Summary
This paper describes updated uncertainties for use with predicted
geomagnetic parameters within magnetic measurement-while-drilling (MWD)
survey-tool-error models. These models are used to define positional-error
ellipsoids along the wellbore, which assist in hitting geological targets and
avoiding collisions with existing wellbores.
The declination, dip angle, and total field strength of the Earth’s magnetic
field are used with magnetic-survey tools for surveying the wellbore. These
values are often obtained from mathematical models such as the British
Geological Survey (BGS) global-geomagnetic model (BGGM). As the Earth’s
magnetic field is continually varying with time, the BGGM is updated annually
to maintain accuracy. However, a global predictive model cannot capture all
sources of the Earth’s magnetic field, which results in uncertainties of the
predicted parameters. The Industry Steering Committee on Wellbore Surveying
Accuracy (ISCWSA) published an MWD-error model in 2000 (Williamson 2000). The
geomagnetic-field uncertainties that are part of this model were derived from
work conducted by the BGS in the early 1990s. Since then, more-accurate data
from magnetic-survey satellites have been introduced into the BGGM, and the
uncertainty of the predicted geomagnetic-field parameters has been reduced.
The original approach to deriving the uncertainties involved separating the
various error sources in the magnetic field and assessing them individually.
This paper uses a simpler approach where clean orientated magnetic downhole
data are simulated using geomagnetic-observatory data. Spot absolute
measurements of the magnetic field made at observatories around the world are
adjusted for the crustal magnetic field to make them more representative of
hydrocarbon geology. The adjusted observatory data are then compared with the
predicted values from the BGGM to assess the uncertainty. The uncertainties do
not fit a normal distribution, so they are expressed as limits for various
confidence levels. They vary with location and, in their derivation, do not
assume any underlying empirical error distribution. While they also vary with
time, we provide time-averaged look-up tables which should be valid for as long
as there are good-quality satellite data on which to base global magnetic-field
models. Options to reduce the uncertainties further using data from local
magnetic surveys [in-field referencing (IFR)] and observatories (interpolation
IFR) are also described.
The use of the revised geomagnetic uncertainty values in the MWD-error model
will reduce wellbore-position uncertainty to reflect the increased accuracy
from recent improvements in geomagnetic modeling. This is demonstrated using
results for the ellipsoids of uncertainty output by an MWD error model for
three standard ISCWSA well profiles.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
28 January 2008
- Meeting paper published:
18 March 2009
- Revised manuscript received:
19 June 2009
- Manuscript approved:
13 November 2009
- Published online:
15 April 2010
- Version of record:
14 June 2010
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