Bottom hole assembly:

(bha) the lower portion of the drillstring, consisting of (from the bottom up in a vertical well) the bit, bit sub, a mud motor (in certain cases), stabilizers, drill collars, heavy-weight drillpipe, jarring devices (“jars”) and crossovers for various threadforms.

The bottomhole assembly must provide force for the bit to break the rock (weight on bit), survive a hostile mechanical environment and provide the driller with directional control of the well.

Oftentimes the assembly includes a mud motor, directional drilling and measuring equipment, measurementswhile- drilling tools, logging-while-drilling tools and other specialized devices.

A simple bha consisting of a bit, various crossovers, and drill collars may be relatively inexpensive (less than $100,000 us in 1999), while a complex one may cost ten or more times that amount.