Th e drill string assembly consists primarily of the kelly, drill pipe, bottom hole assembly (BHA), and drill bit. The drilling fluid and rotational power are transmitted from the surface to the bit through the drill string.
Below Figure-1 shows the usual arrangement of drill string components and bit. The drill pipe section contains conventional drill pipe, and heavy weight pipe. The drill pipe is attached with a square or hexagonal pipe called kelly at the upper end of the drill string. Th e BHA may contain the following items such as: i) drill collars, ii) stabilizers, iii) jars, iv) reamers, v) shock subs, and vi) bit sub. In addition, the drill string may include shock absorbers, junk baskets, drilling jars, reamers, and other equipment. There are some special tools in the BHA or drill pipe, which may include monitor-while-drilling (MWD) tools, and drill stem-testing tools. Finally there exists drill bit at the lower end of the drill string. Heavy walled large-diameter drill collars furnish bit load. >> More Information <<
The drill bit is attached to the drill collars by means of a bit sub. For an eff ective rock cutting, the lower part of the drill collar is stacked onto the drill bit to provide the WOB. The drill cuttings generated by the rock bit are removed from the bottom of the hole by the drilling fl uid, which is circulated inside the drill string and through the drill bit into the annular space between the drill string and the bottomhole wall. Stabilizers are placed above the bit to control the direction in which the drill bit penetrates the formation. Downhole motors with bent subs and rotary-steerable tools are also used for controlling the direction in which the bit drills.
Figure 1: Components of the drill string.