According to OSPAR (OSLO-PARIS) “Recommendation 2001/1”, produced water can be defined as water which is produced during oil and/or gas production operations and is an inevitable by-product of oil and gas production. This includes formation water, condensation water and reproduced injection water; it also includes water used for desalting oil.
As oil fields mature, an increasing amount of water is produced. Currently, for every barrel of oil produced there are approximately five barrels of water co-produced. For offshore oil and gas production, roughly 75% of this produced water is treated and then discharged into the ocean with the rest being reinjected into a reservoir for disposal or for pressure maintenance. In comparison, for onshore production, roughly 90% of this water is treated and reinjected, either for disposal or for reservoir pressure maintenance, with the rest being treated and then reused or discharged.
Produced water may contain a variety of components, including salts, solids, dissolved gases, heavy metals, radioactive materials, and oil content. As some of these may cause potential harm to the marine environment when discharged to the ocean, the discharge of produced water is strictly regulated by government bodies around the world. One of the key parameters that must be measured and reported is the oil content of produced water. In the North Sea, a monthly average of 30 mg/L “dispersed oil”, as agreed by the OSPAR Convention, must be met. For many of the newer installations in the North Sea, however, a reduced figure is often set and agreed between the operator and the corresponding regulatory body/authority of the individual nation.
In general terms, oil in produced water may include dissolved and dispersed. Dissolved oil usually refers to oil-in-water in a soluble form. As aliphatic hydrocarbons generally have a very low solubility in water, it is the single ring or two ring aromatic hydrocarbons, together with compounds like organic acids and phenols, that form the bulk of dissolved oil. Dispersed oil usually means oil-in-water in the form of small droplets, which may range from sub-microns to hundreds of microns in size. Mainly comprised of petroleum hydrocarbons, both aliphatic and aromatic, dispersed oil can also contain heteroatom compounds such as organic acids.
In legal terms, oil in produced water may be defined quite differently. For example, under OSPAR, “dispersed oil” is defined as “the sum of the concentrations of compounds extractable with n-pentane, not adsorbed on Florisil and which may be chromatographed with retention times between those of n-heptane (C7H16) and n-tetracontane (C40H82), excluding the concentrations of the aromatic hydrocarbons toluene, ethyl benzene and the three isomers of xylene (TEX).” Under this definition, organics acids, BTEX (Benzene, Toluene, Ethyl benzene, Xylene) and those hydrocarbons with carbon number above 40 or below seven are not included.
In the USA, oil in produced water is also termed as Total Oil and Grease (TOG) as determined by the EPA method 1664 B. Here anything extracted by n-hexane, and which remains after the n-hexane is evaporated, is included in the measured TOG results. Organic acids, phenols, and those hydrocarbons with a carbon number above 40, that are excluded in the OSPAR definition, will be included.
Oil sheen is a visual phenomenon due to the differences between the refractive index of oil and water.
In the United States, according to the National Pollutant Discharge Elimination System (NPDES) General Permit, “sheen” means a silvery or metallic sheen, gloss, or increased reflectivity, visual colour, or iridescence on the water surface. In Canada, the National Oceanic and Atmospheric Administration (NOAA) defines oil sheens as oil layers less than 2 or 3 µm in thickness, as opposed to slicks, which are greater than 3 µm in thickness. Colourful sheens occur when the thickness of these oil layers approaches the wavelength of visible light (less than a micron).
Sheens caused by the release of oil-in-water can be characterised by their appearance. This characterisation is used to identify the extent of the sheen that has formed and can even be utilised to estimate the thickness of oil on the surface and thus the volume of oil in the area of the sheen.
In the North Sea, the Bonn Agreement is understood to be the oldest regional agreement established by governments for responding to oil pollution incidents. It is a mechanism by which nine governments of the Great North Sea and its wider approaches, together with the European Union, cooperate in dealing with pollution of the North Sea by oil and other harmful substances. Under the Bonn Agreement, oil appearance codes are given in the table below.
In general, it is understood that oil sheen formation from the discharges of produced water, or indeed from any other hydrocarbon discharge sources to the ocean, must be prevented. Oil sheen indicates an apparent oil pollution to the general public, even though the amount of oil involved in forming the oil sheen may be minimal.
