The drilling bit is connected to the lower end of the drilling string and drills the ground by transferring the energy received from the pipe. There are various types of drilling bit in oil and gas industry and selecting appropriate bit type is done by considering important factors such as cost per foot, specific energy, drilling capability, penetration rate, Drilling rig systems and etc. The correct selection of drilling bit type has a significant impact on the performance and efficiency of drilling operations. Therefore, the selection of drilling bit type can be considered as one of the multi-criteria decision issues.

The amount of energy transferred to the bit should be proportional to the hardness of the formation rocks. The material and type of drilling bit must be able to withstand the applied pressures. Drills that have a significant impact on the speed and success of drilling operations, according to the performance are designed based on the following principles:

Crushing-chipping action: The function of these bits is through creating crush and chop  in soft formations such as sticky clays and shales.

Shearing action: these types of bits penetrate rocks by tangential force and Shearing action.

Scraping the rock: the function of these bits is through cutting abrasive action