The Effect of Drilling Fluid Content on Magnetic Shielding of Downhole Compasses in MWDs

Abstract

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.

Ditch Magnet Performance

“…Laboratory experiments confirmed that the magnetic field of the Earth was attenuated when a three-axis probe was lowered into a field-used drilling fluid containing magnetic waste particles [4]. This theory was also confirmed by Tellefsen et al (2012) [5] through their work. The problem is still present today.…”

“…At approximately 6000 seconds, the shielding effect reaches its maximum before it starts dropping again towards its initial value. This effect is anticipated to be cause by small from the work was that magnetic contaminants taken from a ditch magnet showed a significant effect on magnetic shielding [5].…”

“…Some experiments [2,5] were conducted using a magnetometer called Barrington Mag-01H to measure this shielding effect. The phenomenon of shielding appears briefly as a damping of the measured cross-axial components of the Earth`s magnetic field intensity in the tool, and it often dominates over most other relevant error sources [9].…”

“…Normally the azimuth errors reach from 1-2°, but in some cases, the azimuth error may be five to ten times larger if drilling occurs in predominantly east-west direction and/or close to the North-or South Pole [1, 4,5,9].…”

“…Series of experiments conducted by Amundsen et al (2010) [2], and Ding et al (2008) [13] showed that magnetic shielding can have a pronounced dynamic behavior [5]. Figure 3 shows the dynamic behavior of magnetic shielding.…”