China drilled oil wells with bamboo poles in 347 CE
The earliest known oil wells were sunk in China in 347 CE, using bits fixed to bamboo poles and reaching as deep as about 240 metres. The oil was not the prize. It was burned to boil brine down into salt, and by the 10th century bamboo pipelines linked the wells to salt springs.
Elsewhere, Japanese writers of the 7th century called petroleum burning water. Around Baku, on the Caspian shore, oil fields were being worked by the 9th century, and Marco Polo described a seep there yielding enough to fill a hundred ships at once. Baku also saw the first well drilled specifically to look for oil, in 1846, bored with percussion tools to 21 metres. Ignacy Łukasiewicz drilled an early modern well at Bóbrka in Poland in 1854, North America's first commercial well opened at Oil Springs, Ontario, in 1858, and offshore drilling began off the California coast in 1896.
Early rigs worked by repeatedly lifting and dropping a heavy bit on a cable. Rotary drilling replaced that in the 20th century, going deeper and faster; the Kola Borehole, deepest of all, passed 12,000 metres. Until the 1970s wells were mostly vertical, but directional techniques now bend a borehole until it runs sideways through a flat reservoir, exposing far more oil-bearing rock and even tapping deposits kilometres from the rig, beneath towns or sensitive ground.
A modern well is built in stages, like a telescope pushed into the ground. The rig turns a string of pipe with a bit on the end, lengthening it by screwing on sections about 9 metres long. After each stretch, steel casing is lowered and cemented in place to seal off high-pressure zones, and the next stretch is drilled narrower, often through two to five nested sizes. Drilling mud, mostly water and clay, is pumped down to cool the bit, carry rock chips up and hold back fluids in the rock. Crews watch for a kick, when formation pressure beats the weight of the mud, and shut the blowout preventers if one comes. Finally the casing is perforated across the productive layer so oil can flow in.
Old wells linger as a problem. Once unprofitable they may be abandoned or orphaned, and poorly kept wellheads can leak methane and other pollutants. Reuters estimated in 2020 that at least 29 million abandoned wells exist worldwide.
Source: Oil well
