The bottom hole assembly commonly abbreviated BHA, is a drill string component. It usually resides above the drill bit and below the drill pipe. In some instances you can consider the drill bit as part of the BHA. The primary component of the BHA is the drill collar.

The Purpose

–        To protect the drill pipe from excessive bending and torsional load

–        To help with directional and deviation control

–        To help drill straighter, more vertical holes

–        To reduce the severity of doglegs, key-seats and ledges

–        To help ensure that casing can be run safely

–        To increase the drill bit performance

–        To reduce rig and string vibration

–        As a tool in fishing, testing and workovers

Types

Slick – Only drill collars. Cheapest assembly, with the least risk, with regard to fishing and recovery.

Pendulum – Designed to drill holes more vertically and drop inclination. Most BHA theories apply to holes that have an inclination of less the 20°. For this case it is best t consult tables by Lubinsk and Woods.

Packed – Designed to drill straight holes and to reduce doglegs, keyseats and ledges. It provides the best assurance that casing can be run into the hole. Such an assembly is expensive and carries the highest risk with respect to fishing and recovery. The theory comes from that developed by Hoch.

Directional – Designed to turn the hole in a chosen directional or inclination or to maintain a selected course. Based on the principle of levers and fulcrums.

(Mitchell 1995)

Bibliography

Mitchell, Bill. Advanced Oilwell Drilling Engineering. 10 Edition. Richardson, Texas: Society of Petroleum Engineers of the AIME, 1995.