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19. h W AE o Mod ls:Expl n on nd ynh ss
This chapter is included courtesy of Andrew McGregor of Tech21 Engineering Solutions, from a paper produced for ISCWSA. The text and associated appendices are taken from the paper: The ISCWSA Error Models: An Explanation and Synthesis, version 11 .
19.1 Introduction
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 position uncertainty around a wellbore trajectory. However, since many different factors contribute to the final position uncertainty it is not a trivial matter to determine these bounds. The Industry Steering Committee for Wellbore Survey Accuracy (ISCWSA) has produced an error model in an attempt to quantify the accuracy or uncertainly of downhole surveys. The error model is a body of mathematics for evaluating the uncertainty envelope around a particular survey. Originally the error model dealt with only MWD surveys, but it has later been extended to also cover gyro surveys. This document sets out to provide an overview and understanding of these models. The full details can be found in two SPE papers; SPE-67616 and SPE-90408 (see references below). There have also been successive revisions to both models, details of which can be obtained from the ISCWSA website iscwsa.org The error model identifies a number of physical phenomena which contribute to borehole survey errors and provides a mathematical framework for determining in numeric terms the uncertainty region around a particular survey. Typically this error model will be implemented in directional drilling software. The user will select the appropriate tool model for the survey tool that has been run and the error results will be used in anti-collision or target sizing calculations.
Figure 84: ISCWSA error model schematic.
In simple terms, the error model is a means to start from a gyro drift specification in deg/hr or a tool misalignment in degrees and evaluate how that error effects the borehole survey measurements of inclination, azimuth and measured
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