Oil well

An oil well is a drillhole bored into the Earth to extract petroleum hydrocarbons. These hydrocarbons include petroleum oil and associated petroleum gas, which is natural gas released alongside the oil.

Some wells are specifically designed to produce only gas, known as gas wells. Creating these wells is a massive financial undertaking. A single well can cost at least hundreds of thousands of dollars to construct. Costs increase significantly when drilling in difficult locations, such as offshore environments.

Because the value of a well depends on market prices, companies frequently buy and sell them as assets. A well might be unproductive when oil prices are low, but it becomes economically valuable when prices rise.

The drilling process begins with a detailed planning phase. Geologists and geophysicists identify a geologic target to optimize production or confirm a reservoir's existence. Engineers must design a trajectory, which is the planned path from the surface to the target. They must account for variables like lithology, which is the physical characteristics of the rock, and pore pressure.

The team also selects specific drill bits and drilling fluids for the job. These plans often go through many iterations to ensure safety and cost-efficiency.

To create the well, a drilling rig rotates a drill string with a bit attached to the end. The bit cuts into the rock through compressive failure or by shearing off slices.

As the bit moves deeper, workers pump drilling fluid, often called "mud," down the inside of the drill pipe. This fluid exits at the bit to cool it and lift rock cuttings to the surface. The mud also helps prevent the rock from destabilizing and manages fluid pressure.

The cuttings are then screened out by "shakers" so the fluid can be reused. If formation pressure becomes too high, a "kick" can occur, requiring the use of blowout preventers.

Structural integrity is maintained using steel pipes called casing. As the hole is drilled, sections of casing are placed into the borehole.

A cement slurry is then pumped into the annulus, which is the space between the casing and the borehole wall. This process isolates high-pressure zones from the surface and protects the surrounding formations. Modern wells often use two to five sets of progressively smaller casing strings. This layered approach allows engineers to drill deeper into more unstable or high-pressure formations.

Humanity has been extracting oil for centuries, starting with ancient China in 347 CE. Those early wells used bamboo poles and bits to reach depths of up to 200 meters. By the 9th century, the Persian alchemist Muhammad ibn Zakarīya Rāzi distilled petroleum to produce kerosene.

In the 19th century, drilling became more modern. In 1854, Ignacy Łukasiewicz drilled one of the first modern wells in Poland. North America saw its first commercial well in Ontario in 1858. By 1896, the industry moved offshore in the Summerland Oil Field of California.

Technological shifts have changed the shape of wells over time. Early modern wells used percussion drilling, which involved repeatedly dropping a bit on a cable. The 20th century introduced rotary drilling, which is much faster and reaches greater depths. While most wells were vertical until the 1970s, modern directional drilling allows for highly deviated or even horizontal wells.

Horizontal wells are useful because they provide more surface area in the production zone. This increases the production rate and allows companies to reach reservoirs located miles away. Some wells also use hydraulic fracturing, which involves injecting liquid to force more production.

While oil wells provide energy, they also present environmental challenges. Neglected or poorly maintained wellheads can leak methane or toxic substances into the air, water, and soil. This problem is most common with abandoned or "orphaned" wells, which are wells no longer maintained by their owners because they are no longer profitable. A 2020 estimate suggested there are at least 29 million abandoned wells worldwide. These wells are a significant source of greenhouse gas emissions that contribute to climate change.

When a well is active and producing, devices like pumpjacks are used to bring the oil to the surface.