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Points to Ponder
From this section, you can understand why error envelopes re elliptical are big and why they are important. In the next section we will introduce the components that make up the ISCWSA error model but below are a few key lessons about uncertainty to summarise. 1. All observations are wrong. What we really need to know is roughly by how much they could be wrong. 2. Clients, and even other disciplines in our own organisation, think our surveys are right. 3. It is essential that we educate the data users and owners in how big these errors can be. 4. Nearly all components of error can be improved upon. 5. Overlapping ellipses does not mean a collision but the probability rises rapidly as they overlap. 6. When discussing with clients make sure they know the difference between financial risk and health-and- safety risk. 7. Agree the dispensation rules for close drilling well in advance and stick to them even if the client does not. Better to lose a contract than a reputation.
18. h W AE o Mod l: n oduc on
For a detailed description of the ISCWSA error model, please see Chapter 19 by Dr Andrew McGregor.
18.1 Some background to ISCWSA
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.
LINK
ISCWSA website
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