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magnetic Surveys
09 February 2019
Gentlemen and ladies,
In a recent CNN article (I am sure there are other more scientific references available) the migration of the north magnetic north pole has accelerated in the past 3 decades. This has resulted in updating the World Magnetic Model a year ahead of schedule (last update 2015).

https://edition.cnn.com/2019/02/05/health/north-pole-magnetic-moving-russia-scli-intl/index.html

What is the effect on magnetic wellbore surveys and how is this change incorporated in survey models?

4 answer(s)
nbergstrom
Wellbore Positioning Advisor
Magvar
Total Posts: 35
Join Date: 03/07/08
I forgot to mention that the better worldwide geomagnetic models (BGGM, HDGM, MVHD) are updated every year and thus suffer much less from the secular drift of the earth's main magnetic field.  The main field is caused by convection currents in the liquid part of the earth's iron core.   This conductive material moving in the magnetic field generates electric current flows, which in turn generate a magnetic field.  Thus a "self-generating dynamo".
nbergstrom
Wellbore Positioning Advisor
Magvar
Total Posts: 35
Join Date: 03/07/08
The World Magnetic Model (WMM) referenced in the article is usually updated every 5 years to meet its specified accuracy.  This model is sponsored by the US and UK Defense departments and has requirements for accuracy, thus the unscheduled update when the magnetic field moved more than expected prior to the 5-year update.

The only advantage of the WMM (and the similar IGRF model) is that it is available at no cost.   Most oilfield survey providers will be using a better global model such as the BGGM, HDGM, or MVHD.  If there is a requirement for higher accuracy (less positional uncertainty) a local InField Referencing (IFR) model may be used.  IFR type 1 models account for at least part of the local anomaly caused by magnetic minerals in the earth's crust.  IFR type 2 adds to this a geomagnetic observatory to measure and correct for the disturbance field caused by solar flares.

IFR1 is commonly used in drilling long laterals in close proximity to each other, where a small angular error can result in spacing between wellbores that is less than ideal.  IFR2 is most useful at higher latitudes, especially during geomagnetic storms.  If you can see the Northern Lights, you probably should be using IFR2.
Scott_McNeil
Consultant
SPREADAssociates
Total Posts: 119
Join Date: 05/03/08
Hi Dwight,

Further to Martins response, your survey company should also have a nearby Earth reference station that they can cross check with.

The reason for this is that while the North Magnetic pole is moving constantly, solar activity (flares, sunspots) can cause sudden short term changes to the Earths Magnetic field.

On one survey on a highly deviated well, we found that there was a substantial difference between the magnetic azimuths between the in and out survey, building up to 8 deg. by the end of the survey.

Before committing to the time and cost of a second run, the survey company (Sperry Sun, may they RIP) were able to check with their ground reference station and found that a minor solar flare had caused the Earths magnetic field to change several degrees during the run.

The field was still changing, so there was little point in re-running the survey at that point.

We were subsequently able to cross reference the survey readings with the readings from the earth station to provide correction factors throughout the time period and get an acceptable survey.

All the best

Scott
mhayes
Consultant Driling Engineer / ERD Advisor
Stanfield-Hayes Consulting
Total Posts: 36
Join Date: 25/03/11
If you ensure that your magnetic models are up to date, then this shouldn't be a problem. To this end, QC your magnetic survey supplier as usual to ensure they are using a valid magnetic model and ensure all surveys are date referenced. Whilst the reality may have changed faster than the modelled expectations, change is the norm rather than the exception.

It may also be worth referencing SPE 119851 - Confidence Limits Associated With Values of the Earth's Magnetic Field Used for Directional Drilling from Susan Macmillan (BGS Project Leader) and Steve Grindrod.
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