The key parts of any oil well are: a) the drill bit, which fractures the earth and extends the well; b) the pipe or drill string to which the bit is attached and which is gradually lengthened as the well gets deeper; and c) the rig which fulfills the role of superstructure bearing the load of the drill string and also contains machinery to rotate or percuss the drill string.
The earliest oil wells were percussive, that is, holes were drilled simply by hammering at the earth. Very soon, the limited depths which this method can attain meant that rotary drills were introduced. Modern wells drilled using rotary drills can achieve lengths of over 10 kilometres.
Until recently, all oil wells were vertical (or, more specifically, were supposed to be vertical — deviations introduced by different lithologies and mechanical imperfections meant that most wells were at least slightly sub-vertical). However, modern technologies allow strongly deviated wells which can, given sufficient depth, actually become horizontal. This is of great value as the reservoir rocks which contain hydrocarbons are usually horizontal, or sub-horizontal. A well, therefore, which passes along a reservoir (rather than through it, as a vertical well must) can tap a much larger volume.
Oil wells are also known as production wells; wells drilled purely for exploratory purposes (which aren't, therefore, intended to actually produce oil or gas) are known as exploration wells. An appraisal well is used to assess characteristics of a proven hydrocarbon accumulation, such as its flow-rate. A wildcat is a well 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 the abundance of oil in the subsurface meant that the chances of finding it were high. In modern times, oil exploration onshore 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.
| Well location | Typical cost (in millions of £) |
|---|---|
| Northern North Sea | 8–12 |
| West of Shetlands | 5–15 |
| Southern North Sea | 7–12 |
| Irish Sea | 2–3 |
These costs are exclusive of any testing (i.e. flow rate testing) and are clearly extremely high. The absolute cost is largely a reflection of the remoteness of the location being drilled, hence the relative cheapness of the Irish Sea (shallow water, close to coast) in comparison to the West of Shetlands (deep water, long way from coast and other facilities).
Oil companies do not generally own their own oil rigs, rather they tend to rent them from service companies. Typical rent costs for an oil rig in 2000 were upwards of £90,000 per day.