Drilling an Oil Well in US Petroleum Holdings

DRILLING AN OIL WELL

The earliest oil wells were drilled percussively (cable-tool drilling), that is, holes were drilled simply by hammering at the earth. Very soon, the limited depths which this method could attain meant that rotary drilling was introduced. Modern wells drilled using rotary drills can achieve lengths of over 12,000 meters / 38,000 feet.

Until the 1970s, most oil wells were vertical (or, more specifically, were supposed to be vertical – deviations introduced by different lithology and mechanical imperfections meant that most wells were at least slightly deviated). However, modern technologies (directional drilling) allow strongly deviated wells which can, given sufficient depth, actually become horizontal. This is of great value as the reservoir rocks which contain hydrocarbons are usually horizontal, or sub-horizontal. A well, therefore, which passes along a reservoir (rather than through it, as a vertical well must) can tap a larger volume with a much larger surface area (and thus a correspondingly higher production rate). Using deviated and horizontal drilling, it has also become possible to reach reservoirs several kilometers away from the drilling place (Extended Reach Drilling), allowing to produce hydrocarbons from underneath e.g. environmentally sensitive areas or offshore close to the coast line.

Drilling

The well is created by drilling a hole (5 to 30 inches wide) into the earth with an oil rig turning a drill bit. After the hole is drilled, a metal pipe slightly smaller than the hole size (called a ‘casing’) is run into the hole. The outside of the casing is then bonded and secured to the hole with cement. The casing provides structural integrity to the newly drilled wellbore in addition to isolating potentially dangerous high pressure zones from each other and from the surface.

With these zones safely isolated and the formation protected by the casing, the well can be drilled deeper (into potentially more-unstable and violent formations) with a smaller bit, and also cased with a smaller size casing. Modern wells often have 2-5 sets of subsequently smaller hole sizes drilled inside one another, each cemented with casing.