A Drill Bit is a tool attached to the bottom of the drill string (assembly of drill pipe and tools) to excavate the earth's rock. Rotation of the drill string and weight provides power to the drill bit that crushes, scraps or grinds the rock at the bottom of the well as drilling progresses. Due to different formations, the drill bit must be changed after a period of time during which drilling operations must stop.
Drill bits are of various sizes, ranging from 3.5 inches to 30 inches, with bigger sizes being extremely rare. Drill bits are broadly divided into the following categories.
Drill bits of each type may be of various design and geometry, to cater for a particular field application or drilling practices. Some examples are outlined below:
Roller Cone (Tricone) bits are steel body bits with cutters. The cutters (cones) roll at the bottom of the well, together with the rotation of the drill string. Roller cone bits are mostly used in soft and sticky formations or for top-hole drilling. Two major types of roller cone bits are:
- Milled-tooth (MT) - conical cutters (cones) with steel teeth that are fabricated as an integral part of the bit. The teeth vary in size and shape. For soft formations, the teeth are long and widely spaced. For harder formations, the teeth are short and positioned very close to each other.
- Tungsten carbide inserts (TCI) - conical cutters (cones) with tungsten carbide inserts around them. The size of inserts varies depending on application. In soft formation, chisel shaped and long inserts are used, whereas for harder rocks, rounded and short-extension inserts are more effective.
Fixed Cutter bits consist of steel or matrix body and cutting elements. Cutters work like an excavator, by shearing the rock. Cutting elements are fixed and these drill bits do not have any moving or rotating parts. Two types of fixed cutter bits are:
- Polycrystalline Diamond Compact (PDC) - steel or matrix body bits with fixed Polycrystalline Diamond Compact blades.
- Diamond Bits are steel or matrix body bits with fixed synthetic or natural diamond blades. These drill bits are made-to-order and designed for specific applications.
PDC bits have become almost a default selection, unless the expected formation is very sticky or too hard, where other types of bits are used.
Matrix is a very strong composite material whereby tungsten carbide bonded with other metals is used to achieve the very high hardness and high resistance to abrasion and erosion. While the matrix material can hold a high compressive load, impact loading makes the matrix collapse. Matrix PDC bits are preferred over steel PDC bits, where body erosion is a concern.
Diamond-Impregnated is PDC type of bits but with fully embedded diamonds forming a solid matrix. This type of drill bits are used in hard and abrasive formations and proved to be extremely efficient, and able to drill effectively in soft formations as well. Only matrix-body is used for diamond-impregnated bits. These drill bits are made-to-order and designed for specific applications.
Depending on the rock, a different type of drill bit can be used in any given well, with the selection criteria being driven by the type of formation, temperature, well inclination and logging programmes. Association of Drilling Contractors (IADC) has developed a comprehensive classification system that allows engineers to streamline the selection and categorization process.
Below are some key data on drill bits and the difference between them:
Generally, the cost of drill bits is less than 3% of the well cost. However, the performance of the drill bit is vital to drilling a successful well