Quantifying Global and Random Uncertainties in High Resolution Global Geomagnetic Field Models Used for Directional Drilling

Abstract

Summary Total field strength, declination, and dip angle of the Earth's magnetic field, in conjunction with gravity, are used by magnetic-survey tools to determine a wellbore's location. Magnetic field values may be obtained from global models that, depending on the model, have a wide range of spatial resolution at the Earth's surface from large scale (3000 km) to small scale (28 km). The magnetic field varies continuously in both time and space, so no model can fully capture the complexity of all sources; hence, there are uncertainties associated with the values provided. The SPE Wellbore Positioning Technical Section/Industry Steering Committee on Wellbore Surveying Accuracy (ISCWSA) published their original measurement-while-drilling (MWD) error model in 2000. Such models and uncertainties define positional error ellipsoids along the wellbore, which assist the driller in achieving their geological target, in addition to aiding collision avoidance. With the recent update to Revision 5 of the ISCWSA error model, we have reassessed the uncertainties associated with our latest high-resolution global magnetic field model. We describe the derivation of location-specific global and random uncertainties for use with predicted geomagnetic values from high-resolution models within magnetic MWD survey-tool-error models. We propose a sophisticated approach to provide realistic values at different locations around the globe; for example, we determine separate errors for regions where the models have high spatial resolution from aeromagnetic data compared to regions where only satellite data are available. The combined uncertainties are freely available via a web service with which the user can also see how they vary with time. The use of the revised uncertainty values in the MWD-error model, in most cases, reduces the positional error ellipsoids and allows better use of the increased accuracy from recent improvements in geomagnetic modeling. This is demonstrated using the new uncertainty values in the MWD-error model for three standard ISCWSA well profiles. A fourth theoretical well offshore Brazil where the vertical magnetic field is weak shows that with drillstring interference correction relying on the more uncertain magnetic dip, the positional error ellipsoids can increase. This is clearly of concern for attaining geological targets and collision avoidance.

Magnetic Maps and Models for Alternative Navigation

“…The commercial HDGM [13] and BGGM [14] models, as well as the research EMM model, are spherical harmonic models incorporating a combination of satellite data for core wavelengths and coefficients derived from the longest wavelengths of survey-based grids. The commercial models are used in directional drilling applications.…”

Evidence-based uncertainty estimates for the International Geomagnetic Reference Field

“…It is also to be noted that the more rapidly time-varying external field is explicitly not considered in this study as the IGRF does include it, though it will be present in most measurements. As the effect of the ionosphere and magnetosphere is latitude-dependent (e.g., Beggan et al 2021;Chulliat et al 2020) quantifying the external field effects is not difficult. However, as these uncertainties are time-dependent (daily, seasonal, solar cycle) providing an 'average' error for these sources is difficult though possible, as long as it is clear to the user what the uncertainties represent (Beggan et al 2018).…”

An account for the geomagnetic disturbances due to space weather events during offshore directional drilling. A proper use of the adjacent land-based observatory magnetic field data

“…Buchanan et al (2013)) or by using tools manufactured with nonmagnetic materials. The core and crustal fields (being mostly static) can be accounted for through high-resolution models of the Earth's internal field, such as the High Definition Geomagnetic Model (Nair et al, 2021) and British Geological Survey Global Geomagnetic Model (Beggan et al, 2021). However, electric currents in the ionosphere and magnetosphere and their counterparts induced in the conducting Earth give rise to diurnal and irregular variations observable in the MWD.…”