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Types of Oil Wells

Oil 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 it undergoes pressure reduction from the reservoir to the surface. Unwanted natural gas can actually be quite 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 wellsite, but due to environmental concerns this practice is becoming less and less common. Often, unwanted (or 'stranded'; gas without a market) gas is pumped back into the reservoir with an 'injection' well for disposal or repressurizing the producing formation. Another solution is to export the natural gas as a liquid. Of course, in locations such as the United States with a high natural gas demand, pipelines are constructed to take the gas from the wellsite to the end consumer.

Another obvious way to classify oil wells is by land or offshore wells. There really is very little difference in the well itself; an offshore well simply targets a reservoir that also happens to be underneath an ocean. Also, due to logistics, drilling an offshore well is far more costly than an onshore well. By far the most common type of well is of the onshore variety. Another way to classify oil wells is by their purpose in contributing to the development of a resource. They can be characterized as:

• production wells when they are drilled primarily for producing oil or gas, once the\producing structure and characteristics are established
• appraisal wells when they are used to assess characteristics (such as flowrate) of a proven hydrocarbon accumulation
• exploration wells when they are drilled purely for exploratory (information gathering) purposes in a new area
• wildcat wells when a well is drilled, based on a large element of hope, in a frontier area where very little is known about the subsurface. In the early days of oil exploration in Texas, wildcats were common as productive areas were not yet established. In modern times, oil exploration in many areas has reached a very mature phase and the chances of finding oil simply by drilling at random are very low. Therefore, a lot more effort is placed in exploration and appraisal wells.

At a producing well site, active wells may be further categorized as: