Types of Drill Bits
Drill bits are broadly classified into two main types according to their primary cutting mechanism. Rolling cutter bits drill largely by fracturing or crushing the formation with “tooth” shaped cutting elements on two or more cone-shaped elements that roll across the face of the borehole as the bit is rotated. Fixed cutter bitsemploy a set of blades with very hard cutting elements, most commonly natural or synthetic diamond, to remove material by scraping or grinding action as the bit is rotated.
Modern commercial rolling cutter bits usually employ three cones to contain the cutting elements, although two cone or (rarely) four cone arrangements are sometimes seen.These bits mainly fall into two classes depending on the manufacture of the cutting elements or “teeth”. Steel-tooth bits have cones that have wedge-shaped teeth milled directly in in the cone steel itself. Extremely hard tungsten carbide material is often applied to the surfaces of the teeth by a welding process to improve durability.Tungsten carbide insert (TCI) bits have shaped teeth of sintered tungsten carbide press-fit into drilled holes in the cones. Some types of steel-tooth bits also have TCI elements in addition to the milled teeth. The cones rotate on roller or journal bearings that are usually sealed from the hostile down-hole drilling fluid environment by different arrangements of o-ring or metal face seals. These bits usually also have pressure compensated grease lubrication systems for the bearings.
The first commercially successful rolling cutter drill bit design was disclosed in US patents granted to Howard R. Hughes, Sr. on August 10, 1909. This bit employed two conical steel rolling elements with milled teeth that engaged the formation, when the device was rotated, to produce the cutting action. This design represented a significant improvement in drilling performance over the so-called “fish tail” scraper type bits commonly used in rotary drilling at the time, and over the next two decades, rotary drilling with rolling cutter bits largely replaced all other drilling methods in the oilfield. The significance of the Hughes Two-Cone Drill Bit was recognized on its 100th “birthday” when it was designated a Historic Mechanical Engineering Landmark by the American Society of Mechanical Engineers (ASME).
Fixed cutter bits were the first type of drill bit employed in rotary drilling, and they are mechanically much simpler than rolling cutter bits. The cutting elements do not move relative to the bit; there is no need for bearings or lubrication. The most common cutting element in use today is the polycrystalline diamond cutter (PDC), a sintered tungsten carbide cylinder with one flat surface coated with a synthetic diamond material. The cutters are arranged on the blades of the bit in a staggered pattern with the diamond coated cutter surface facing the direction of bit rotation to provide full coverage of the borehole bottom. Other fixed cutter bits may employ natural industrial-grade diamonds or thermal stable polycrystalline diamond (TSP) cutting elements.
There is also currently available, a “hybrid” type of bit that combines both rolling cutter and fixed cutter elements.