Water injection or water flooding refers to the method in the oil industry where water is injected into the reservoir, usually to increase pressure and thereby stimulate production. Water injection wells can be found both on- and offshore, to increase oil recovery from an existing reservoir.
Water is injected (1) to support pressure of the reservoir (also known as voidage replacement), and (2) to sweep or displace oil from the reservoir, and push it towards a well.
Normally only 30% of the oil in a reservoir can be extracted, but water injection increases that percentage (known as the recovery factor) and maintains the production rate of a reservoir over a longer period.
Waterflooding began accidentally in Pithole, Pennsylvania by 1865. Waterflooding became common in Pennsylvania in the 1880s.〔Abdus Satter, Ghulam M. Iqbal, and James L. Buchwalter, ''Practical Enhanced Reservoir Engineering'' (Tulsa, Okla.: Pennwell, 2008) 492.〕
== Sources of injected water ==
Any and every source of bulk water can be, and has been, used for injection. The following sources of water are used for recovery of oil:
Produced water is often used as an injection fluid. This reduces the potential of causing formation damage due to incompatible fluids, although the risk of scaling or corrosion in injection flowlines or tubing remains. Also, the produced water, being contaminated with hydrocarbons and solids, must be disposed of in some manner, and disposal to sea or river will require a certain level of clean-up of the water stream first. However, the processing required to render produced water fit for reinjection may be equally costly.
As the volumes of water being produced are never sufficient to replace all the production volumes (oil and gas, in addition to water), additional "make-up" water must be provided. Mixing waters from different sources exacerbates the risk of scaling.
Seawater is obviously the most convenient source for offshore production facilities, and it may be pumped inshore for use in land fields. Where possible, the water intake is placed at sufficient depth to reduce the concentration of algae; however, filtering, deoxygenation and biociding is generally required.
Aquifer water from water-bearing formations other than the oil reservoir, but in the same structure, has the advantage of purity where available.
River water will always require filtration and biociding before injection.
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