In the oil industry, waterflooding or water injection is where water is injected into the oil field, 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 to support pressure of the reservoir, and also 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 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.
Filters
The filters must clean the water and remove any impurities, such as shells and algae. Typical filtration is to 2 micrometres, but really depends on reservoir requirements. The filters are so fine so as not to block the pores of the reservoir. Sand filters are a common used filtration technology to remove solid impurities from the water. The sand filter has different beds with various sizes of sand granules. The sea water traverses the first, coarsest, layer of sand down to the finest and to clean the filter, the process is inverted. After the water is filtered it continues on to fill the de-oxygenation tower. Sand filters are bulky, heavy, have some spill over of sand particles and require chemicals to enhance water quality. A more sophisticated approach is to use automatic selfcleaning backflushable screen filters because these do not have the disadvantages sand filters have. The importance of proper water treatment is often underestimated by oil companies and engineering companies. Especially with river-, and seawater, intake water quality can vary tremendously which will have significant impact on the performance of the water treatment facilities. If not addressed correctly, water injection may not be successful. This results in poor water quality, bioclogging of the reservoir and loss of oil production.
De-oxygenation
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. 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 and ammonium bisulphite. Another option is to use membrane contactors. Membrane contactors bring the water into contact with an inert gas stream, such as nitrogen, to strip out dissolved oxygen. Membrane contactors have the advantage of being lower weight and compact enabling smaller system designs.
Water injection pumps
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.
