The Industry Steering Committee on Wellbore Survey Accuracy (ISCWSA) is seeking to dispel the confusion and secrecy currently associated with wellbore surveying and to enable the industry to produce consistent, reliable estimates of survey-tool performance in today’s wells. They believe this will be achieved through the production and maintenance of standards covering the construction and validation of tool error models.
Work focused initially on MWD systems. They provide a large proportion of the total directional survey data world-wide and, because of their similarities between suppliers, are more amenable to specification standardisation than other types of survey tool. The results of the work on an error model for a basic directional MWD service has been presented in SPE 56702 ‘Accuracy Prediction for Directional MWD’ by Hugh Williamson, at the 1999 SPE Annual Technical Conference and Exhibition in Houston, Texas, held from the 3-6 October 1999.
An updated version of this paper (SPE 67616) has been published in the December 2000 edition of SPE Drilling and Completion (Volume 15, Number 4, pages 221 to 233).
A gyro error model has been produced and published in paper SPE 90408 ‘Prediction of Wellbore Position Accuracy When Surveyed with Gyroscopic Tools’ by Torgeir Torkildsen, Stein Havardstein, John Weston and Roger Ekseth.
Depth issues have been investigated and the results published as SPE 95611, “Quantification of Depth Accuracy”, which was presented at the SPE Annual Technical Conference, Dallas, 9-12 October 2005.
Up till now, the process has been explained very simply to demonstrate how angular errors combine to produce elliptical uncertainty envelopes. In practice there are several sources of error all affecting the wellbore position in different ways and the ISCWSA error model provides a rigorous mathematical approach to combining these various sources into one 3D ellipse.
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