Local Geomagnetic Field Modelling Closes the Gap Between MWD and Gyro Directional Surveying

A method for estimation of the influence of local geomagnetic anomalies on magnetic directional surveys has been developed. The method has been verified by comparisons between gyro and magnetic positioning. The method relies upon aeromagnetic survey data, and estimates the 3 components of the local geomagnetic field along the wellpath from field data at flying altitude, and from downward continuation of the field data to reservoir depth. The field components can be predicted at depths down to the shallowest magnetic layer, and applied to enhance the accuracy of magnetic surveying. With access to core data the influence of magnetic material above the reservoir can also be included. A comparison has been run with real magnetic and gyro data from 11 wells at 3 different fields offshore Norway. The results indicate a reduction in lateral position uncertainty by 50% typically. This is very significant when trying to hit small targets. The method has been tested by major service companies and will soon be available as a part of their drilling services. This will benefit drilling with tight tolerances, especially in areas with large local magnetic anomalies such as the North Sea and Arctic regions. Local field strength anomalies can exceed 500 nanoTesla, declination errors can be more than 2 degrees and dip errors more than 0.5 degrees in these areas The lateral position difference between magnetic and gyro surveying has been reduced from 20-50 m to 1-25 m in actual cases. The method has so far been applied in post-processing to verify the results, and is now being applied while drilling for correction of surveys.