A Bottom hole assembly is a component of a drilling rig. It is the lower part of the drill string, extending from the bit to the drill pipe. The assembly can consist of drill collars, subs such as stabilizers, reamers, shocks, hole-openers, and the bit sub and bit.


The bottom-hole assembly (BHA) can be:

  1. slick with no stabilizers
  2. a pendulum bottom-hole assembly for reducing the deviation of a well
  3. packed with stabilizers for a straight hole
  4. a fulcrum for increasing the deviation of the well

Bottom-hole assemblies are also described as, a) specialized, such as articulated, b) steerable, and c) oriented or non-oriented.