All rotary BHA’s have a tendency to build and directional control is maintained by selecting a BHA which gives the desired build up rate (BUR).

Due to the strong build tendency, it is preferable to err on the low side of vertical section as it is it will always be simpler to make a build correction than a drop correction. The maximum desired BUR is 2.1/2 to 3 degrees / 100 ft. The abrasive nature of the formation is such that stabiliser gauge can wear sufficiently during the course of a bit run to significantly change the BHA behaviour.

Deviation Control

Only the lower 60 to 90 ft of the BHA provides deviation control. Additional weight is provided by adding more drill collars

Additional stabilisers are provided to provide stand off from the well bore and to prevent drill collar ‘wobble’ during drilling.

Building and Maintaining Hole Inclination

Some fields have strong formation dips and local knowledge and experience is used to a great degree in selecting BHA’s to build or maintain hole inclination.

Such assemblies are more susceptible to the azimuth tendencies imposed by such strong dipping formations. Stabiliser sizes should be varied at the discretion of the Directional Contractor depending upon conditions encountered in individual wells.

Weight of BHA in Air

The following formula is used when calculating the weight of BHA in air:

Note: The Safety Factor will depend upon the type of BHA stabilisation and can    vary from 50% to 90% depending upon the drilling situation.