When it comes to discharge of produced water offshore, many of the limits around the world were set on the assumption that if one achieves the target using the best available treatment technology, oil sheens are unlikely to be seen from such discharges.
Sheen formation associated with the discharge of produced water is a known and accepted phenomenon. For most countries around the world, there are no specific regulatory requirements relating to oil sheen formation resulting from the discharge of produced water. This is certainly the case, for example, in Australia, Canada, and in the North Sea.
An exception to the above is the USA, where the operator, permit holder is required to: “monitor free oil using the visual sheen test method on the surface of the receiving water. Monitoring shall be performed daily when discharging, during conditions when observation of a sheen on the surface of the receiving water is possible in the vicinity of the discharge, and when the facility is manned. If a sheen is observed in the course of required daily monitoring, or at any other time, the operator must record the sheen and assess the cause of sheen. The operator must keep records of findings and make the record available for inspector’s review. The operator must report total number of days of sheen observed during the reporting period.”
Overall, with the exception of the USA, there are no specific sheen formation requirements resulting from the discharge of produced water. The perception had been that if the discharge of produced water meets the oil-in-water discharge limit or performance standard set by the regulatory bodies, then oil sheen should not occur. In reality, however, oil sheen can occur regularly even if the oil content in the discharged produced water is well within the discharge limit and/or a performance standard set by the regulatory bodies.
Sheens from the discharge of produced water in the North Sea have been observed regularly. This is evident from annual aerial surveillance reports that have been published by the Bonn Agreement. These reports were compiled following aerial surveillances undertaken by Bonn Agreement Contracting Parties each year. They have shown that oil sheen can occur even when the oil in the discharged produced water is well within the monthly performance standard of 30 mg/L. Calm weather conditions have often been linked to sheen formation and observation.
“oil sheen can occur even when the oil in the discharged produced water is well within the monthly performance standard of 30 mg/L”
The detection of oil sheens is a way of identifying problems and issues that may have occurred with oil production and produced water discharges, to help prevent more potentially serious oil pollutions. Persistent oil sheen occurrence from the discharge of produced water will be taken notice of by the regulators who may ask the operators to conduct investigations and take mitigation actions.
Occasional sheens were observed on Canada’s east coast due to offshore oil and gas production. Sheen monitoring was recommended as a means of complying with Canada Oil and Gas Drilling and Production Regulations, and Newfoundland Offshore Petroleum Drilling and Production Regulations. However, there are no specific protocols outlined in the Regulations. As a result, most operators direct platform and rig personnel to watch for sheens as part of their daily activities, investigate sources if sheens are observed, and report to the Canada-Newfoundland and Labrador Offshore Petroleum Board (C-NLOPB) within 24 hours of the incident. Over a 12-year observation period between 2003 to 2014, on average there were 24 reported observations of oil sheens around the three oil production platforms in the Grand Banks. Again, the observed number of sheens indicated that sheens may be a regular occurrence around platforms when oceanic conditions are favourable for formation (calm seas).
It is generally accepted that the formation of oil sheen from the discharge of produced water is predominantly related to its dispersed oil content, i.e. oil droplets. Since produced water is essentially a continuous discharge, the ocean acts as a large separator with a longer residence time than typically utilised for oil and water separation during treatment on offshore platforms. Due to buoyancy force, with oil being in general lighter than water, these oil droplets will then rise, with some reaching the ocean surface and spreading.
Literature suggests that sheens are most likely to occur when oil droplets entrained in the produced water are large enough to rise to the surface without becoming dispersed in the water column, but are small enough to avoid breakup due to natural turbulence caused by the weather and ocean conditions such as tide and wave. However, whether an oil droplet will rise to the surface will depend on many factors. Oil droplet size is one of the key parameters; the larger the oil droplets, the faster they rise to the surface.
As soon as the oil starts to spread over the surface of the sea, it is technically classified as oil sheen. But oil sheen formation is also linked to the characteristics of the receiving water, e.g. temperature; current; wave; weather conditions such as wind and sunlight; the rate and depth of discharge; and the characteristics of the produced water such as temperature, types of oil, concentration level of both dispersed and dissolved oil and the size of oil droplets. There have been very few studies in the literature into how oil sheens are formed and under what circumstances they will form in relation to the discharge of produced water from offshore oil and gas installations.
