Rotary drilling involves using a large drill at the end of a hollow pipe. The drill bit is constructed of very strong steel wheels, typically in a set of three, that are attached to the end of the pipe or a solid bit that can be encrusted with diamonds. As the pipe is spun in a circle, the rotary drilling bit turns and chews the earth and rock out of the way. Rotary drilling is able to bore much farther down through the earth than any other form of drilling.
The hollow pipe attached to the drill bit allows for fluid to be pumped into the pipe and down to the rotary drilling bit. This fluid does three things. First and foremost, it cools the drill bit, as rotary drilling through rock generates a great deal of heat. The fluid also lubricates the bit and the pipe to allow it to pass more easily through the earth and rock. Perhaps the most helpful trait of the fluid is that is helps to carry the cut stone and earth to the surface through the pipe, which allows the rotary drilling rig to continuously cut and move downward without stopping to clean the hole.
The type of well drilled stipulates what type of drilling fluid will be used. The most common type of drilling fluid is called mud—this fluid is mixed with bits of clay and helps to seal the drill casing to the earth. This is an important step, as a leaking case can cause much difficulty and damage to the area around the well. When drilling a fresh water well, the drilling crew would not use saltwater, as it could contaminate the fresh water. Conversely, when rotary drilling through underground salt deposits, fresh water used as drilling fluid could cause the salt to erode and collapse the well.
Lubrication of the drill bit is considered important. A burnt or overheated drill bit will not cut through rock as fast as a fresh and sharp drill bit. The drill bit can be changed, but this is a time-consuming effort. To change the bit, all of the drilling pipe known as the drill string must be removed section by section until the bit reaches the surface. Careful planning of the area to be drilled can prevent the drill from being forced too swiftly through the earth.