A drill bit is what actually cuts into the rock when drilling an oil or gas well. Located at the tip of the drillstring, below the drill collar and the drill pipe, the drill bit is a rotating apparatus that usually 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.
In contrast to percussion drilling, which consists of continuously dropping a heavy weight in the wellbore to chip away at the rock, rotary drilling uses a rotating drill bit to grind, cut, scrape and crush the rock at the bottom of the well. The most popular choice for drilling for oil and gas, rotary drilling includes a drill bit, drill collar, drilling fluid, rotating equipment, hoisting apparatus and prime mover.
The prime mover is the power source for the drilling, while the hoisting equipment handles lifting the drill pipe to either insert it into the well or lift it out of the well. Rotating equipment is what sets the whole system in motion. Before the early 1900s, drilling equipment was spun using livestock and a wooden wheel, but now, the rotating equipment is put in motion by a rotary table, which is connected to a square-shaped hollow stem, called a Kelly. Connected to the Kelly is the drill collar, which puts pressure and weight on the drill bit to make it drill through the rock and sediment. Capping off the drillstring is the drill bit, and encompassing the drilling process is drilling fluid, which helps to provide buoyancy to the drill string, lubricate the drilling process and remove cuttings from the wellbore.
Types of Drill Bits
There are a number of different types of drill bits. Steel Tooth Rotary Bits are the most common types of drill bits, while Insert Bits are steel tooth bit with tungsten carbide inserts. Polycrystalline Diamond Compact Bits use synthetic diamonds attached to the carbide inserts. Forty to 50 times stronger than steel bits, Diamond Bits have industrial diamonds implanted in them to drill extremely hard surfaces. Additionally, hybrids of these types of drill bits exist to tackle specific drilling challenges.
Various drilling designs are also employed for different results, including core bits, which gather formation cores for well logging; mill bits, which help to remove cuttings from the well; and fishtail bits, which enlarge the drill hole above the drill bit.
Different configurations work better on different formations; so a number of different drill bits may be inserted and used on one well. Additionally, drill bits have to be changed due to wear and tear. Drilling engineers choose the drill bits according to the type of formations encountered, whether or not directional drilling is required, for specific temperatures, and if well logging is being done.
When a drill bit has to be changed, the drill pipe (typically in 30-feet increments) is hoisted out of the well, until the complete drill string has been removed from the well. Once the drill bit has been changed, the complete drill string is again lowered into the well.