The error model is designed to be a practical method that can be relatively easily implemented in software and then used by well planners and directional drillers. It is intended to be applied to a range of tools, used worldwide and accordingly attempts to give good representative survey uncertainties, without the need to model every single variation of tool or running conditions.
The model only applies to surveys run under normal industry best-practise procedures which include:
a. rigorous and regular tool calibration,
b. a maximum of 100ft survey intervals.
c. field QC checks, such as total magnetic field, gyro drifts , total gravity field and magnetic dip angle on each survey measurement,
d. the use of non-magnetic spacing for MWD surveys according to industry norms,
e. for MWD, surveys taken in a magnetically clean environment away from casing and adjacent wells.
It should be recognised that the model cannot cover all eventualities and works on a statistical basis and so does not say anything specifically about any individual survey. The results can be interpreted as meaning that if a well was properly surveyed a number of times by a variety of different tools with the same specification, then the results would be expected to be randomly distributed with a range of values corresponding to the error model uncertainty results.
The model cannot cover gross blunder errors such as user error in referencing gyros, defective tools or finger trouble entering surveys into a database. The model does not cover all variations and all possibilities in borehole surveying, For example survey data resolution is not modelled.
To qualify under the assumptions, the survey interval should be no more than 100ft. If the survey interval is greater than 100ft then strictly the model is not applicable. Hence, the error model does not include penalty terms for intervals greater than 100ft, nor does it model any improvements for shorter survey spacing.
MWD surveys which are subject to external magnetic interference will generally fail QC checks. The effect on survey accuracy of magnetic interference from adjacent wells or from casing, can vary enormously and in many instances are impossible to quantify. The error model does not attempt to determine the size of the error for any surveys which are subject to this kind of interference.
Finally, a major misconception is that the ISCWSA provides certified error models for specific survey tools. The published ISCWSA papers only define the process and equations to work from a set of error model parameters to an estimate of position uncertainty. The ISCWSA committee does not define, approve or certify the tool codes containing the actual error model magnitudes which drive the error model. These should be obtained from the survey contractor who provides the tool, since they are the ones best placed to understand the specifications and limitations of their tools.
Only in the specific cases of a standard MWD tool, or a MWD tool with an axial (short-collar) interference correction applied does the ISCWSA specify any parameter values.
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