Oil Well Drilling and Production
An oil well is a steel-lined hole drilled to bring oil and gas to the surface. How it is drilled and completed, how it produces, how deep it goes and where the first ones were.
An oil well is a hole drilled into the ground to bring oil and gas to the surface, lined with steel pipe and sealed in place with cement. It is usually between 12 cm and 1 m across, and it can run from a few hundred metres to well over 10 km.[1] The work falls into four stages: planning, drilling, completion and production. What most people picture, a nodding pump in a field, is only the visible top of a structure that may extend for kilometres underground.
Anatomy of a Well
A modern well is built as a telescope. Drilling proceeds in two to five sections, each narrower than the last, and each section is lined with casing, the steel pipe cemented against the rock.[1] Every string has a specific job.
| String | Job | Typical size |
|---|---|---|
| Conductor | Holds back loose ground near the surface and returns drilling mud to the rig | 18 to 30 in |
| Surface casing | Isolates fresh water, and is the most tightly regulated string | 13⅜ in |
| Intermediate casing | Separates formations that cannot share one mud weight | Varies |
| Production casing or liner | Lines the reservoir section. A liner hangs from the string above and stops short of the surface | Liners often 7 in |
| Tubing | Carries the fluid up, inside the last casing string | Varies |
Source: Casing (borehole) entry.[4]
Cemented casing protects groundwater, stops weak rock from caving in, and keeps zones at different pressures apart.[4] At the top sits the production tree, known in the trade as the Christmas tree, a stack of valves that controls pressure and flow.[1]
Well Types
Wells are classified by purpose. An exploration well, or wildcat, tests whether a structure holds oil at all. An appraisal well measures the size and limits of what was found. A production well brings oil out, while an injection well works in reverse, pumping water or gas into the reservoir to hold up pressure and sweep oil towards the producers.
They are also classified by shape. A vertical well crosses the reservoir over a short interval. Since most reservoir rock lies roughly flat, a well that turns horizontal stays in contact with it for far longer. The Permian Basin in the US shows the scale of the change. Average lateral length on new horizontal wells rose from 6,149 feet in 2015 to 10,867 feet in 2025, about 3.3 km, while the basin's output climbed from 2.9 million to 11.2 million barrels of oil equivalent a day.[5]
Drilling the Hole
Geologists and geophysicists pick the target from seismic data, and engineers then design a path to it from the surface, based on estimates of pore pressure and fracture pressure along the way. A drilling rig then turns a bit on the end of a string of pipe, adding joints of about 9 m as the hole deepens.[1]
Mud is what makes this possible. Pumped down inside the pipe, it leaves through the bit and returns up the space outside the pipe, cooling the bit, carrying cuttings to the surface, supporting the walls of the hole and balancing the pressure of the fluids in the rock.[1] The cuttings caught on the shakers also tell the geologist which formation the bit has reached.
Mud Weight and Pressure
The column of mud exerts a hydrostatic pressure that depends only on its density and vertical height:
P = ρ × g × h
Mud at 1,300 kg/m³ at a true vertical depth of 3,000 m gives 1,300 × 9.81 × 3,000 = 38.3 MPa, or about 383 bar. That figure has to stay above the pressure of the fluid in the rock and below the pressure that would fracture it.[4] Too low, and oil, gas or water enters the hole, an event known as a kick. Too high, and the rock cracks and mud is lost. When no single mud weight suits every formation in an open section, the shallower zones are sealed off behind intermediate casing before drilling continues.
Well Control
The final barrier is the blowout preventer (BOP), a set of valves that can close the well if a kick gets out of hand. The best-known failure is Deepwater Horizon in the Gulf of Mexico, where 11 people died in the explosion of 20 April 2010. BP's own investigation listed inadequate cementing, valves that did not close and faults in the BOP system among the causes.[6] Cement, mud and valves form one chain, and that accident broke several of its links at once.
While drilling, logging tools lowered into the hole read the porosity of the rock and the fluids in it. If oil shows, the well is tested for flow. If the result is uneconomic, the well is plugged without spending money on completion.
Completion and Production
A finished hole cannot yet produce. To complete it, crews make small perforations in the casing across the reservoir so that oil can flow into the tubing.[1] Rock that does not flow well is treated with acid or fractured hydraulically. Türkiye's Raman-8 well shows how much difference that can make: it started in 1948 at about 45 barrels a day and reached about 450 that same year after acid treatment.[2]
With enough reservoir pressure, oil rises by itself. As pressure falls, artificial lift takes over through pumpjacks, downhole pumps or gas lift.[1] The fluid reaching the surface is usually a mix of oil, gas and water, split in separators at the site.
Much of the oil never comes out. The US Department of Energy puts primary recovery at about 10% of the oil originally in place, secondary recovery with water or gas injection at 20 to 40%, and enhanced recovery using heat, gas or chemicals at 30 to 60% or more.[7]
Well Depth
Two measures are easily confused. True vertical depth is the straight-down distance from surface to bottom, while measured depth is the length of the hole along its path. On a vertical well they nearly match. On an extended-reach well they can differ by a factor of many.
