Error Properties of Magnetic Directional Surveying Data

The primary tools for directional surveying while drilling (MWD) are based on the determination of the magnetic azimuth. This paper presents the results from a study that highlights several aspects of the quality of directional surveying with magnetic tools. Survey data from 66 North Sea well sections have been investigated in order to evaluate the overall quality of the sensor readings and the applicability of different estimation and error detection techniques. The surveys cover 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, some of the error term accuracies are normally better than the modelled ones. The applicability of multi-station estimation techniques is demonstrated. There is always a chance of misinterpretation due to several reasons. These are: poor geometry, low redundancy, high random noise level, presence of gross errors, and errors in the Earth's gravity and magnetic field references. Since the estimated results are very dependent on the selected parameter model, the major issues related to the selection procedure are discussed. Procedures are also derived for performing multi-station estimation of parameters in a reliable and robust manner. Finally, there is a practical demonstration of the application of a new error detection procedure. This method is used to detect outliers that might cause errors in the multi-station estimation. This is a significant potential source of error as such outliers cannot be detected when applying conventional methods. It is demonstrated how much these extended quality control procedures can improve the final wellbore positions.