These error sources are multiple and varied. In the ISCWSA error model there are dozens of error sources. Each will affect a given survey station in different way. For example the accuracy with which we know our magnetic north would be a simple azimuth effect. This will not have any effect on either inclination or measured depth observations. “Weighting Functions” are used to determine the effect any error source will have on MD, inclination and azimuth respectively. We say that the weighting functions for a compass reference error are (0, 0, 1) i.e. they have no effect on MD, no effect on inclination and a full effect on azimuth.
However a tool misalignment due to a bent housing in the assembly might affect inclination or azimuth depending on the toolface. In this case the weighting function would be (0, cos(Toolface), sin(Toolface)). A drillpipe stretch error would have a weighting function of (1, 0, 0) as you might expect and a BHA sag correction error would have a weight function of (0, 1, 0). The ISCWSA error model SPE paper 67616 (from the ISCWSA website – iscwsa.org) details all the weighting functions for all the error sources.
The next step is to determine the effect of the observation error on the well path position. In this diagram you can see that a measured depth error could affect North East and TVD.
But an azimuth error only affects North and East, so we get the following:
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