Water injection (oil production)
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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.
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[edit] Injected Water Sources
Generally, water produced from the reservoir (at the same time as the oil) is preferred for use as an injection fluid. Using produced water avoids the potential of causing formation damage due to incompatible fluids. Also, the produced water, being environmentally unfriendly, must be disposed of in some manner, and this method has an economic benefit. As the volumes of water being produced are not sufficient to replace all the production volumes (oil & gas), make-up water must be used to ensure that all the reservoir voidage is replaced.
When it is ocean water, it is pumped through large pipes which extend far out into the ocean. It is collected in large basins at a seawater intake pumping station. The seawater is pressurized and passed through filters to the de-oxygenation tower.
[edit] Filters
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.
[edit] De-oxygenation tower
Oxygen must be removed from the water because it encourages certain bacteria to grow. If bacteria grows in the oil, it will block the porous rock in the oil reservoir. The filtered water drops into the de-oxygenation tower, splashing onto various trays causing gases to be released. The gas is sucked out of the tower with vacuum pumps and the water fills the bottom of the tower. The de-oxygenated water is taken by boosting pumps to the high pressure water injection pumps xxx.
Several systems exist to remove the oxygen from the water, all based on Henry's law. They consist in lowering the oxygen partial pressure in the gas phase in contact with the water. Gas stripping towers or vacuum towers can be used. Proprietary designs (e.g. MINOX) based on two-stages cocurrent static mixers and promoting "more compact" solutions also exist.
[edit] 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. For example, if the water injection is stopped for a day, the production rate does not change, unlike with gas lifts.
Gas lifts are the opposite, if the injection is interrupted the production will stop immediately, because gas lifts have only a short term effect.
