Drill bit (well)

In the oil and gas industry, a drill bit is a tool designed to produce a generally cylindrical hole (wellbore) in the earth’s crust by the rotary drilling method for the discovery and extraction of hydrocarbons such as crude oil and natural gas. This type of tool is alternately referred to as a rock bit, or simply a bit. The hole diameter produced by drill bits is quite small, from about 3.5 inches (8.9 cm) to 30 inches (76 cm), compared to the depth of the hole, which can range from 1,000 feet (300 m) to more than 30,000 feet (9,100 m). Subsurface formations are broken apart mechanically by cutting elements of the bit by scraping, grinding or localized compressive fracturing. The cuttings produced by the bit are most typically removed from the wellbore and continuously returned to the surface by the method of direct circulation.

A brief survey of the types of drilling machines is useful in understanding the types of drill bits that have found use.

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 bits employ 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 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.

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Wikipedia