A Bottom hole assembly is a component of a drilling rig

Drilling rig

A drilling rig is a machine which creates holes or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person...

. It is the lower part of the drill string

Drill string

A drill string on a drilling rig is a column, or string, of drill pipe that transmits drilling fluid and torque to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit...

, extending from the bit to the drill pipe

Drill pipe

Drill pipe, is hollow, thick-walled, steel piping that is used on drilling rigs to facilitate the drilling of a wellbore and comes in a variety of sizes, strengths, and weights but are typically 30 to 33 feet in length...

. The assembly can consist of drill collars, subs such as stabilizers, reamers, shocks, hole-openers, and the bit sub and bit.

Variations

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.