Drill bit (oil and gas)
A drill bit is a rotating apparatus attached to the end of the drill string that breaks apart, cuts or crushes the rock formations when drilling a well. It consists of two or three cones made up of the hardest of materials (usually steel, tungsten carbide, and/or synthetic or natural diamonds) and sharp teeth that cut into the rock and sediment below. The drill bit is hollow and has jets to allow for the expulsion of the drilling fluid, or "mud", at high velocity and high pressure to help clean the bit and, for softer formations, help to break apart the rock .
Drill bit types
Roller cone bits
Roller cone bits have cones that roll as the bit turns and projections in the cones gouge, scrape, and crush rock as they roll. Most roller cone bits have three cones and are called tri-cone bits. There are two basic types of roller cone bits, milled tooth bits and tungsten carbide insert bits.
Milled tooth bits, also called steel tooth bits, use steel teeth to gouge through rock. Milled tooth bits are best in softer formations and are generally less expensive than other types of bits.
Tungsten carbide insert or TCI bits have tungsten carbide treated inserts that gouge, chip, and crush rock. Tungsten carbide is one of the hardest materials known and TCI bits are capable of drilling some of the hardest and most abrasive formations.
Tungsten carbide inserts come in variety of shapes. Because of their appearance, TCI bits are often called button bits.
Fixed Cutter Bits
Fixed cutter bits do not have moving parts and it work primarily by sheering and scraping through the rock. Fixed cutter bits include Polycrystalline Diamond Compact Bit, Natural Diamonds Bits and Impregnated Bits.
Polycrystalline Diamond Compact
Polycrystalline Diamond Compact or PDC bits have tungsten carbide cutters topped with hard caps of diamond composite material. The cutters are angled and arranged to sheer channels in the rock. The diamond caps are made by heating and compressing artificial diamond grid with tungsten carbide and other metallic binders.
PDC bits come in variety of designs that can be used for an extensive range of drilling requirements. PDC bits are much more expensive than roller cone bits but can generally penetrate faster and last longer than roller cone bits, which saves drilling cost.
Natural Diamond Bits
Natural Diamond Bits have industrial grade diamonds, set in the bit surface, to create an abrasive cutting place. They're primarily used in hard or highly abrasive formations that would be more damaging to other bit types. They're not as effective in softer formations because of there smoother surface profile.
Diamond Impregnated Bits
Diamond impregnated bits had PDC cutters protruding straight out the bit body, where regular PDC bits have cutters bonded to the outside and angled to the cutting face. The impregnated design increases cutters stability and the lateral angle keeps cutters sharp as they wear.
Diamond impregnated bits are good in formations with intermittent layers of soft and hard rock because they can effectively cut through both.
There are many other types of specialty bits including bits coring, sidetracking bits, reaming bits and other applications.
- The Abrasive Wear Of Rock Drill Bit Materials, by A.P. Mouritz (Materials Research Laboratory) | I.M. Hutchings (U. of Cambridge), SPE-24222-MS, 1991
- A New Approach to Drill Bit Selection, by H. Rabia (U. of Newcastle Upon Tyne) | M. Farrelly (U. of Newcastle Upon Tyne) | M.V. Barr (British Petroleum plc), SPE-15894-MS, European Petroleum Conference, 20-22 October, London, United Kingdom, 1986
- Fluid Dynamics in a Diamond Drill Bit, by Merle H. Meyers (Shell Oil Co.) | James E. Funk (U. Of Kentucky), SPE-1696-PA, 1967