19.3.4 MWD Error Magnitudes

The MWD model is the only case for which the ISCWSA defines error magnitudes and provides complete tool models, if only for a limited number of situations. This is because it is considered that the performance of most modern MWD tools is quite similar and a generic model is valid.

Much of the MWD paper [1] is given over to a justification of error magnitudes for the MWD error sources and a complete tool models are given both standard MWD surveys and for surveys which are corrected for axial interference for the combinations situation where the tools are run from a fixed land rig with fixed depths and for an offshore floating platform.

So effectively there are four defined tool models:

The application of survey correction techniques to improve the accuracy of MWD surveys will generally change the magnitudes of one or two error sources. So for example, running sag-correction software will reduce the magnitude of the sag error term; conducting an IFR survey will improve the knowledge of the Earth’s magnetic field and lead to reduced magnitudes for the four reference field error terms. The use of the error model as implemented in directional-drilling software is the appropriate way to evaluate the effect of these techniques and to determine an appropriate survey program for a well. However, the ISCWSA has not defined or standardized values to be used to model these.