DRILLING

Once an area has been selected and the right to drill thereon has been obtained, actual drilling may begin. The most common method of drilling in use today is rotary drilling. Rotary drilling operates on the principle of boring a hole by continuous turning of a bit. The bit is the most important tool. The rest of the rig (a derrick and attendant machinery) is designed to make it effective. While bits vary in design and purpose, one common type consists of a housing and three interlocking movable wheels with sharp teeth, looking something like a cluster of gears. The bit, which is hollow and very heavy, is attached to the drill stem, composed of hollow lengths of pipe leading to the surface. As the hole gets deeper, more lengths of pipe can be added at the top. Almost as important as the bit is the drilling fluid.

Although known in the industry as mud, it is actually a repaired chemical compound. The drilling mud is circulated continuously down the drill pipe, through the bit, into the hole and upwards between the hole and the pipe to a surface pit, where it is purified and recycled. The flow of mud removes the cuttings from the hole without removal of the bit, lubricates and cools the bit in the hole, and prevents a blow out which could result if the bit punctured a high pressure formation.

The cuttings, which are carried up by the drilling mud, are usually continuously tested by the petroleum geologist in order to determine the presence of oil.