1 Bridge: an obstruction in the borehole usually caused by the borehole wall caving in.

2 Building assembly: a BHA specially designed to increase the inclination (drift angle) of the wellbore.

3 Build up rate: the rate at which drift angle is increasing as the wellbore is being deviated from vertical. Usually measured in degrees per 1 foot drilled.

4 Build up section: that part of the wellbore’s trajectory where the drift angle is increasing.

5 Bumper sub: a drilling tool,placed in the BHA, consisting of a short stroke slipjoint which allows a more constant WOB to be applied when drilling from afloating rig.

6 Cable tool drilling: an earlier method of drilling used before the introduction of modern rotary methods. The bit was not rotated but reciprocated by means of a strong wire rope.

7 Cap rock: an impermeable layer of rock overlying an oil or gas reservoir and preventing the migration of fluids.

8 Cased hole: that part of the hole which is supported by a casing which has been run and cemented in place.

9 Casing: large diameter steel pipe which is used to line the hole during drilling operations.

10 Casing head Housing: a large recepticle which is installed on top of the surface casing string. It has an upper flanged connection. Once it is installed it provides: a landing shoulder for the next casing string: and a flanged connection for the BOP stack to be connected to the well.