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
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
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
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
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
© 2008. Society of Petroleum Engineers
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- Original manuscript received:
6 January 2005
- Meeting paper published:
26 September 2004
- Revised manuscript received:
4 September 2007
- Manuscript approved:
9 September 2007
- Version of record:
20 March 2008