Improving models of the Earth’s magnetic field for directional drilling applications

Abstract

Over the past 20 years, directional borehole drilling has become increasingly important for improving the optimal extrac­tion of reserves from challenging targets and for reducing wellbore collisions. Very long wells drilled using borehole steering methods can take weeks to months to complete and rely on accurate models of the Earth’s magnetic field which necessarily include a parameterization of its time variation. Magnetic field models used in the hydrocarbon industry, such as the BGS Global Geomagnetic Model (BGGM), are computed from data collected by a network of ground-based magnetic observato­ries and from low Earth-orbiting satellites. Magnetic field models provide snapshots looking back in time, but to be useful to industry, they also need to predict how the field will change in the future. Previously, predictions of magnetic variation have been based on relatively simple extrapolation of the observed changes. We introduce a physics-based technique to forecast the changes in the field by deducing large-scale flow of the iron-rich liquid at the top of the outer core and use this to advect the present magnetic field forwards in time. We demonstrate that this method produces valuable improvements in the accuracy of magnetic field models and hence an improved tool for directional drilling.