The drilling of holes for exploration and extraction of crude oil and natural gas. Deep holes and high pressures are characteristics of petroleum drilling not commonly associated with other types of drilling. In general, it becomes more difficult to control the direction of the drilled hole as the depth increases, and additionally, the cost per foot of hole drilled increases rapidly with the depth of the hole. Drilling-fluid pressure must be sufficiently high to prevent blowouts but not high enough to cause fracturing of the borehole. Formation-fluid pressures are commonly controlled by the use of a high-density clay-water slurry, called drilling mud. The chemicals used in drilling mud can be expensive, but the primary disadvantage in the use of drilling muds is the relatively low drilling rate which normally accompanies high bottom-hole pressure. Drilling rates can often be increased by using water to circulate the cuttings from the hole; when feasible, the use of gas as a drilling fluid can lead to drilling rates as much as 10 times those attained with mud. Drilling research has the objectives of improving the utilization of current drilling technology and the development of improved drilling techniques and tools.
The hole direction must be controlled within permissible limits in order to reach a desired target at depths as great as 25,000 ft (7600 m). Inclined layers of rocks with different hardnesses tend to cause the direction of drilling to deviate; consequently, deep holes are rarely truly straight and vertical. The drilling rate generally increases as additional drill-collar weight is applied to the bit by adjusting the pipe tension at the surface. However, crooked-hole tendency also increases with higher weight-on-bit. A so-called packed-hole technique has been used to reduce the tendency to hole deviation. One version of this technique makes use of square drill collars that nearly fill the hole on the diagonals but permit fluid and cuttings to circulate around the sides. This procedure reduces the rate at which the hole direction can change.
In mountainous terrain, it is difficult to construct well locations over each subsurface drilling target, and from offshore drilling platforms it is necessary to drill many wells from a single surface location. For these situations, technology has been developed that permits wells to be drilled directionally from the single surface location to the desired subsurface point.
Advances in technology and the need to accomplish special objectives have led to drilling horizontal wells in deep oil or gas reservoirs. The angle of the well is successively built up in order to reach the ultimate horizontal course of the well. The program for drilling the well is developed before hand, based on the technology for actually increasing the angle and then continuing the drilling at the desired angle.
Novel drilling methods include studies of rock failure by mechanical, thermal, hydraulic, fusion and vaporization, and chemical means. Jet piercing is widely used for drilling very hard, spallable rocks, such as taconite. Other methods include the use of electric arc, laser, plasma, spark, and ultrasonic drills. See also Oil and gas well completion; Petroleum geology; Rock mechanics; Turbodrill; Well logging.
