Knowledge Centre
Produced Water
Source of emissions
Formation water is naturally trapped in oil and gas reservoirs and despite all efforts to produce the hydrocarbons selectively, a fraction of this water is brought to the surface admixed with oil and gas. Additional water can be injected into the reservoir to help force the oil to the surface. As a result both formation and injected water are eventually produced along with the hydrocarbons. The water is separated from the oil and gas in the first stage of processing of the hydrocarbons and separated produced water is generally discharged to sea after treatment.
Produced water has a complex chemistry. It contains naturally occurring dispersed oil, dissolved organic compounds including aromatic hydrocarbons, organic acids, phenols, inorganic compounds as well as traces of chemicals added during the production /separation process. Naturally occurring radioactive materials (NORM) also occur in produced water. The most abundant are usually radium-226 and radium-228. Produced water chemical composition varies over a widely depending mainly on the reservoir geology and may also change during the production lifetime of the reservoir.
Potential environmental impacts
To be of environmental concern, the components of produced water must exhibit persistent, toxic or bioaccumulative properties. However, for these components to actually be harmful they must be available to marine organisms.
Following its discharge to sea produced water undergoes several weathering processes, partly influenced by the behaviour of the discharge plume which may be dense and sinks toward the seabed or buoyant and rises to the surface. The effluent dilutes rapidly upon discharge to well-mixed seawater. Low molecular weight organic compounds will either volatilise into the air or be degraded by indigenous micro-organisms present in seawater. Many constituents will precipitate on discharge after undergoing oxidisation processes (e.g. cationic metals). Higher molecular weight organic particles tend to absorb onto suspended solid and sediment. Individually or collectively, these processes tend to reduce to concentrations of produced water compounds in the receiving environment and thereby decrease their potential toxicity and bioavailability to marine organisms. [OGP, 2005]
Research into the effects of produced water discharges has focused on components that could result in chronic biological effects, in particular polycyclic aromatic hydrocarbons (PAHs) and high molecular weight phenols. PAHs are known to have mutagenic, carcinogenic and teratogenic properties but at the concentrations found in the receiving environment many marine organisms have the ability to metabolise and detoxify PAHs. In the laboratory, high molecular weight phenols can be shown to exhibit endocrine disruption potential explaining the difficulty to establish a clear connection between exposure and biological effects. However, for both PAHs and phenols it has not been possible to identify any significant biological effects resulting from exposure to produced water discharge.
Trace metals in produced water discharges tend to bind with particulate matter and are transported to the seabed sediment where they are immobilised and hence are not bioavailable. Similarly, radionuclides in the discharge form insoluble complexes in seawater which reduces their availability to marine organisms (see section on NORM Scale).
The environmental effect of chemical additives in produced water is discussed in the chemical discharges section.
Extensive research has been undertaken to assess the environmental effects of produced water discharges in many of the oil producing regions of the world. The overall conclusion to date is that no significant impact can be found, indicating that the assimilative capacity of the marine environment is not being exceeded.
Key control and mitigation measures
OSPAR Recommendation 2001/1 has the objective to prevent and eliminate pollution by oil and other substances from the discharges of produced water into the sea and required a reduction of oil in produced water of 15% compared with a year 2000 baseline. At the target date of 2006 installations on the UKCS had achieved a 24% reduction. The Recommendation also expects individual installations to meet an oil in water concentration limit of 30 mg/l. The average concentration across the UKCS is currently 19.78 mg/l.
In the UK, Recommendation 2001/1 is implemented by the Offshore Petroleum Activities (Oil Pollution Prevention and Control) Regulations 2005 which use a permit process to regulate discharges at each installation.
Conditions attached to the permit require daily sampling and analysis of hydrocarbon concentrations, a bi-annual sampling and analysis of the aromatics and other chemicals components and quarterly samples and analysis for NORM. The permit also includes a requirement to report through EEMS.
OSPAR Recommendation 2001/1 also advises operators to regularly assess the application of BAT (Best available techniques) and BEP (Best Environmental Practices) in relation to the management of produced water to ensure that each individual installation takes account of latest developments in BAT and BEP. A number of technologies are available to remove potentially hazardous components from produced water discharges but their cost and space and weight limitations on offshore installations must be balanced against the environmental benefit. Produced Water Re-injection (PWRI) is often seen as the ultimate solution as it means no discharge to the marine environment. However, it is not technically feasible on all installation and has a high energy requirement. The balance between a minor improvement to the marine environment and increased emissions of greenhouse gases is difficult to assess.
References
- OGP (2002) Aromatics in produced water: occurrence, fate & effects, and treatment. Report No. I.20/324, pp30
- OGP (2005) Fate and effects of naturally occurring substances in produced water on the marine environment. Report No. 364, pp42
Updated: November 2009
