Bottom hole assembly (BHA) design for directional control - PetroWiki

Petrowiki 

The bottom hole assembly (BHA) is a portion of the drillstring that affects the trajectory of the bit and, consequently, of the wellbore. The BHA design objective for directional control is to provide the directional tendency that will match the planned trajectory of the well. The bit side force is the most important factor affecting the drilling tendency. The direction and magnitude of the bit side force determine the build, drop, and turn tendencies. Note: Rotary Steerable Assemblies are a notable exception to the comments below and are used as directional assemblies that can be steered and are used to build, drop, or hold angle and can be controlled from surface.