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DRILLING AN OIL WELL

The earliest oil wells were drilled percussively (cable-tool drilling), that is, holes were drilled simply by hammering at the earth. Very soon, the limited depths which this method could attain meant that rotary drilling was introduced. Modern wells drilled using rotary drills can achieve lengths of over 12,000 meters / 38,000 feet.

Until the 1970s, most oil wells were vertical (or, more specifically, were supposed to be vertical - deviations introduced by different lithology and mechanical imperfections meant that most wells were at least slightly deviated). However, modern technologies (directional drilling) 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 production 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.

Drilling

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 slightly smaller than the hole size (called a 'casing') is run into the hole. The outside of the casing is then bonded and secured to the hole with cement. The casing provides structural integrity to the newly drilled wellbore in addition to isolating potentially dangerous high pressure zones from each other and from the surface.

With these zones safely isolated and the formation protected by the casing, the well can be drilled deeper (into potentially more-unstable and violent formations) with a smaller bit, and also cased with a smaller size casing. Modern wells often have 2-5 sets of subsequently smaller hole sizes drilled inside one another, each cemented with casing.
 
To drill the well:
   
. The drill bit, aided by rotary torque and the compressive weight of drill collars above it, breaks up the earth.
. Drilling fluid is pumped down the inside of the drill pipe and exits at the drill bit and aids to break up the rock, as well as clean, cool and lubricate the bit.
. The generated rock "cuttings" are swept up by the drilling fluid as it circulates back to surface outside the drill pipe.
. The pipe or drill string to which the bit is attached is gradually lengthened as the well gets deeper by screwing in several 30' joints of pipe at surface.
   
This process is all facilitated by a drilling rig which contains all necessary equipment to circulate the drilling fluid, hoist and turn the pipe, control downhole pressures, remove cuttings from the drilling fluid, and generate onsite power for these operations.

Completion

After drilling and casing the well, it must be 'completed'. Completion is the process in which the well is enabled to produce oil or gas.
   
In a cased-hole completion, holes (called perforations) are made in the casing that covers the reservoir section to provide a path for the oil to flow from the surrounding rock into the well bore. In open hole completion, often 'sand-screens' or a 'gravel pack' is installed in the last drilled, uncased reservoir section. These tools are to maintain structural integrity of the wellbore in the absence of casing, while still allowing flow from the reservoir into the wellbore. Screens also control the migration of formation sands into production tubulars and surface equipment, which can cause washouts and other problems, particularly in unconsolidated sand formations in offshore fields.
   
After a flow path is made, acids or other fluids are often pumped into the well to fracture, clean, or otherwise prepare and stimulate the reservoir rock to optimally produce hydrocarbons into the wellbore. Finally, the area above the reservoir section of the well is packed off inside the casing, and connected to the surface via a smaller diameter pipe called tubing. This arrangement provides a redundant barrier to leaks of hydrocarbons as well as allowing damaged sections to be replaced. Also, the smaller diameter of the tubing produces hydrocarbons at an increased speed, in order to overcome the hydrostatic effects of heavy fluids such as water.
     
In most wells, the natural pressure of the subsurface reservoir is high enough to push the oil or gas all the way to surface. However, this is not always the case, especially in depleted fields where the pressures have been lowered by other producing wells. Installing a smaller diameter tubing may be enough to help the production, but other types of artificial lift can also be used. Common solutions are downhole pumps, gas lift, or surface pump-jacks - the "nodding donkey" pumps dotting the countryside in old oil fields in Texas and Oklahoma. The use of artificial lift technology in a field is often termed as "secondary recovery" in the industry.

Production

The production stage is the most important stage of a well's life, when the oil and gas are produced. By this time, the oil rigs and workover rigs used to drill and complete the well have moved off the wellbore, and the top is usually outfitted with a collection of valves called a "Christmas Tree". These valves regulate pressures, control flows, and allow access to the wellbore in case further completion work needs to be performed. From the outlet valve of the Christmas Tree, the flow can be connected to a distribution network of pipelines and tanks to supply the product to refineries, natural gas compressor stations, or oil export terminals.
   
As long as the pressure in the reservoir remains high enough, this Christmas Tree is all that is required to produce the well. If the pressure depletes and it's considered economically viable, an artificial lift method mentioned in the completions section can be employed.
   
Workovers are often necessary in older wells, which may need smaller diameter tubing, scale or paraffin removal, repeated acid matrix jobs, or even completing new zones of interest in a shallower reservoir. Such remedial work can be performed using workover rigs-also known as puling units-to pull and replace tubing, or by the use of a well intervention technique called coiled tubing.
      
Enhanced recovery methods such as water-flooding, steam flooding, or CO2 flooding may be used to increase reservoir pressure and provide a "sweep" effect to push hydrocarbons out of the reservoir. Such methods require the use of injection wells (often picked from old production wells in a carefully determined pattern), and are used when facing problems with reservoir pressure depletion, high oil viscosity, or can even be employed early in a field's life; in certain cases-depending on the reservoir's geomechanics-reservoir engineers may determine that ultimate recoverable oil may be increased by applying a water-flooding strategy early in the field's development rather than later. The application of such enhanced recovery techniques is often termed as "tertiary recovery" in the industry.
   
Abandonment

Finally, when the well no longer produces or produces so poorly that it is a liability to its owner, it is abandoned. In this simple process the wellbore is filled with cement so that the flow path from the reservoir to the surface is plugged.
    
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 byproduct 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 because it must be piped to the end user. The easy way to get rid of it was to burn it away at the well site, 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 another 'injection' well for disposal. Another solution is to export the natural gas as LNG. Of course, in locations such as the United States with a high natural gas demand, pipelines are constructed to take the gas from the well site 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. These wells dot the Southwestern United States, and are also the most common type of well in the Middle East.
    
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 flow-rate) 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.

Re-Entry Projects:

Re-entry Projects involve locating well bores that were drilled in the past that were plugged and abandoned. The process generally involves finding the specific location of a previously drilled well and uncovering the site. Sites are frequently buried under several feet of soil and are often overgrown with trees and brush. Once uncovered, a rig is moved into the site and begins drilling through cement "plugs" that were placed during the plugging process. Sometimes there are numerous plugs in the well bore, each of which must be drilled out, down to the zone of interest for the re-entry.

Once the desired total depth is reached, work begins on the placement of casing, or pipe, in the hole. Frequently, casing already exists and must simply be extended or slightly modified to create a producing well.

Various other completion tasks such as perforation, stimulation (fracturing, acidizing, etc.) and swabbing are then performed to create a well bore from which hydrocarbons can be recovered.

Advantages:

Re-entry and re-completion have exciting advantages over speculation and drilling of new wells, and the United Stated Department of Energy has fully endorsed this new approach. Furthermore, the state of Texas currently has and incentive program that grants an exemption from 10 years of state severance tax for these types of wells.

New cutting-edge technologies have made re-entry a profitable venture for independent exploration companies. Geophysists can now map subterranean rock far more accurately, and new revolutionary imaging techniques greatly increase the possibilities or petroleum discovery. The newest generation of powerful microprocessors on the market is now able to support the processor-intensive imaging software.

For most re-entry candidates, exploration geologists and/or geophysicists have already identified petroleum-producing zones, and the exploration companies that abandoned the sites had only partially extracted hydrocarbons from the wells they drilled.

Re-entries usually cost much less than new drills and are procedurally advantageous. They often significantly reduce risk by using geographically known petroleum production zones, and can generate quicker returns than new drilling ventures. Re-entries average significantly less time to completion versus drilling a new well.