BHA

Bottom Hole Assembly, also known as 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 collar, 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, measurements-while-drilling tools, logging-while-drilling tools, and other specialized devices.