A bottom hole assembly is a set of components that used in various drilling operations. Bottom hole assemblies are located at the lower ends of drill strings, below the drill pipes. A typical assembly consists of a drill bit, collars, and various other items such as mud motors. One main purpose served by bottom hole assemblies is to provide the weight necessary for the bit to drill into whatever substance it is meant to penetrate. The bottom hole assembly is also responsible for providing the drill operator with the ability to control the direction of a well.
In the type of deep drilling operations that are commonly used for hydrocarbon exploration and recovery, there are a few common designs. These drilling operations typically make use of drill strings, which are pipes that transmit both drilling fluid and the torque necessary to rotate a drill bit. The top end of a drill string typically has the machinery necessary to pump drilling fluid, and either a top or kelly drive to provide torque for the drill bit. Pipe typically encases the string below that, in order to contain the drilling fluid. The lower end of the string consists of the drill bit, collars, and other components that are collectively known as the bottom hole assembly.
The main component of a bottom hole assembly is the drill bit, which a top or kelly drive rotates in order to penetrate rock formations. Above the drill bit a number of drill collars are typically used to provide tension on the string, and the weight that is necessary for the bit to remain in contact with the formations it has to penetrate. These collars and all of the other components are typically held together by threaded connections. When dissimilar threads must be joined together in a bottom hole assembly, components known as subs are used.
A number of other components can also be found in various bottom hole assemblies, above the drill bit. Drilling stabilizers are often included to ensure the assembly remains centered, and a downhole or mud motor can provide the drill bit with additional power in some circumstances. Mud motors can also be useful in directional drilling, though additional directional control can be provided by a rotary steerable system. Other components can be added to a bottom hole assembly if it is necessary to gather certain information about the process. A measurement while drilling (MWD) component can sending pressure, temperature, and other types of data back to the surface, and some are even capable of taking samples of formations.