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Degaussing Mud MotorsNeil Bergstrom, MagVAR

Neil Bergstrom, P.E. Senior Wellbore Positioning Technical Advisor

▪ Magnetic Variation (MagVAR) – part of H&P Technologies

– BA, Physics (Colorado College)

– Professional Engineer (Petroleum) Colorado #40847

– Devon, Halliburton, Scientific Drilling, COLOG, Mt. Sopris, EDCON

– Specialized in

• Wellbore Positioning and Anti-Collision

• Magnetic Interference

• Magnetic Ranging and Wellbore Intercepts

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Magnetic Variation Services (MagVAR)

▪ A division of Helmerich and Payne Technology

▪ Wellbore Survey Management Services

– Real-time and/or batch mode corrections and QC

▪ High-precision magnetic reference models of earth’s field

– MVHD MagVAR High Definition Global magnetic model

– IFR1 local magnetic models derived from aeromagnetic data

▪ MVRO MagVAR Remote GeoMagnetic observatories

▪ Drone surveys for locating buried wellhead and pipelines

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Drill String Interference (DSI)

▪ Depends on BHA residual magnetism and non-mag spacing

▪ Changes the Z-Axis (Along Hole Axis) Magnetic measurement

▪ Changes measured Azimuth and QC Values

– QC Values: BTotal, magnetic Dip angle

– Changes depend upon direction the Z-axis is pointing

– Largest Azimuth Changes at Horizontal East or West

– Smallest BTotal and Dip changes at Horizontal East or West

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Magnetic Drillstring Interference (DSI)

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The effect of DSI is to change the

measured azimuth and the QC Values

If uncorrected this results in well

placement errors.

Examples of MWD Error from 1000 nT DSI

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These examples assume

mid-continent magnetic

reference values.

These are all horizontal

wells.

As the azimuth approaches due east (or west), the azimuth errors

become larger.

At the same time the QC Values (BTotal and Dip) become smaller.

Since corrections depend on the QC Values, corrections are unreliable

for horizontal wells in the East or West direction.

Inclination

Magnetic

Azimuth

Azimuth Error

(Degrees)

Lateral Error

(in 10,000 ft)

dBTotal

(QC)

dDip

(QC)

90 0 0.0 0 508 1.1

90 10 0.4 75 501 1.0

90 30 1.2 209 442 0.9

90 60 2.1 366 260 0.6

90 80 2.4 419 98 0.2

90 90 2.5 436 10 0.0

What can be done about DSI?

▪ Increase non-mag spacing

– Moves MWD sensor further from bit

▪ Apply axial corrections (single or Multi-Station)

– Unreliable near horizonal East or West direction

▪ Reduce residual magnetism on BHA

– Requires effective degaussing

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Motors and BHA parts are magnetized during inspection

▪ The residual magnetism adds to the earth’s magnetic field

▪ This is called Drillstring Interference (DSI)

▪ DSI changes the azimuth measured by the MWD instrument

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▪ Magnetic inspection requires the part to be magnetized

▪ If not degaussed, there will be residual magnetism

Steel is made of microscopic magnetic domains

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Magnetized

Most domains pointing in

same direction

Superficially demagnetized

Some outermost domains

reversed. Temporary.

Only end is demagnetized

End domains pointing in

reverse directions

Temporary

AC Degaussing with a Coil

AC Degaussing: Pass the part slowly through the coil.

Each magnetic domain will experience a gradually

reducing and reversing field as it moves away from the

center of the coil.

A low frequency is required because a changing

magnetic field generates eddy currents in a conductive

material that oppose the changing field.

Used with permission from Vallon: http://www.vallon-degaussing.com/pdf/EM-Degaussing-brochure_EN.pdf

https://www.tdworld.com/substations/reliable-demagnetization-transformer-cores

To Effectively Degauss

▪ Each magnetic domain should “see” a reversing and

gradually reducing field.

▪ Initial reversing field should be at least as large as

magnetizing field.

▪ DC: Requires variable current coil

– Multiple passes reversing and reducing current stepwise

▪ AC: Requires low frequency ~ 1 Hz.

– Need not be a sine wave.

– Part moved slowly through the coil – one pass

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Applicable Standards

▪ ASTM E-1444 Standard Practice for MPI Section 6.7.1

– No more than +/- 3 gauss measurable anywhere on the part

▪ T.H. Hill DS-1 4th Edition Section 3.36

– 3 gauss maximum. 10 Gauss on agreement with client.

▪ Temporary reductions in magnetic field do not achieve the

intended purpose.

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Conclusion

▪ Degaussing motors and subs reduces need for non-mag

– See www.iscwsa.net > meeting minutes > ISC48 #14

▪ Small parts are more easily degaussed

– Motor parts should be degaussed before assembly

▪ Superficial degaussing leaves high internal magnetic

fields which come back over time

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Thank You