5.2.4 NMDC Length Selection Charts

Using the formulae from SPE 11382 by S.J. Grindrod and C.J.M. Wolff, NMDC charts can be constructed for various well inclinations and azimuths and for a maximum acceptable azimuth error. The latter is taken as 0.25 degrees as the limit for good magnetic surveying practice. By varying the DIP and B for local conditions, charts can be prepared for various areas of the world.

An example chart for a bit and stabiliser BHA is given below:

The charts can be used in two ways.

1. To estimate the recommended length of NMDC for a particular situation.
2. If a different length was used, an estimate of the possible azimuth error can be obtained.

To find the recommended length of NMDC for a particular BHA, the azimuth from North or South and the inclination are used to arrive at a point on the selection chart. For example a section of a well being drilled at 60° inclination and 35° azimuth requires 24 m of NMDC.

This is demonstrated on the example chart above, with the 24 m being found by visually interpolating between the 20 m and 30 m length lines.

Where inadequate lengths of NMDC are used, (or when reviewing past surveys where insufficient NMDC was used) it is possible to estimate the resulting compass error: –

Possible Azimuth Error for length of NMDC used = Acceptable Azimuth Error x (Length required)2 / (Length used)2

Example:

If only 2 NMDC’s with a total length of 18.9 m (62 ft) were used instead of the recommended
NMDC total length of 24 m (69 ft) we have: -

Estimated possible azimuth error = 0.25 × (24)2 / (18.9)2 = 0.4 deg

Note that it is not valid to deliberately cut back on NMDC usage and plan to theoretically correct a survey by the above formulae. This is because the formula assumes pole strengths for the BHA components and actual pole strengths are not generally measured in the field.