Types of Wells

A way to classify wells is by their purpose. 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

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, 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.

Water Injectors
injecting water into the formation either 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 let alone into a fresh water source, in the case of onshore wells. Frequently, water injection has an element of reservoir management and produced water disposal.

Aquifer Producers
intentionally producing reservoir water for re-injection to manage pressure. This is in effect moving reservoir water from where it is not as useful, to where it is more useful. These wells will generally only be used if produced water from the oil or gas producers is insufficient for reservoir management purposes. Using aquifer produced water rather than sea water is due to the chemistry.

Gas Injectors
injecting gas into the reservoir often as a means of disposal or sequestering for later production, but also to maintain reservoir pressure.