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.
Discharge To Sea
After treatment, produced water still contains traces of oil and, because of this, discharge into the sea is strictly controlled by legislation. Under the terms of an international convention which takes into account the need to safeguard the marine environment, the permitted level of oil in produced water that can be discharged from an installation is 40 ppm, averaged over a month. Operators must analyse the oil-in-water level twice each day and report the results to the regulatory authorities. In addition to their obligation to meet this legal requirement, UKOOA members are committed to achieving an annual average oil content of 30 ppm at company level. In 1998, the industry total average oil in produced water was 22ppm, reflecting a trend of improvement since 1992.
Figure 4 shows the quantities of water discharged from offshore installations over the last three years, along with the amounts of oil discharged with the produced water. The progressive increase in produced water results from a combination of an increasing number of producing installations and increasingly aging fields, while the gradual decrease in the concentration of oil in water arises mainly from improvements in the technology used for separating small quantities of oil from water.
||Number of installations
||Water quantity (millions of tonnes)
||Oil levels (ppm)
||Oil quantity (tonnes)
The small quantities of water that arise from gas reservoirs may contain traces of natural gas liquids, called condensate, and Figure 5 shows the quantities of water discharged from offshore gas-producing installations over the last three years, along with the amounts of condensate discharged with the produced water. The rise in the amount of produced water is for the same reasons mentioned above.
||Number of installations
||Total produced water (tonnes)
||Average condensate content (ppm)
||Total condensate discharged (tonnes)
Environmental Impacts Of Oil In Produced Water
The agreed average oil level was established to prevent adverse effects on marine life. In addition, produced water may contain residues of chemicals added during the production process, along with dissolved organic and inorganic compounds that were present in the geological formation. After it is discharged, produced water is first diluted by the turbulence close to the installation, then widely dispersed by marine currents.
In addition to laboratory tests which show that produced water is not toxic to marine life, the profuse growth on submerged structures and the large populations of fish that inhabit the waters below offshore installations provide ample evidence that produced water discharges do not adversely effect marine life.
Oil discharges are generally thought of in terms of large accidents that result in stranded oil and damage to marine life. Discharges of oil in produced water cannot be compared with large oil spills because the small amounts of oil are dispersed within the sea and do not form a surface slick. Long term or chronic effects are also very unlikely given the miniscule levels of contaminants in produced water discharged.