How Does a Drilling Rig Work?
Man originally used drilling rigs to access underground water supplies. With the development of the automobile, oil drilling rigs became more numerous and more advanced. Today, drilling rigs operate on land and in the depths of the ocean. They search for oil, gas, water and other minerals.
Drilling Rig Basics
There are many types of well drilling rigs but all of them work under the same basic premise. A rig is designed to bore a hole into the ground to retrieve a substance. The rig uses a tool to break or grind the underground rock. The broken rock is then brought to the surface. This process is repeated to the desired well depth.
Types of Wells
Auger Rig
Auger drilling dates back over 3,000 years. These rigs use a helical screw as a bit to grind through rock by rotating the screw. The blades of the screw lift the broken rock to the surface. This type of drilling is designed for softer rock or dirt at shallower depths. Originally used for water drilling, it is now used mostly for construction, environmental and probe drilling.
Cable Tool Rig
A cable tool rig was originally used for water wells and later for oil wells in the 19th and early 20th centuries. An impact bit is secured to the rig with a metal cable. The bit is dropped into the hole, where it repeatedly hits the rock, breaking the rocks into pieces. The rock pieces are collected into a container, called a cutting basket and brought to the surface. After the basket is emptied, the bit is returned to the hole to repeat the process.
The cable tool rig is now considered obsolete, although it is still used in Africa and the Middle East for water well drilling. It takes a long time to drill with a cable tool rig, as repeated trips in and out of the hole delay the actual deepening of the hole. It also can only drill shallower wells.
Rotary Rig
The rotary drilling rig is the most commonly used rig today. It is more efficient and can be used to greater depths than the cable tool rig. A very sharp rotating drill bit is attached to a special pipe, called a drill pipe, which is then attached to a rotation tool called a rotary. The rotary is attached to a power source, called a prime mover, which is a diesel engine or electric turbine. The rotary, drill pipe and bit, when attached, are called a drill string. The drill string is attached to a swivel that suspends it in the air and allows the drill string to rotate freely. Drill pipe comes in 30-foot lengths and more lengths of pipe are added from the top as needed for depth.
One important component of the rotary drilling rig is the circulating system. The circulating system injects fluid into the well for several purposes:
- Cool and lubricate the drill bit.
- Remove rock and debris.
- Control pressure inside the well.
- Coat the well walls.
The circulating fluid is called mud, although it is made up of many chemicals and components. The mud is pumped through the swivel and down the drill pipe until it reaches the bit. As more mud is pumped in the well, it rises to the top between the drill pipe and the rock wall of the well. As it travels to the surface, it collects the rock and debris in the hole. When it reaches the surface, it is collected and filtered.
Some drilling rigs employ an air-drilling technique, which uses air, natural gas or engine exhaust as a circulating substance instead of fluid. Air drilling is less expensive and can cut the drilling time.
Periodically, in a rotary drilling rig, the bit must be changed because it is damaged, worn or there is a need for a different type of bit. At that point, the entire drill string must be removed from the hole, pipe by pipe. This is called "coming out of the hole." Since each drill pipe is 30 feet long, on a 15,000-foot well 500 pipes must be unscrewed from each other until the bit is removed. After the bit is changed, the 500 drill pipes must then be re-attached to the drill string.This is a time consuming part of the rotary drilling process.
There are numerous variations of these basic types of drilling rigs. The drilling industry is constantly revising and refining technological advances to rigs. The general concept remains the same: dig a hole and remove the contents.