Magnetic Shielding During MWD Azimuth Measurements and Wellbore Positioning

Abstract

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.

Ditch Magnet Performance

“…Magnetic particles in the drilling fluid may also contribute significantly to errors in directional surveying of wellbores [2]. Errors in the directional surveying of the wellbore position, caused by azimuth errors, may cause the directional driller (DD) to miss his pre-set target of a long deviated well section within the range of 1-200m.…”

“…Errors in the directional surveying of the wellbore position, caused by azimuth errors, may cause the directional driller (DD) to miss his pre-set target of a long deviated well section within the range of 1-200m. Small changes in azimuth ranging from 1-2° is typically caused by the magnetic contaminated drilling fluid, but errors five to ten times larger may occur under unfavorable conditions [2]. Missing the target with a distance of 1-200m may cause a significant cost increase to the operation as the original reservoir section in a worst case scenario needs to be cemented back and re-drilled.…”

“…(1), (2), (3) and (4) given above remain valid in the presence of magnetic materials, except that the relation between B and H is not necessarily simple.…”

“…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].…”

“…Magnetic contamination of the drilling fluid caused by magnetic erosion waste particles from the well and drilling additives, are anticipated to contribute significantly to errors in directional surveying of wellbores [2]. This includes damage to mud pumps, damage to downhole tools and casing/drill-pipe wear.…”