| Well | Year | Depth | Note |
|---|---|---|---|
| Drake, Pennsylvania | 1859 | 21 m | Drilled using salt-well techniques |
| Raman-8, Batman | 1946 | About 1,360 m | Türkiye's first long-lived producer |
| Tiber, Gulf of Mexico | 2009 | 35,050 ft (about 10,680 m) vertical | In 4,132 ft of water |
| Chaivo, Sakhalin-1 | 2017 | 15,000 m measured | 14,129 m horizontal reach |
| Kola SG-3, Russia | 1989 | 12,262 m vertical | A scientific borehole, not an oil well |
Source: company and institutional statements.[2][8][9][10][11]
Heat is one of the main limits. At Kola, drilling stopped in 1992 after temperatures of 180 °C were met at about 11,900 m, higher than expected.[11]
The First Wells
Which well came first depends on what counts as a well. The oldest known were sunk in China in 347 CE, reaching about 240 m with bits on bamboo poles.[1] The modern candidates arrive within a few years of each other.
| Year | Place | Claim |
|---|---|---|
| 1846 | Baku | Drilled with percussion tools to 21 m |
| 1854 | Bóbrka, Poland | Ignacy Łukasiewicz, one of the first modern wells |
| 1858 | Oil Springs, Ontario | First commercial oil well in North America |
| 1859 | Titusville, Pennsylvania | Edwin Drake strikes oil on 27 August |
| 1896 | Summerland, California | First offshore oil wells |
Source: Oil well entry; ASME.[1][8]
Drake's well reached 69 feet and averaged 1,000 gallons a day for three years, around 24 barrels. He borrowed from salt-well drilling and developed the method of driving iron pipe to keep the hole open. The American Society of Mechanical Engineers treats the well as the start of the modern petroleum industry.[8] Its importance owes less to being first than to the wave of drilling, refining and trade that followed.
Oil Wells in Türkiye
Oil was confirmed in Türkiye in 1940 at 1,048 m on Raman Mountain in Batman province. Raman-1 averaged about 100 barrels a day until rising water output forced its abandonment.[3] Lasting production came with Raman-8, spudded on 11 February 1945. It found commercial oil at about 1,360 m in 1946 and began producing on 8 March 1948, and in 2018 it was reported to be still giving about 200 barrels a day.[2][3] The nearby Garzan field followed in 1951.[3]
The weight of production has since moved to Gabar in Şırnak. National output reached 23.1 million barrels in the first half of 2026, of which state-owned TPAO produced 21.8 million and Gabar supplied 60%. In August 2026 the energy minister announced a new daily record of 83,300 barrels at the field.[12]
End of a Well's Life
A well is shut in once production no longer covers its running costs, and that point moves with the oil price. Abandonment means sealing sections with cement so that water and gas zones cannot communicate, removing the wellhead and restoring the site. Costs range from a few thousand dollars for a shallow land well to millions offshore. A 2020 Reuters estimate counted at least 29 million abandoned wells worldwide.[1]
Associated gas produced along the way must also go somewhere. Without a pipeline it is flared at the site. Otherwise it is sold into a grid or reinjected to support reservoir pressure.
From the well, crude oil travels by tanker or pipeline to an oil refinery. The fuel oil and gas oil that ships burn are among the last products in that chain.
Frequently asked questions
What is an oil well?
An oil well is a hole drilled into the ground to bring oil and gas to the surface, lined with steel pipe and sealed in place with cement.
How is an oil well drilled?
The target is chosen from seismic data, a rig is set up and a rotating bit cuts the hole. Mud pumped down the pipe cools the bit and carries the cuttings up. The hole is lined with steel casing and cemented, and perforations across the reservoir then let the oil flow.
How deep is an oil well?
It depends on the field. Drake's well was 21 m and Raman-8 about 1,360 m. Tiber reached about 10,680 m vertically, and the Chaivo well measures 15,000 m along its path.
Where was Türkiye's first oil well?
On Raman Mountain in Batman province. Oil was confirmed at Raman-1 in 1940, and long-term production began at Raman-8 in 1948.
Can all the oil in a reservoir be recovered?
No. The US Department of Energy puts primary recovery at about 10% and enhanced recovery at 30 to 60%.
How does a pumpjack work?
The beam at the surface moves a string of rods up and down. That motion drives a pump at the bottom of the well, which lifts the fluid up the tubing. It is used once reservoir pressure is no longer enough.
Sources
- Wikipedia. Oil Well. Used as a secondary source; the abandoned well count rests on a 2020 Reuters estimate.
- Memurlar.net. 70 Yıldır Kesintisiz Petrol Çıkıyor, 4 September 2018.
- İşçi Haber. Türkiye, Kuruluşunun 100. Yılında Petrolde Sınıf Atladı, 20 October 2023.
- Wikipedia. Casing (Borehole). Used as a secondary source.
- US Energy Information Administration (EIA). Longer Wells Boost Permian Crude Oil and Natural Gas Production. Today in Energy, 19 August 2026.
- Wikipedia. Deepwater Horizon Explosion. Used as a secondary source for summaries of BP's investigation report of 8 September 2010.
- US Department of Energy. Enhanced Oil Recovery. Office of Fossil Energy and Carbon Management.
- American Society of Mechanical Engineers (ASME). Drake Oil Well, Landmark No. 40.
- Oil & Gas Journal. BP's Tiber One of Industry's Deepest Wells, 2 September 2009.
- Rosneft. Press Release on the Chaivo Field Well, 16 November 2017.
- Wikipedia. Kola Superdeep Borehole. Used as a secondary source.
- Milliyet Uzmanpara. Yurt İçi Petrol Üretimi 6 Ayda 23,1 Milyon Varili Buldu, 16 August 2026; Hürriyet Daily News, 17 August 2026; Milliyet, Gabar'da Yeni Rekor, August 2026.
