Depending upon the found reservoir different types of wells are drilled.
Exploratory Wells
Exploratory wells which are drilled in areas where neither oil nor gas has been previously found are called “wildcats.” Those wells which strike oil or gas are called “discovery wells.” Other exploratory wells, known as “step-out” or “appraisal” wells, are drilled to determine the limits of a field following discovery, or to search for new oil- and gas-bearing formations next to, or beneath, those already known to contain the product. A well which does not find any oil or gas, or finds too little to produce economically, is called a “dry hole”.
When multiple producing formations are discovered when drilling a single well, a separate string of pipe may be run into a single well for each individual formation.
Developmental Wells
After discovery, the area of the reservoir is roughly determined with a series of step-out or appraisal wells. Developmental wells are then drilled to produce gas and oil. The number of developmental wells to be drilled is determined by the expected definition of the new field, both in size and in productivity. Because of the uncertainty as to how reservoirs are shaped or confined, some developmental wells may turn out to be dry holes. Occasionally, drilling and producing occurs simultaneously.
Geopressure/geothermal Wells
Geopressure/geothermal wells are those which produce extremely high-pressure (7,000 psi) and high-temperature (149 °C) water which may contain hydrocarbons. The water becomes a rapidly expanding cloud of hot steam and vapors upon release to the atmosphere from a leak or rupture.
Stripper Wells
Stripper wells are those which produce less than ten barrels of oil a day from a reservoir.
Multiple Completion Wells
When multiple producing formations are discovered when drilling a single well, a separate string of pipe may be run into a single well for each individual formation. Oil and gas from each formation are directed into its respective piping and isolated from one another by packers, which seal the annular spaces between the piping string and the casing. These wells are known as multiple completion wells.
Injection Wells
Injection wells pump air, water, gas or chemicals into reservoirs of producing fields, either to maintain pressure or move oil toward producing wells by hydraulic force or increased pressure.
Service Wells
Service wells include those used for fishing and wire-line operations, packer/plug placement or removal and reworking. Service wells are also drilled for underground disposal of salt water, which is separated from crude oil and gas.
