Confidence limits associated with values of the Earth's magnetic field used for directional drilling

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 missing 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 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 a MWD error model in 2000 (Williamson, 2000). The geomagnetic field uncertainties that are part of this model were derived from work done 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 down-hole 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 time and with location and, in their derivation, do not assume any underlying empirical error distribution. Options to further reduce the uncertainties using data from local magnetic surveys (In-Field Referencing) and observatories (Interpolation In-Field Referencing) 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 modelling