Accurate wellbore positioning is a major challenge in the Frade field, a deepwater heavy oil project offshore Brazil that has been historically technically and economically challenging. The inherent subsurface and surface complexities alone might have prevented the development of this asset, as the structure is a low-relief anticline with two main fault blocks, consisting of three stacked reservoirs, and spanning an area of 20 km2.

The key challenge in magnetic surveying in Brazil is the large discrepancy between downhole tool readings and the British Geological Survey (BGS) Global Geomagnetic Model (BGGM), which provides the magnetic field at a 400-km resolution. A better understanding of natural variations in the local magnetic field is essential for a successful development of the field. As such, a new method of mapping the natural variations was developed.

With its recent refinements, geomagnetic referencing can now produce significant savings in overall project cost by providing accurate, real-time data on wellbore position while corrections to trajectory are still possible, preventing the costly sidetracks often required when errors are discovered in a post-drilled survey. It can also eliminate the need for costly troubleshooting of downhole magnetic tools in real time, caused by inaccuracy of the geomagnetic model used.

This paper outlines collaboration among operator, contractors and academic experts, and the development of the High- Definition Geomagnetic Model (HDGM) by the United States National Geophysical Data Center, which improves the spatial resolution to 30 km. The large-scale magnetic field study was integrated with the Bacia de Campos aeromagnetic survey to account for the entire spatial spectrum of the geomagnetic field, down to the kilometer scale. Using ellipsoidal harmonic functions and an equivalent source technique, two 3D geomagnetic references were produced for comparison purposes, and the results were validated by observations from the downhole measurement-while-drilling (MWD) tool reading.