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Types of Wells: Fossil-fuel wells come in many varieties

Fossil-fuel wells come in many varieties. By produced fluid, there can be wells that produce oil, wells that produce oil and natural gas, or wells that only produce natural gas. Natural gas is almost always a by-product of producing oil, since the small, light gas carbon chains come out of solution as they undergo pressure reduction from the reservoir to the surface, similar to uncapping a bottle of soda pop where the carbon dioxide effervesces. Unwanted natural gas can be a disposal problem at the well site. If there is not a market for natural gas near the wellhead it is virtually valueless since it must be piped to the end user. Until recently, such unwanted gas was burned off at the well site, but due to environmental concerns this practice is becoming less common. Often, unwanted (or ‘stranded’ gas without a market) gas is pumped back into the reservoir with an ‘injection’ well for disposal or re-pressurizing the producing formation. Another solution is to export the natural gas as a liquid. Gas to liquid, (GTL) is a developing technology that converts stranded natural gas into synthetic gasoline, diesel or jet fuel through the Fischer-Tropsch process developed in World War II Germany. Such fuels can be transported through conventional pipelines and tankers to users. Proponents claim GTL fuels burn cleaner than comparable petroleum fuels. Most major international oil companies are in advanced development stages of GTL production, e.g. the 140,000 bbl/d (22,000 m3/d) Pearl GTL plant in Qatar. In locations such as the United States with a high natural gas demand, pipelines are constructed to take the gas from the well site to the end consumer.

Raising the derrick
Another obvious way to classify oil wells is by land or offshore wells. There is very little difference in the well itself. An offshore well targets a reservoir that happens to be underneath an ocean. Due to logistics, drilling an offshore well is far more costly than an onshore well. By far the most common type is the onshore well. These wells dot the Southern and Central Great Plains, Southwestern United States, and are the most common wells in the Middle East.

Another way to classify oil wells is by their purpose in contributing to the development of a resource. They can be characterized as:

  • wildcat wells are those drilled outside of and not in the vicinity of known oil or gas fields.
  • exploration wells are drilled purely for exploratory (information gathering) purposes in a new area.
  • appraisal wells are used to assess characteristics (such as flow rate) of a proven hydrocarbon accumulation.
  • production wells are drilled primarily for producing oil or gas, once the producing structure and characteristics are determined.
  • Abandoned well are wells permanently plugged in the drilling phase for technical reasons.

Oil extraction in Boryslav in 1909
At a producing well site, active wells may be further categorized as:

  • oil producers producing predominantly liquid hydrocarbons, but mostly with some associated gas.
  • gas producers producing almost entirely gaseous hydrocarbons.
  • water injectors injecting water into the formation to maintain reservoir pressure, or simply to dispose of water produced with the hydrocarbons because even after treatment, it would be too oily and too saline to be considered clean for dumping overboard offshore, let alone into a fresh water resource in the case of onshore wells. Water injection into the producing zone frequently has an element of reservoir management; however, often produced water disposal is into shallower zones safely beneath any fresh water zones.
  • aquifer producers intentionally producing water for re-injection to manage pressure. If possible this water will come from the reservoir itself. Using aquifer produced water rather than water from other sources is to preclude chemical incompatibility that might lead to reservoir-plugging precipitates. These wells will generally be needed only if produced water from the oil or gas producers is insufficient for reservoir management purposes.
  • gas injectors injecting gas into the reservoir often as a means of disposal or sequestering for later production, but also to maintain reservoir pressure.

Lahee classification
New Field Wildcat (NFW) – far from other producing fields and on a structure that has not previously produced.
New Pool Wildcat (NPW) – new pools on already producing structure.
Deeper Pool Test (DPT) – on already producing structure and pool, but on a deeper pay zone.
Shallower Pool Test (SPT) – on already producing structure and pool, but on a shallower pay zone.
Outpost (OUT) – usually two or more locations from nearest productive area.
Development Well (DEV) – can be on the extension of a pay zone, or between existing wells (Infill).

Source: Company Management