The primary tools for directional surveying while drilling (MWD are based on determination of magnetic azimuth. This paper presents the results from a study where several aspects of the quality of directional surveying with magnetic tools are highlighted. Survey data from more than 50 North Sea well sections has been investigated in order to estimate the overall quality of the sensor readings and the applicability of different estimation and error detection techniques. The surveys are covering different wellbore geometries and are performed by several service companies.

The Industry Steering Committee on Wellbore Survey Accuracy (ISCWSA has developed error models for magnetic directional surveys, which now have become an industry standard. These error models are derived for two standard single-station processing techniques. The error term values are settled with the basis in input from several service companies. The results from the analysis of the survey data, presented in this paper, confirm the systematic error term values in general. However, the magnetometer accuracies are normally better than modeled.

The applicability of multi-station estimation techniques, when applied on real measurements, is demonstrated. Due to several reasons; poor geometry, low redundancy, high random noise level, presence of gross errors and errors in the Earth gravity and magnetic field references, the probability for misinterpretation is relatively high. The estimated results are very dependent on the chosen parameter model, and the major trends are discussed. Procedures for performing multi-station estimation in a reliable and robust way are derived.

Finally, a practical application of a new error detection procedure is demonstrated. This method is used to detect outliers that might be harmful for the multi-station survey estimation, and which even can not be detected when applying conventional methods. It is demonstrated how much these extended quality control procedures may improve the final wellbore positions and statistical properties.

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