Oil well
An oil well is a drillhole boring in Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released as associated petroleum gas along with the oil. A well that is designed to produce only gas may be termed a gas well. Wells are created by drilling down into an oil or gas reserve and if necessary equipped with extraction devices such as pumpjacks. Creating the wells can be an expensive process, costing at least hundreds of thousands of dollars, and costing much more when in difficult-to-access locations, e.g., offshore. The process of modern drilling for wells first started in the 19th century but was made more efficient with advances to oil drilling rigs and technology during the 20th century.
Wells are frequently sold or exchanged between different oil and gas companies as an asset – in large part because during a drop in the price of oil and gas, a well may be unproductive, but if prices rise, even low-production wells may be economically valuable. Moreover, new methods, such as hydraulic fracturing have made some wells viable. However, peak oil and climate policy surrounding fossil fuels have made fewer of these wells and costly techniques viable.
However, neglected or poorly maintained wellheads present environmental issues: they may leak methane or other toxic substances into local air, water and soil systems. This pollution often becomes worse when wells are abandoned or orphaned – i.e., where a well is no longer economically viable, so are no longer maintained by their owners. A 2020 estimate by Reuters suggested that there were at least 29 million abandoned wells internationally, creating a significant source of greenhouse gas emissions worsening climate change.
History
The earliest known oil wells were drilled in China in 347 CE. These wells had depths of up to about and were drilled using bits attached to bamboo poles. The oil was burned to evaporate brine producing salt. By the 10th century, extensive bamboo pipelines connected oil wells with salt springs. The ancient records of China and Japan are said to contain many allusions to the use of natural gas for lighting and heating. Petroleum was known as burning water in Japan in the 7th century.According to Kasem Ajram, petroleum was distilled by the Persian alchemist Muhammad ibn Zakarīya Rāzi in the 9th century, producing chemicals such as kerosene in the alembic, and which was mainly used for kerosene lamps. Arab and Persian chemists also distilled crude oil in order to produce flammable products for military purposes. Through Islamic Spain, distillation became available in Western Europe by the 12th century.
Some sources claim that from the 9th century, oil fields were exploited in the area around modern Baku, Azerbaijan, to produce naphtha for the petroleum industry. These places were described by Marco Polo in the 13th century, who described the output of those oil wells as hundreds of shiploads. When Marco Polo in 1264 visited Baku, on the shores of the Caspian Sea, he saw oil being collected from seeps. He wrote that "on the confines toward Geirgine there is a fountain from which oil springs in great abundance, in as much as a hundred shiploads might be taken from it at one time."
In 1846, Baku the first ever well was drilled with percussion tools to a depth of for oil exploration. In 1846–1848, the first modern oil wells were drilled on the Absheron Peninsula north-east of Baku, by Russian engineer Vasily Semyonov applying the ideas of Nikolay Voskoboynikov.
Ignacy Łukasiewicz, a Polish pharmacist and petroleum industry pioneer drilled one of the world's first modern oil wells in 1854 in Polish village Bóbrka, Krosno County, and in 1856 built one of the world's first oil refineries.
In North America, the first commercial oil well entered operation in Oil Springs, Ontario in 1858, while the first offshore oil well was drilled in 1896 in the Summerland Oil Field on the California Coast.
The earliest oil wells in modern times were drilled percussively, by repeatedly raising and dropping a bit on the bottom of a cable into the borehole. In the 20th century, cable tools were largely replaced with rotary drilling, which could drill boreholes to much greater depths and in less time. The record-depth Kola Borehole used a mud motor while drilling to achieve a depth of over.
Until the 1970s, most oil wells were essentially vertical, although lithological variations cause most wells to deviate at least slightly from true vertical. However, modern directional drilling technologies allow for highly deviated wells that can, given sufficient depth and with the proper tools, actually become horizontal. This is of great value as the reservoir rocks that contain hydrocarbons are usually horizontal or nearly horizontal; a horizontal wellbore placed in a production zone has more surface area in the production zone than a vertical well, resulting in a higher production rate. The use of deviated and horizontal drilling has also made it possible to reach reservoirs several kilometers or miles away from the drilling location, allowing for the production of hydrocarbons located below locations that are difficult to place a drilling rig on, environmentally sensitive, or populated.
