This paper describes updated uncertainties for use with predicted geomagnetic parameters within magnetic measurement-whiledrilling (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 that 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.