Scope
This document details the mathematical framework underpinning the ISCWSA error model for wellbore positioning. The aim is to define the current version of the error model mathematics in one concise document and as such, it brings together material that was previously available in a number of SPE papers and ISCWSA documents. This document is intended for implementers and those who wish to understand the details of the model rather than for users of the model’s results. A familiarity with the basic concepts of borehole surveying is assumed.
The document is broken down into twelve sections.
Firstly there is an introduction and overview of the constituent elements of the ISCWSA error model and some comments on what the model does and does not include. Secondly the derivation of the error model mathematics is described. There then follows some guidance for implementers which summarises the core model section. Then particular details of the MWD and gyro models are discussed. Finally, the ISCWSA test wells are specified.
Background
Like all measurements, borehole surveys are subject to errors and uncertainties which mean that a downhole survey result is not 100% accurate. For many applications, such as anti-collision and target sizing, it is very important to be able to quantify the uncertainty in position along a wellbore. However, since many different factors contribute to the final position uncertainty, determining these bounds is not a trivial matter.
The Industry Steering Committee for Wellbore Survey Accuracy (ISCWSA) (also known as the SPE Wellbore Positioning Technical Section) has developed an error model in an attempt to quantify the accuracy or uncertainty of downhole surveys. This error model consists of a body of mathematics for evaluating the uncertainty envelope around the survey. The aim is to provide a method of evaluating well bore position uncertainties based on a standardised and generalised set of equations, which will cover most scenarios and which can be implemented in a consistent manner in well planning and directional software.
The model starts from identified physical phenomena which contribute to survey errors, and then evaluates how these phenomena effect the survey measurements at each station and how these errors then build up along a survey leg and ultimately along the entire wellbore. Typically the mathematics are implemented in directional drilling software in which the user selects the appropriate tool model for use, along with the wellbore surveys or plan in order to obtain an uncertainty or anti-collision report.
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Definition of the ISCWSA Error Model - Revision 4.3