Definition - What does Bottom Assembly mean?
The name itself indicates the assembly (equipment of specific function) that is present in the bottom of the string, i.e., all the parts below the drill pipe are simply known as BHA.
Parts and the length of BHA keep on varying depending on the type of operation performed in the well. Generally the length of a BHA may vary from one meter to 200 meters
Petropedia explains Bottom Assembly
During a drilling operation, BHA will contain a bit to drill the formation, a bit sub to connect the bit to a drill collar or drill pipe and X-over’s to connect various threads of pipes; this combination makes a simple BHA. The weight of BHA should be enough to break the formation rock and withstand the pressure and temperature conditions of a well. Technical properties like Hole shape, direction of the hole (vertical well or horizontal well) and characteristics of the well depend on BHA.
Depending on the type of operation required, simple BHA is combined with other equipment like mud motors, stabilizers, hole-openers, reamers, scrappers, measurements-while-drilling tools, logging while drilling tools and other specialized devices which make the BHA.
During the reservoir testing and production phase the BHA contains safety valves, circulation valves, X-Over’s and gas injection system connected to the production tubing