Confidence Limits Associated With Values of the Earth's Magnetic Field Used for Directional Drilling
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 119851, "Confidence Limits Associated With Values of the Earth's Magnetic Field Used for Directional Drilling," by Susan Macmillan, British Geological Survey; Allan McKay, Petroleum GeoServices; and Steve Grindrod, SPE, Copsegrove Developments, prepared for the 2009 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 17-19 March. The paper has not been peer reviewed.
Uncertainties have been updated for use with predicted geomagnetic parameters in magnetic measurement-while-drilling (MWD) survey-tool-error models. These models define positional-error ellipsoids along the wellbore that assist in hitting geological targets and missing existing wellbores. The declination D, dip angle I, and total field strength F of the Earth's magnetic field are used with magnetic-survey tools for surveying the wellbore. The use of revised geomagnetic-uncertainty values in the MWD-error model could reduce wellbore-position uncertainty.

Introduction
The Earth's magnetic field is a vector quantity that depends on position and time and may be expressed as the vector sum of three sources: the field generated in the Earth's core, the crustal field from local rocks, and a combined external field from electrical currents flowing in the upper atmosphere and in the magnetosphere. Global models used in directional drilling [e.g., the British Geological Survey Global Geomagnetic Model (BGGM)] generally represent a combination of the main (i.e., internal) field and the undisturbed external field arising from ever-present magnetospheric currents. The values computed for the Earth's magnetic field at a given location and time from the BGGM will differ from actual values because of a combination of model errors, crustal fields, and external fields. This study investigated the magnitude of the uncertainty.

Data and Analysis
Directional-drilling and survey-management companies require accurate geomagnetic-field estimates, along with knowledge of associated uncertainties, when magnetic-survey tools are used to survey wellbores. Magnetic-survey tools measure the direction of the wellbore relative to the direction of the local geomagnetic field. In addition, the geomagnetic-field I and F are required for use in algorithms that reduce the error caused by the magnetic field associated with the drillstring. The uncertainties are used to compute positional-error ellipsoids along the wellbore to help intercept increasingly small geological targets and miss other wellbores.
Geomagnetic-observatory data were used to estimate BGGM uncertainty. The difference was computed between model predictions and observatory data not used in the construction of a given BGGM. A "BGGM year" is the year starting soon after the release of a BGGM in May of each year. For example, the 2006 BGGM year spans 2006.5 to 2007.5. The majority of dates entered for magnetic-field computations that use a given release of the BGGM are expected to be within this period. The overall performance of BGGM revisions has changed over the years.