The water injection method used in oil production is where water is injected back into the reservoir usually to increase pressure and thereby stimulate production. Water injection wells can be found both on- and offshore. This method is used to increase oil recovery from an existing reservoir.
Water is injected for two reasons:
1. For pressure support of the reservoir (also known as voidage replacement).
2. To sweep or displace the oil from the reservoir, and push it towards an oil production 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 of time.
Any and every source of bulk water can be, and has been, used for injection. Some aspects to consider when selecting an injection source are as follows:
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 & 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 filtering and biociding before injection.
The filters must clean the water and remove any impurities, such as shells and algae. Typical filtration is to 2 micrometres. The filters are so fine so as not to block the pores of the reservoir. Sand filters are the easiest to use, because there is an automatic system with Delta P which cleans the filter with a backwash when the sand filter is dirty. The sand filter has different beds with various sizes of sand granules. The sea water traverses the first, finest, layer of sand down to the coarsest and to clean the filter, the process is inverted. After the water is filtered it continues on to fill the de-oxygenation tower.
Oxygen must be removed from the water because it promotes corrosion and growth of certain bacteria. Bacterial growth in the reservoir can produce toxic hydrogen sulfide, a source of serious production problems, and block the pores in the rock.
A deoxygenation tower brings the injection water into contact with a dry gas stream (gas is always readily available in the oilfield). The filtered water drops into the de-oxygenation tower, splashing onto a series of trays, causing dissolved oxygen to be lost to the gas stream.
An alternative method, also used as a backup to deoxygenation towers, is to add an oxygen scavenging agent such as sodium bisulfite.
The high pressure, high flow water injection pumps 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.