19.3.5 Relative Contribution of the Various Error Terms

For the ISCWSA#1 test well (typical North Sea Extended reach well), the plots below show the relative contribution of the various error sources to the overall ellipse dimensions. The values below have been calculated based upon revision 2 of the MWD model. All the values here are quoted in Earth referenced (NEV) frame and it must be remembered that being independent the terms are summed by the RSS method.

North Axis Errors:

East Axis Errors:

Vertical Axis Errors:

From these graphs it is easy to see that as expected, the biggest error contribution in the vertical is from SAG. For this well the north and east errors are dominated by axial interference, followed by uncertainty in the magnetic model (i.e. declination uncertainty).