Potential Environmental Impact
There have been very few studies done on the subject of the environmental impact of oil sheens resulting from the discharge of produced water. Most of the work done to date has come from Canada.
“there have been very few studies of the environmental impact of oil sheens resulting from the discharge of produced water”
Early work in Canada suggested that there would be little environmental impact of oil sheens resulting from produced water discharge. However, later research work from the same country has shown that seabirds may be impacted by thin sheens forming around offshore petroleum production facilities, where discharged produced water has contained currently admissible concentrations of hydrocarbons.
The work demonstrated that sub-visible sheens could result in damage to seabirds’ feather microstructure, which can result in reduced buoyancy, and (or) water penetration and increased metabolism. Depending on other stressors such as cold weather, disruption of feather microstructure from sheens could cause death from hypothermia or starvation. In addition to metabolic disruption, low levels of external oiling could also alter seabirds’ behaviour, leading to more time preening and less time feeding and tending nests. Also, sub-lethal effects are thought to be likely, impacting health and reproduction. Sheens from offshore oil and gas platforms are therefore considered to be a probable contributor to the cumulative effects of anthropogenic stressors on marine birds. It should be pointed out that the impact of oil sheens resulting from the discharge of produced water offshore and the impact of the discharge of produced water on the marine environment in general are two separate issues. A lot of work has already been done regarding the impact that the discharge of produced water may have on the marine environment in general, but there has been little in the literature regarding the impact of oil sheens on the environment.
Prevention and Mitigation
Whilst oil sheens may be linked to a very small amount of oil on a water surface and may be minimal in terms of causing actual environmental harm, public perception is that they are evidence of oil pollution. No operators or regulators wish to see oil sheens; therefore, their occurrence needs to be prevented and mitigated.
Whilst there is very little that one can do to change the ocean conditions below and above surface, actions can be taken to prevent and mitigate oil sheen formation associated with the discharge of produced water. These may include:
- Reducing the amount of water being produced and being brought to the surface through the deployment of water shut off technologies both at reservoir and downhole level or through the use of downhole separation and produced water reinjection or subsea separation and produced water reinjection technologies.
- Increasing the amount of produced water being reinjected. In the North Sea, there has been a steady increase in the amount of produced water being reinjected over the past two decades, either for reservoir pressure maintenance or for disposal. Approximately 30% of the total produced water is now reinjected in the North Sea. A similar figure is found elsewhere in the world.
- Reducing the amount of oil content in the discharged produced water. This may be accomplished by using both mechanical and chemical technologies for produced water treatment.
- Optimising the topside multiphase separation and produced water treatment processes. For example, by setting up the right interface level inside multiphase separators, optimising pressure differential ratio across hydrocyclones and dosing rate of chemicals such as demulsifier, deoiler etc.
- Deploying online oil-in-water monitoring tools that can detect process upset conditions and help optimise produced water treatment processes easily.
- Discharging produced water at a depth well below the ocean surface to allow oil droplets to disperse and degrade before they have had the chance to rise to the surface.
In summary, produced water is an inevitable by-product and a waste stream from oil and gas production. Its discharge is strictly regulated by government bodies around the world, however, oil sheen resulting from the discharge of produced water is understood to be a common occurrence, and is accepted by industry and regulatory bodies to a large extent.
Detecting of oil sheens is carried out by operators and regulators, as it offers a method of identifying more serious oil pollution from illegal discharges and/or from accidental oil spills. While oil sheens resulting from the discharge of produced water used to be considered a mere nuisance, recent research work from Canada has indicated that these oil sheens could have a detrimental impact on seabirds, and it is therefore important to prevent and mitigate oil sheen formation.
There are various methods that offshore oil and gas operators can potentially implement to prevent and mitigate oil sheen formation, including reducing the amount of water being produced, increasing the amount of produced water being reinjected, or reducing the oil content level in the discharged produced water, as well as optimising topside production processes and lowering the location of discharge point to well below the ocean surface.