Life of a well
Planning
- For a production well, the target is picked to optimize production from the well and manage reservoir drainage.
- For an exploration or appraisal well, the target is chosen to confirm the existence of a viable hydrocarbon reservoir or to learn its extent.
- For an injection well, the target is selected to locate the point of injection in a permeable zone that may support disposing of water or gas and/or pushing hydrocarbons into nearby production wells.
Before a well is drilled, a geologic target is identified by a geologist or geophysicist to meet the objectives of the well.
When the well path is identified, a team of geoscientists and engineers will develop a set of presumed characteristics of the subsurface path that will be drilled through to reach the target. These properties may include lithology pore pressure, fracture gradient, wellbore stability, porosity and permeability. These assumptions are used by a well engineering team designing the casing and completion programs for the well. Also considered in the detailed planning are selection of the drill bits, bottom hole assembly, and the drilling fluid. Step-by-step procedures are written to provide guidelines for executing the well in a safe and cost-efficient manner.
With the interplay with many of the elements in a well's design, trajectories and designs often go through several iterations before the plan is finalized.
Drilling
The well is created by drilling a hole 12 cm to 1 meter in diameter into the earth with a drilling rig that rotates a drill string with a bit attached. At depths during the process, sections of steel pipe, slightly smaller in diameter than the borehole at that point, are placed in the hole. Cement slurry will be pumped down the inside to rise in the annulus between the borehole and the outside of the casing. The casing provides structural integrity to that portion of the newly drilled wellbore, in addition to isolating potentially dangerous high pressure zones from lower-pressure ones, and from the surface.With these zones safely isolated and the formation protected by the casing, the well can be drilled deeper with a smaller bit, and then cased with a smaller size pipe. Modern wells generally have two to as many as five sets of subsequently smaller hole sizes, each cemented with casing.
To drill the well
- The rotating drill bit, aided by the weight of the drill string above it, cuts into the rock. There are different types of drill bits; some cause the rock to disintegrate by compressive failure, while others shear slices off the rock as the bit turns.
- Drilling fluid, a.k.a. "mud", is pumped down the inside of the drill pipe and exits at the drill bit. The principal components of drilling fluid are usually water and clay, but it also typically contains a complex mixture of fluids, solids and chemicals that must be carefully tailored to provide the correct physical and chemical characteristics required to safely drill the well. Particular functions of the drilling mud include cooling the bit, lifting rock cuttings to the surface, preventing destabilisation of the rock in the wellbore, and overcoming the pressure of fluids inside the rock so that these fluids do not enter the wellbore. Some oil wells are drilled with air or foam as the drilling fluid.
- The generated rock "cuttings" are swept up by the drilling fluid as it circulates back to the surface inside the casing and outside of the drill pipe. The fluid then goes through "shakers" that screen the cuttings out of the fluid, which is returned to the pit for reuse. Watching for abnormalities in the returning cuttings and monitoring pit volume or rate of returning fluid are imperative to catch "kicks" early. A "kick" is when the formation pressure at the depth of the bit is greater than the hydrostatic head of the mud above, which if not controlled temporarily by closing the blowout preventers followed by increasing the density of the drilling fluid would allow formation fluids to enter the annulus uncontrollably.
- The drill string to which the bit is attached is gradually lengthened as the well gets deeper by screwing in additional 9 m sections or "joints" of pipe under the kelly or top drive at the surface. This process is called "making a connection". The operation called "tripping" is when pulling the bit out of the hole to replace the bit, and running back in with a new bit. Joints are usually combined for more efficient tripping by creating stands of multiple joints. A conventional triple, for example, has three joints at a time racked vertically in the derrick. Some modern rigs, called "super singles", trip pipe one at a time, laying it out on racks as they go.