Effects of Geomagnetic Disturbances on Offshore Magnetic Directional Wellbore Positioning in the Northern Auroral Zone

Abstract

This thesis focuses on how disturbances in the geomagnetic field, offshore northern Norway, may affect the accuracy of magnetic directional wellbore surveying. The topics discussed are, however, applicable to the entire northern auroral zone. Suggestions on how to manage the effect of increased geomagnetic activity on magnetic directional wellbore survey operations in and near the auroral zone are described. The results from our study of the geomagnetic conditions in the Norwegian Sea clearly indicate that the direction from a monitoring station to a drilling site matters when using data from the monitoring station to quality control or correct downhole directional measurements affected by geomagnetic disturbances. While the deviations in total field and dip correspond well over large distances along the same geomagnetic latitude in east-west direction, the declination variations correlate better in the north-south direction. When analysing observatory data in the vicinity of the Barents Sea we discovered that the deviations in declination during disturbed periods can be constantly offset from the quiet level for several hours. Magnetic directional surveys taken during such conditions will result in an unmodeled position-bias, if not corrected for. Based on observatory data from all over the northern auroral zone, and a new application of an external magnetic field model, we show that there can be significant differences in the geomagnetic conditions along concurrent geomagnetic latitudes, in addition to the known latitudinal variations.