The magnitudes of certain significant error terms vary with latitude, for both magnetic and gyroscopic surveying instruments. This implies an increase in surveyed azimuth uncertainty, as one moves into northern regions.

This work analyzes the influence of the uncertainty increase on the required separation distance between wellbores. An expansion factor Fφ2/φ1 is presented, defined as the factor that scales a separation distance from latitude φ1 to latitude φ2, provided that the confidence level, and thus the collision risk, remains the same.

Uncertainty analysis incorporating a broad range of wellbore profiles and several magnetic and gyroscopic surveying instruments has been carried out. The results show that Fφ2/φ1 can range from 1.35 to above 2.0, when moving from 60°N (North Sea region) to 75°N (Barents Sea region). If expansion factors of these magnitudes are ignored, the collision risk typically increases by a factor of 10 to 50, respectively. These numbers show that it is of vital importance to apply the correct latitude dependency in wellbore surveying error models, when planning drilling activities in arctic regions.

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