In the petroleum industry, a field is an area underlain without substantial interruption by one or more reservoirs of commercially valuable oil or gas, or both. A single reservoir (or group of reservoirs which cannot be separately produced) is a pool. Several pools separated from one another by barren, impermeable rock may be superimposed one above another within the same field. Pools have variable areal extent. Any sufficiently deep well located within the field should produce from one or more pools. However, each well cannot produce from every pool, because different pools have different areal limits.
Development of a field includes the location, drilling, completion, and equipment of wells necessary to produce the commercially recoverable oil and gas in the field.
General considerations
Oil and gas production necessarily are intimately related, since approximately one-third of the gross gas production in the United States is produced from wells that are classified as oil wells. However, the naturally occurring hydrocarbons of petroleum are not only liquid and gaseous but may even be found in a solid state, such as asphaltite and some asphalts.
Where gas is produced without oil, the production problems are simplified because the product flows naturally throughout the life of the well and does not have to be lifted to the surface. However, there are sometimes problems of water accumulations in gas wells, and it is necessary to pump the water from the wells to maintain maximum, or economical, gas production. The line of demarcation between oil wells and gas wells is not definitely established. Most gas wells produce quantities of condensable vapors, such as propane and butane, that may be liquefied and marketed for fuel, and the more stable liquids produced with gas can be utilized as natural gasoline.
Production methods in producing wells
The common methods of producing oil wells are (1) natural flow; (2) pumping with sucker rods; (3) gas lift; (4) hydraulic subsurface pumps; (5) electrically driven centrifugal well pumps; and (6) swabbing.
However, most wells are not self-flowing and various lifting methods must be employed. Approximately 90% of the wells made to produce by some artificial lift method in the United States are equipped with sucker-rod–type pumps. In these the pump is installed at the lower end of the tubing string and is actuated by a string of sucker rods extending from the surface to the subsurface pump. The two common variations are mechanical and hydraulic long-stroke pumping. Other lifting mechanisms are the gas lift, hydraulic subsurface pumps, swabs, bailers, jet pumps, and sonic pumps. See also Oil and gas well completion.
Production instruments
The commoner and more important instruments required in petroleum production operations are the following:
Gas meters, which are generally of the orifice type, are designed to record the differential pressure across the orifice, and the static pressure.
Recording subsurface pressure gages small enough to run down 2-in. (3-cm) ID (inside diameter) tubing are used extensively for measuring pressure gradients down the tubing of flowing wells, recording pressure buildup when the well is closed in, and measuring equilibrium bottom-hole pressures.
Subsurface samplers designed to sample well fluids at various levels in the tubing are used to determine physical properties.
Oil meters of various types are utilized to meter crude oil flowing to or from storage.
Dynamometers are used to measure polished-rod loads.
Liquid-level gages and controllers are used. They are similar to those used in other industries, but with special designs for closed lease tanks.
