What is Produced Water?

Almost all offshore oilfields produce large quantities of contaminated water that can have significant environmental effects if not handled properly.  

Oil and gas reservoirs have a natural water layer (called formation water) that, being denser, lies under the hydrocarbons.  Oil reservoirs frequently contain large volumes of water, while gas reservoirs tend to produce only small quantities.  To achieve maximum oil recovery, additional water is usually injected into the reservoirs to help force the oil to the surface.  Both formation and injected water are eventually produced along with the hydrocarbons and, as an oil field becomes depleted, the amount of produced water increases as the reservoir fills with injected seawater.

At the surface, produced water is separated from the hydrocarbons, treated to remove as much oil as possible, and then either discharged into the sea or injected back into the wells.  In addition, some installations are able to inject produced water into other suitable geological formations. 

Water is present in some form in the majority of reservoirs before oil production takes place. In the most common sort of reservoir, oil accumulates above large volumes of water. This water is known as an aquifer, and the reservoir is a water-drive reservoir.

Produced water is therefore a by-product of oil production. New fields generally have a very small fraction of water produced with the oil extracted. The ratio of water present to total production is referred to as the ‘water cut’. As fields mature and the hydrocarbon reservoir is produced, the level of oil in a reservoir is depleted. The water beneath the oil therefore moves higher within the reservoir, and so more water is produced with the oil. Consequently, in older fields, where the water cut is high, produced water can account for up to 95% of the fluids produced. Indeed, in many older fields produced water re-injection (PWRI) is used whereby pressure in the reservoir is increased by pumping produced water back in; thus making extraction of remaining oil reserves all the more easier.

Understandably, this mix of fluids must be separated so that the oil produced can be moved onshore. Fluids from the reservoir are fed into separator units, where oil and water are allowed to settle out. There are strict limits to the concentration of oil that is allowed to be discharged in produced water. The removal of oil from water can therefore involve many stages to adequately reduce the oil content before the water is discharged overboard.

Produced water contains a mixture of inorganic and organic components derived from the reservoir rocks and hydrocarbons, together with a range of chemicals which are added to improve production and to assist the separation of oil from the water.

On discharge to the sea, produced water is immediately dispersed and diluted to the point where no immediate environmental effect can be detected. This apparent lack of effect has resulted in several studies to examine the bioaccumulation and endocrine disruption potential of produced water components. The complex composition of produced water makes direct assessment of these properties in the laboratory difficult. Oil & Gas UK is working with regulators and CEFAS in a programme to identify the components of produced water which have the potential for biological activity and to see if they can be found in the environment surrounding installations.

Oil & Gas UK is active on the issue of oil in produced water, and recently with DBERR, facilitated the advent of an oil in produced water (OIPW) trading scheme - essentially a cap-and-trade scheme which encourages operators of offshore installations to sell on spare allowances for oil discharge, should their annual discharge fall below their allowance.

Many installations on the UKCS are now in their mature phase and produce significantly more water than hydrocarbons. With a favourable oil price, wells that are producing 95% water can still remain commercially viable. It is technically challenging and in most cases simply not feasible, to reduce the quantity of water being produced from a reservoir. Mitigation options are, therefore, ‘end of pipe’ and include removal of hazardous components prior to discharge, or re-injection back into subsurface strata.

Given that a large installation could be discharging up to 40,000 tonnes of water per day and there is no capacity for holding tanks, any treatment of hazardous components must be a continuous process. Currently available treatment technologies are aimed at reducing the oil content of produced water discharges in line with OSPAR strategy.

In addition to oily content, some of the soluble radionuclides and particles of NORM scale will pass through the system and be discharged with the produced water. We are not aware of, nor are we sponsoring any research into technologies for the removal of radionuclides from produced water.

Please see links for Produced Water Studies (Flow Analysis) and PSI.