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 Environment Index
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Produced Water
What is Produced Water?
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.
What do we currently know about the effects of produced water discharges
on the marine environment?
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.
What action is Oil & Gas UK taking on produced water?
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.
What is the current legislation regarding produced water?
There are over 100 installations on the UKCS which discharge produced water. In
2005 these installations discharged approximately 241 million tonnes of
water, containing 4,908 tonnes of oil.
Despite a lack of evidence of harm from this small amount of oil, OSPAR
strategies are driving operators towards the re-injection of produced water
in order to meet oil in water reduction targets. Clearly, re-injection will
also eliminate the discharge of radionuclides but it is not a universal
solution. Re-injection can either be back into the hydrocarbon reservoir or
into a specially drilled disposal well. Not all reservoirs are able to take
re-injected water and in those that can, there may be a need to filter the
produced water prior to re-injection to prevent blocking of the reservoir
rock. This may result in a secondary waste stream in the form of
contaminated filter material.
Similarly, not all installations have suitable strata available to take a
disposal well. If an installation was originally built without injection
pumps it is likely that there will be space and weight constraints which may
prevent retrofitting. It is also important to note that re-injection may not
be available 100% of the time and during this downtime it will be necessary
to discharge to sea to enable the installation to continue production.
(Although an oil installation can stop production quickly, it can be a
lengthy and complex process to restart production.) Re-injection is an
energy intensive process with significant CO2 emissions which must be
accounted for in the EU Emissions Trading Scheme. |
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Environment Index
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