The well is created by drilling a hole (5 to 30 inches wide) into the earth with an oil rig turning a drill bit . After the hole is drilled, a metal pipe called 'casing' is cemented into the hole. In order to get access to the hydrocarbon producing interval, the casing and cement are either perforated ('cased hole completion') or additional section of earth is drilled below the casing ('open hole completion'). In most cases several casings are set in the well, starting with large shallow casing, and then deeper casings are set in smaller holes drilled through the upper casings.
To drill the well,
- the drill bit, breaks up the earth (which is then washed out of the hole with the 'drilling mud') and extends the well;
- the pipe or drill string to which the bit is attached is gradually lengthened as the well gets deeper;
- the oil 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 drilled percussively , that is, holes were drilled simply by hammering at the earth. Very soon, the limited depths which this method could attain meant that rotary drills were introduced. Modern wells drilled using rotary drills can achieve lengths of over 10 kilometres.
Until the 1970s , most oil wells were vertical (or, more specifically, were supposed to be vertical â€” deviations introduced by different lithology and mechanical imperfections meant that most wells were at least slightly deviated). 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 larger volume with a much larger surface area (and thus a correspondingly higher productive rate. Using deviated and horizontal drilling, it has also become possible to reach reservoirs several kilometers away from the drilling place (Extended Reach Drilling), allowing to produce hydrocarbons from underneath e.g. environmentally sensitive areas or offshore close to the coast line.
After drilling the well, 'tubing' (smaller diameter pipe, from 2.5 to 7 inch diameter) is typically run into the well and packed off at the base, inside the casing. The well produces up through the tubing. This arrangement provides a redundant barrier to leaks of hydrocarbons as well as allowing damaged sections to be replaced.
Oil wells 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 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 have 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.
Modern oil wells can be extremely expensive to build and maintain, due partly to the cost of the technologies in active use today, but also to the increasingly inclement climates and harsh environments that are today being explored for oil and gas. The following is a quick comparison of average well costs for the UK Continental Shelf (UKCS), based on values from March 1998 :
|Well location||Typical cost (in millions of Â£ )|
|Northern North Sea||8â€“12|
|West of Shetlands||5â€“15|
|Southern North Sea||7â€“12|
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).
Onshore wells can be considerably cheaper, particular if the field is at a shallow depth. Here costs range from less than one million $US to $15 million for the very deep and difficult wells.
Oil companies do not generally own their own oil rigs, rather they tend to rent them from service companies. Typical rent costs for an offshore oil rig in 2000 were upwards of $160,000 per day. For a land rig upwards from $10,000 to about $35,000 per day 2005 depending on size and equipment.
Offshore platforms, the well's supporting stucture, produce artificial reefs .