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depth at a particular survey station. The model then propagates and combines these error contributions, taking into account all of the important physical errors, at all of the survey stations from all of the survey runs within a well. The final results are error ellipse dimensions, in metres or feet, within which the actual wellbore is expected to be found to a specified level of confidence. It is common to talk about the MWD model and the gyro model, but in fact these both of these models share a common framework for describing and propagating the errors. This document will present some of the background information on the ISCWSA models before presenting this basic mathematical framework on which they are based. The MWD model is described in some detail, followed by a discussion of the gyro error model. There follows a section which is concerned with implementation details, which will mainly be of interest to software developers. The last section discusses standardisation of the model. There is some maths in these early sections but it should still be possible to follow the thread of the discussion without worrying too much about the details of the equations. For those with a thirst for more, four appendices present in detail the mathematics of the framework and all of the weighting function equations. To follow the discussion, the reader should at least be familiar with the basic concepts of borehole surveying. A good starting point would be an understanding of chapters 5 and 10 of this book which describe the principles of MWD and gyro tools and also chapters 18 and 20 which present an introduction to error modelling and error propagation.
The sections following 19.1.1 in the previous edition have been moved to a new appendix at the end of the book.
20. An coll s on chn qu s This section deals with the survey aspects of Anti-collision monitoring and does not venture into the various mitigation methods such as the use of jetting or non-aggressive bits to minimise the impact of collision. There are techniques available to monitor the proximity of adjacent wells by measuring the external magnetic interference from nearby casing but these are beyond the scope of this document.
20.1 Minimum Separation Methods and Limits
The point in an adjacent well at which minimum separation occurs is where the separation vector hits the well at right angles. This is determined by conducting a 3D closest approach scan from the planned well. Relics of early well planning software are scans that can be done either at right angles or in horizontal planes from the planned well. Both of these alternatives should be avoided as they do not represent the true geometrical separation between the wells. Figure 84 illustrates this fact. In particular, scanning at right angles to the well bore can completely miss a vertical appraisal well when drilling horizontally. Best policy is that only 3D closest approach will be used for the definitive anti-collision report. Clients may also require horizontal or perpendicular scans to use in matching against their checking software.
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