The production of petroleum oil is basically a very simple process – a hole is drilled down into the earth’s crust (on land or under the sea) until an oil-bearing rock formation is reached, whereupon the oil is forced up the drilled hole to the surface. The consequent “gusher” is a sight familiar to magazine readers or movie-goers.
The actual process is a good deal more complex than this, but the first stage of petroleum production can be regarded in this simple way, with the natural pressure of the subterranean zone being sufficient to force the oil to the surface, in what is known as the primary phase of production. Eventually the pressure falls as the oil is extracted, and the production rate starts to fall. At this point, the oil discharge is boosted by the use of downhole pumps, although such pumps (like the familiar “nodding donkey”) will usually have been fitted at, and may have been used from, the start of operation. The deeper the oilbearing formation, the more likely that some form of artificial lift will be needed.
When the flow of oil out of the rock formation into the bottom of the well is too low for its further discharge by pumping, then that is the end of the primary production phase. The amount of oil produced in this primary phase can be as little as 10% of the total content of the reservoir and as much as 30%. For most production wells, the oil is recovered as a mixture with water, and this “produced water” can be up to five times, or more, the volume of recovered oil. A major wellhead process then is the separation of the oil from its accompanying produced water.
As pumped production rates start to fall, the oil production process can then move into a secondary phase, in which a fluid is injected into the reservoir to repressurise it and force out more of the contained oil. This can be done by waterflooding or by gas injection. In waterflooding, clean filtered water is injected into the rock layer through specially drilled injection wells (or through disused production wells), to push residual oil along to the operating wells. An injection of gas into the gas cap above the formation achieves the same effect. Primary and secondary production together can recover a total of 15 to 40% of the original oil in the reservoir.
When production starts to fall at the end of the secondary phase, a tertiary phase can be embarked upon – also called enhanced oil recovery (EOR) – although this is still only used in a minority of cases. EOR can employ
specific gas injection, or thermal methods such as cyclic steam injection or steam flooding, or even, in the extreme, in-situ combustion. Among the specific gas injection processes is the use of carbon dioxide, which then offers a very valuable means of carbon sequestration. Microbial injection, although still only a method under development, shows considerable promise for the future. By the use of EOR, the total recovery from an oil-bearing rock formation can be increased to 30 to 60% or more.
This three-stage production process applies to liquid oil trapped below ground, which can be pumped to the surface. A significant proportion of the world’s total reserves of hydrocarbon fuels (some say as much again as the current total proved liquid oil reserves) exists as tar sands, in the form of a mixture of heavy oil and bitumen with sand. Huge deposits of this material lie in Canada, and production of “synthetic” crude oil from them is now a well-established process. If the deposits are at or close to the surface, then open-cast mining is used, followed by hot water processing and flotation to release the oil. For subterranean deposits, recovery is achieved by hot water injection.