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Non-Magnetic Drill Collars

Requirements relating to NMDC use when surveying with magnetic instruments..

NMDC Length

Every magnetic compass placed in a steel BHA will exhibit an azimuth error. This is caused by the interference between the magnetic field of the Earth and any BHA components which cannot be demagnetised (e.g. mudmotors).

To reduce this error to an acceptable level, the correct length of non-magnetic drill collars (NMDC) must be run in every BHA in which magnetic surveys are to be taken.

The selection of the correct length of NMDC and optimum compass spacing will keep the magnetic compass (azimuth) error typically below 0.25°. The NMDC length requirement varies with both the azimuth and inclination of the well.

The calculations for NMDC length shall be carried out using Surveying Contractor software.

Note: Such NMDC’s shall be a minimum of 30 ft long.

Compass Spacing

The optimum compass spacing is dependent on the components in the BHA below the NMDC. The guidelines for spacing of the compass from the bottom of the NMDC section as a percentage of the total continuous length of NMDC’s are:

bit + stabiliser       -   30% up from base of NMDC section

bit + mudmotor     -   at least 40% up from base of NMDC section.

Note: The compass should not be placed within 3 ft of a connection. If tandem instruments are run, the bottom compass should be spaced according to the requirements detailed above.

Magnetic Interference

Magnetic interference calculations based on region and well orientation shall be performed by the Directional Contractor to determine the minimum length of non- drillstring material required for clean magnetic surveys.

The calculation for axial magnetic interference from the drillstring for magnetic survey tools shall be carried out using the Surveying Contractor software.

In practice this value is known for the Company operational area, and need only be re-calculated if a different well orientation is planned.

Typical Configurations

Vertical Wells

Configurations vary. Typically, vertical wells require:

  • one NMDC for magnetic single shots or multi-shots or:
  • one NMDC above MWD when it is run

 

Deviated Wells

Typically, deviated wells require:

low angle (< 15° inclination):

  • 8ft NM (pony) DC below MWD plus 1 NMDC above MWD

 

moderate and high angle (16° - 60° inclination) with mudmotor

  • one NMDC below MWD plus 2 NMDC above MWD

moderate and high angle (16° - 60° inclination) without mudmotor

  • 16ft NW (pony) DC below MWD plus 2 NMDC above MWD.