drilling deviated wells. These errors may be in the range of 1-200m if drilling long, deviated intermediate sections. Therefore, these effects represent a significant cost to be mitigated. The error becomes even more pronounced if drilling occurs in arctic regions close to the magnetic North pole. Added clays, weight materials and the tubular wear are anticipated to distort the geomagnetic field at the location of the magnetometers. The effect on the magnetometer readings is obviously linked to the amount of magnetic material in the drilling fluid. The problem has been studied both by laboratory experiments and analyses of downhole survey data. However, there are several inconsistencies in the results, and the phenomenon is not fully understood.

In the following it is shown how the magnetic distortion relates to some drilling fluid additives. A series of experiments have been conducted to increase the understanding of the effects. First, a series of experiments showed that presence of free iron ions do not contribute to magnetic distortion. Next, a series of experiments with bentonite-based fluids were conducted, showing the effect of bentonite on magnetic shielding. Measurements on a series of clean, laboratory made oil-based drilling fluids showed that the magnetic shielding did not increase by the addition of organophilic hectorite clays. Finally, eroded steel from an offshore drilling location was added into the oil-based drilling fluid. These swarf and steel fines significantly increased the magnetic shielding of the drilling fluid. This paper also outlines how much the drilling direction may be distorted by the presence of these additives and contaminants.

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