Prediction of Wellbore Position Accuracy When Surveyed With Gyroscopic Tools


This paper describes a new method for the estimation of wellbore position accuracies when using gyroscopic tools. The developed method represents a solution to the industry’s need for a general and flexible error model which is applicable for all gyroscopic surveying tools and services.

The general gyro error model consists of a new set of error terms and a mathematical description of how the different error sources contribute to position uncertainties dependent on sensor configurations and operational modes. The model is suitable for appropriate modeling of most gyro surveying services.

The description of the model and the attached numerical examples are demonstrated to be sufficient to implement the model. The model has now been implemented in a number of commercial well planning and survey-management software packages.

The paper is a product of a collaborative work within the SPE Wellbore Positioning Technical Section (SPE WPTS), formerly the Industry Steering Committee on Wellbore Surveying Accuracy (ISCWSA).


The analysis of wellbore position accuracies has been developed during the last few decades as a key factor to ensure safe and economical drilling operations.

Harvey, Walstrom, and Eddy (1971) summed up their work on analysis of errors in directional-survey calculations. In this pioneering work, it was concluded that wellbore position uncertainties were dominated by random errors and effects.

Wollf and de Wardt (1981) showed that systematic errors are the most significant contributors to wellbore position uncertainty. The presented model(s) became an industrial standard for directional surveying.

The applications of error models were emphasized by Thorogood (1990). He also addressed the importance of ensuring that the actual survey quality is validated in accordance with the applied error model.

The necessity of describing the wellbore position accuracies and statistical characteristics in a proper way has increased, along with the recent advances in surveying and drilling techniques. Several companies and persons have significantly contributed to the development of new methods and applications (Brooks and Wilson 1996; Ekseth et al. 1997; Torkildsen et al. 1997).
Ekseth’s Dr. ing. thesis (1998) has become the basis for subsequent developments of error models and estimation techniques.

Work in recent years by a group of industry experts, (members of the SPE WPTS) culminated in the publication of an error model for magnetic measurement while drilling (MWD) survey tools, that has become widely accepted and used within the oil industry (Williamson 2000). The work described here was born out of a desire to extend that model to encompass the full range of surveying techniques available to the industry, and, specifically, to include gyro survey tools. The formative work that has led to this paper took place within the meetings of the SPE WPTS, with subsequent detailed developments undertaken by a gyro working group within that committee.

Gyro tools are widely used for completion surveys and to control the drilling of wellbores in regions of high magnetic interference where the magnetic tools become less reliable. Recent advances in gyro technology have led to the application of gyro survey tools during drilling operations (the MWD gyro).