Well completion

Casing. If the preliminary tests show that one or more of the formations in the bore hole will be commercially productive, the well must be prepared for the continuous production of the oil or gas. First, a large outside pipe, or casing, slightly smaller in diameter than the drill hole, is inserted to the full depth of the well. A cement slurry is forced between the outside of the casing and the inside surface of the drill hole. When set, this cement forms a seal so that fluids cannot pass from one portion of the well to the other through the bore hole. The casing is usually about 23 centimeters (9 inches) in diameter. It creates a permanent well through which the productive formations may be reached. After the casing is in place, a production string of smaller (about eight centimeters [three inches] in diameter) tubing is extended from the surface to the productive formation with a packing device to seal the productive interval from the rest of the well. If multiple productive formations are found, as many as four production strings of tubing may be hung in the same cased well. If a pump is needed to lift oil to the surface, it is placed on the bottom of the production string.

Since the casing is sealed against the productive formation, opening must be made to allow the oil or gas to enter the well. A down-hole perforator uses an explosive to shoot holes through the casing and cement and into the formation. The perforator tool is lowered through the tubing on a wire line. When it is in the correct position, the charges are fired electrically from the surface. Such perforating will be sufficient if the formation is quite productive. If not, an inert fluid may be injected into the formation at pressures high enough to fracture the rock around the well and thus open more flow passages for the petroleum. In early times, nitroglycerin was exploded in the well bore for the same purpose. An acid that dissolves rock may be used in place of the inert fluid.

Christmas tree valves. When the subsurface equipment^: is in place, a network of valves (called a Christmas tree) is installed on the surface, arranged so that flow from the well may be regulated and measured and so that tools to perform subsurface work may be introduced through the tubing. Christmas trees may be very simple, such as those found on a low-pressure well that must be pumped, or they may be very complex, as in the case of a high-pressure well with multiple producing strings.

Auxiliary apparatus. Artificial lift. Many oil wells do not have a formation pressure high enough to push the oil (which weighs about 0.7 kilogram per liter [six pounds per gallon]) to the surface. In these cases some artificial means for lifting the oil to the surface must be installed. The most common installation involves a motor and «walking beam» (like a seesaw) on the surface that operates the pump on the bottom of the production string. A chain of solid metal rods connects the beam and the pump. Another method, called gas lift, uses the buoyancy of a bubble of gas in the tubing to push the oil to the surface. A third type of artificial lift forces some of the produced oil down the well at high pressure to operate a pump at the bottom of the well. Even though an oilfield may have enough pressure to produce naturally at first, artificial lift will usually be required in the later stages of production. Gas wells that produce little or no liquid do not need artificial lift devices.

Surface equipment. Water often flows into the well along with oil and gas. Surface equipment separates the water from the petroleum and separates the oil and the gas for storage and transportation. The production from a number of wells also must be gathered at a central point for distribution. A settling tank is usually used to separate the water, although either heat or chemical treatment may be necessary to break down emulsions. Gas is separated from the oil by allowing the produced stream to pass through a series of pressure-reducing valves and removing the liberated gas to storage. The remaining crude oil is kept at normal pressures in cylindrical tanks after being brought through pipelines from the individual wells. Gas is usually piped directly to a central processing plant (called a gasoline plant), where remaining liquids are removed before it is fed to the consumer pipeline.