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.
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.
The high pressure, high flow water injection pumps in partnership with HPHT Scale inhibitors complete the required solution for High Pressure EOR (Enhanced Oil Recovery) packages. Usually these packages are placed near to the de-oxygenation tower and boosting pumps. They fill the bottom of the reservoir with the filtered water to push the oil towards the wells like a piston. The result of the injection is not quick, it needs time.
Water injection is used to prevent low pressure in the reservoir. The water replaces the oil which has been taken, keeping the production rate and the pressure the same over the long term.