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 that is then mounted with an extraction device such as a pumpjack which allows extraction from the reserve. Creating the wells can be an expensive process, costing at least hundreds of thousands of dollars, and costing much more when in hard to reach areas, e.g., when creating offshore oil platforms. The process of modern drilling for wells first started in the 19th century, but was made more efficient with advances to oil drilling rigs during the 20th century.

Wells are frequently sold or exchanged between different oil and gas companies as an asset – in large part because during falls in 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 (a process of injecting gas or liquid to force more oil or natural gas production) have made some wells viable. However, peak oil and climate policy surrounding fossil fuels has made fewer of these wells and costly techniques viable.

However, the large number of neglected or poorly maintained wellheads is a large environmental issue: they may leak methane emissions or other toxic emissions into local air, water or soil systems. This pollution often becomes worse when wells are abandoned or orphaned – where wells no longer are economically viable, and no longer are maintained by a company. A 2020 estimate by Reuters suggested that there were at least 29 million abandoned wells internationally, creating a significant source of greenhouse gas emissions causing climate change.