The bits and its performance are what rotary drilling is all about. When the bit is on bottom and making hole, it is making money-but only as long as it is an effective cutting tool.
To be an effective cutting tool, the bit must be in good condition. Weight must be applied to make the bit drill, and supply this weight is one function of the drill collars. The bit must be rotated, and rotation is the function of the drill stem and the rotary. Finally, the drilling fluid must cool and lubricate the bit as it removes chips and cuttings from the bottom of the hole.
Many variables affect bit performance, particularly the type of formations being drilled. These variables usually involve questions of economics, especially in the selection of a bit type that can drill most economically. Drillers want a bit that has a good rate of penetration (ROP), lasts a reasonable number of rotating hours, and drills holes the same size as the bit (true-togauge).
Essentially, the driller is looking for a bit that averages the most feet per hour and lasts the most hours possible. If the sides of the bit wear down, it will drill and undersize, or undergauge, hole. Out-of-gauge holes cause lost time for reaming, but can also stick the drill stem, cause a fishing job, and thus increase drilling costs.
In most situations, the objective is to get all the footage possible from a bit, thereby minimizing the number of round trips needed for bit changes. Situations occur in which only one or two bits are needed before pulling out for a survey running casing. For example, when making hole to set surface casing in extremely soft formations, only one bit may be needed, and occasionally, that bit may be used for several wells. Deeper drilling in harder rocks is more difficult. Trip time increases, and the driller will generally run a bit designed to drill these formations as well as to drill the surface casing cement left in the hole. In some instances, the cement may be drilled and then bits changed to drill that particular formation.
Formations vary in hardness and abrasiveness. If the bedded strata did not change, one bit best suited for that formation could be selected, and drilling ahead could begin. Usually, however, the strata are made up of alternating layers of soft material, hard brittle rocks, and hard, abrasive sections. Instead of changing bits every time a new type of formation is encountered a compromise bit that can drill through all types of formation is chosen. If drilling is taking place in a known field, information about the formations is available to the driller. If a wildcat well is being drilled, a process of trial and error will have to be followed in selecting bits.
The different types of bit that are generally available are Rock Cone bit, Diamond bit, polycrystalline diamond compact (PCD) bits and Drag bits. Roller cone, or rock, bits have cone shaped steel devices called cones that turn as the bit rotates . Most roller cone bits have three cones, although some have two and some have four. Teeth can be cut out of the cones, or very hard tungsten carbide buttons can be inserted into the cones. The teeth or tungsten carbide inserts will actually cut or gouge out the formation as the bit is rotated.
Diamond bits do not have cones or teeth. Instead, several diamonds are embedded in the bottom and sides quite efficiently. Drag bits are used to drill shallow, soft formations close to the surface. All bits have passages drilled through them to permit drilling fluid to exit.