Confidence Limits Associated With Values of the Earth's Magnetic Field Used for Directional Drilling
- Susan Macmillan (British Geological Survey) | Steve Grindrod (Copsegrove Developments)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2010
- Document Type
- Journal Paper
- 2010. Society of Petroleum Engineers
- 1.9 Wellbore positioning, 1.6.1 Drilling Operation Management, 4.1.5 Processing Equipment, 1.6.6 Directional Drilling, 4.1.2 Separation and Treating, 4.3.4 Scale, 1.12.1 Measurement While Drilling, 1.9.4 Survey Tools, 1.6 Drilling Operations
- earth's magnetic field, ISCWSA, non-Gaussian error, BGGM, MWD error model
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This paper describes updated uncertainties for use with predictedgeomagnetic parameters within magnetic measurement-while-drilling (MWD)survey-tool-error models. These models are used to define positional-errorellipsoids along the wellbore, which assist in hitting geological targets andavoiding collisions with existing wellbores.
The declination, dip angle, and total field strength of the Earth's magneticfield are used with magnetic-survey tools for surveying the wellbore. Thesevalues are often obtained from mathematical models such as the BritishGeological Survey (BGS) global-geomagnetic model (BGGM). As the Earth'smagnetic field is continually varying with time, the BGGM is updated annuallyto maintain accuracy. However, a global predictive model cannot capture allsources of the Earth's magnetic field, which results in uncertainties of thepredicted parameters. The Industry Steering Committee on Wellbore SurveyingAccuracy (ISCWSA) published an MWD-error model in 2000 (Williamson 2000). Thegeomagnetic-field uncertainties that are part of this model were derived fromwork conducted by the BGS in the early 1990s. Since then, more-accurate datafrom magnetic-survey satellites have been introduced into the BGGM, and theuncertainty of the predicted geomagnetic-field parameters has been reduced.
The original approach to deriving the uncertainties involved separating thevarious error sources in the magnetic field and assessing them individually.This paper uses a simpler approach where clean orientated magnetic downholedata are simulated using geomagnetic-observatory data. Spot absolutemeasurements of the magnetic field made at observatories around the world areadjusted for the crustal magnetic field to make them more representative ofhydrocarbon geology. The adjusted observatory data are then compared with thepredicted values from the BGGM to assess the uncertainty. The uncertainties donot fit a normal distribution, so they are expressed as limits for variousconfidence levels. They vary with location and, in their derivation, do notassume any underlying empirical error distribution. While they also vary withtime, we provide time-averaged look-up tables which should be valid for as longas there are good-quality satellite data on which to base global magnetic-fieldmodels. Options to reduce the uncertainties further using data from localmagnetic surveys [in-field referencing (IFR)] and observatories (interpolationIFR) are also described.
The use of the revised geomagnetic uncertainty values in the MWD-error modelwill reduce wellbore-position uncertainty to reflect the increased accuracyfrom recent improvements in geomagnetic modeling. This is demonstrated usingresults for the ellipsoids of uncertainty output by an MWD error model forthree standard ISCWSA well profiles.
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