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Discharge of produced water is strictly regulated, although discharge standards may vary from location to location. The quality of produced water is most widely expressed in terms of its oil content, which acts as a “surrogate” for other pollutants.
As a result, measurement of oil in produced water concentration becomes an important subject, not only for regulatory compliance monitoring but also for operations, as well as data collection for the development of future government regulations and corporate environmental policy. To calculate the total amount of oil that is discharged via the discharge of produced water, the volume of produced water also needs to be measured accurately.
As a National Measurement Institute (NMI), over the past two decades NEL has supported many produced water related activities, aimed at improving the produced water measurement and management practices. Ultimately these activities help minimise the environmental impact of the discharge of produced water for the benefit of society.
Water compositions and potential impact
Produced water is an inevitable by-product of oil and gas production. It is co-produced along with oil and gas during normal production operations. In a mature field, a water cut level of 95% or more is not uncommon. Worldwide, on average it is estimated that for every barrel of oil produced, there are roughly four to five barrels of water co-produced. Often water production becomes a limiting factor for continuing oil production economically for a specific field.
Produced water contains a wide range of contaminants. These include:
- Hydrocarbons: aliphatic and aromatic hydrocarbons including Polycyclic Aromatic Hydrocarbons (PAHs) – they are present in produced water in the forms of dispersed and dissolved
- Soluble organics: naphthenic acids, carboxylic acids and phenols – they can present in both dispersed and dissolved forms, depending on the pH of the water
- Total Dissolved Solids (TDS)
- Total Suspended Solids (TSS)
- Production chemicals
- Heavy metals
- Radioactive materials • Dissolved gas, e.g. H2S, CO2
With the contaminants listed above, it is not difficult to see that discharge of produced water could potentially cause harm to the marine environment.
Produced water management practice
Management of produced water varies with operators, geographical locations and field history. It involves many aspects, including using water shut-off technologies inside the reservoir, to prevent water getting into the wellbore; and downhole and subsea separation, to stop water being brought to the surface. If water has to be brought to the surface, then it must be treated and either discharged or re-injected.
Good produced water management practice would involve different disciplines, including reservoir engineers, production engineers, production chemists, and environmental engineers. It would also require an integrated approach, taking into consideration the different options, as well as production history and profile.
The key to produced water management is to prevent water getting into the wellbore, and to minimise the water being brought to the surface. Failing this, produced water must then be treated, re-injected and / or discharged or reused.
Produced water club
To help industry tackle these types of issues, our Produced Water Club has welcomed more than 100 organisations around the world over the last two decades. It helps both individual member organisations and the industry as a whole with business developments, formulation of new research ideas, information sharing and the promotion of industrywide best practices.
Its core aims are to:
- Keep up to date with legislative and technology developments
- Provide a central contact point on the subject of oil-in-water measurement and monitoring
- Promote best practices aimed at reducing oil pollution
- Bridge the various industry stakeholders, including regulators, operators, technology providers and R&D organisation
The club uses an open industry forum to discuss measurement and management challenges and technology developments, with information and practices being shared. From these meetings, research projects including Joint Industry Projects (JIPs) were formulated to tackle vital oil and gas industry challenges identified by the group.
Oil-in-water analysis methods
Oil-in-water concentration used to be measured by extracting oil from water using Freon-113 (a solvent that was known to cause ozone depletion and damage to the environment) and then analysed using an Infrared absorption-based instrument.
As OSPAR (Oslo and Paris) planned to introduce a new reference oil-inwater measurement method based on Gas Chromatography and Flame Ionisation Detection (GC-FID), there was a need to prepare the industry by developing a guidance on how to implement such a method. GC-FID is a laboratory based analytical method, which is not ideally suited for offshore operations for various reasons. It was therefore envisaged that alternative methods to the GC-FID method would be used by operators in the North Sea. However, such methods must be correlated to the GC-FID method or demonstrated to produce results that are equivalent to the GC-FID method.
“the study was to prepare for a smooth transition from the use of an infrared based reference method to the new GC-FID based reference method”
The JIP was supported by both the UK and Dutch regulators, together with eight offshore operators. The objective of the study was to prepare both the regulators and the industry for a smooth transition from the use of an infrared based reference method to the new GC-FID based reference method. Specifically, the project aimed at the following:
- To identify the best practical means to implement the new reference method
- To establish best practice guidelines for sample taking and handling
- To develop a realistic set of acceptance criteria for alternative methods
- To advise on how to relate data from the GC-FID method to the existing Infrared based method
The key outcome of the JIP was the development of the OSPAR1 and the UK DECC (Department of Energy and Climate Change, now Department of Business, Energy and Industrial Strategy – BEIS) guidelines on oil-in-water sampling and measurement2. These guidelines are still being used today.
Produced water volume determination JIP
As mentioned earlier, to calculate the total amount of oil being discharged into the sea via the discharge of produced water, one requires the accurate measurements of both oil in produced water concentration and discharge volume.
One of our early surveys indicated that estimations of produced water discharge volume were done using a variety of methods, from direct measurement with a flow meter to using well test information or pressure differential data of a hydrocyclone. As a result, it was understood that there was a huge variation in terms of uncertainties associated with the measurement of the produced water volume that was discharged.
At the time, the UK government introduced the Oil Pollution Prevention Control (OPPC) Regulation, which requires that the uncertainty in volume measurement of produced water be within ±10%. Also, the UK Offshore Operators Association (now Oil and Gas UK) was considering the introduction of a produced water trading scheme. Both of these initiatives had increased the importance of the accurate determination of produced water discharge volume.
This JIP was thus initiated to improve the understanding of existing practices, and to develop produced water volume verification procedures and guidelines. Ultimately, the project was aimed at helping the operators to improve produced water management and to assist the UK in meeting a 15% reduction target in discharge of oil via the discharge of produced water. The JIP was supported by the UK regulator together with five operators, and its outcome was a sponsorwide best practice guidance on how to determine produced water volume.
Other JIPs
In addition to the OIWAM and ProVol JIPs, we have instigated a number of other JIPs. Between 2009 and 2017 three JIPs were specifically conducted on behalf of operators in helping develop a subsea water quality measurement sensor for produced water. These projects not only helped advance water quality measurement technology for subsea applications, but also helped improve the reliability and accuracy of those technologies for surface applications. The advancements of these measurement technologies in turn helps improve the oil and gas production process and therefore minimise the environmental impact by using less production chemicals, reduced contamination levels, and less sampling and laboratory analyses.
Summary
Produced water is an inevitable by-product of oil and gas production, but its discharge could potentially harm the environment. Good management and measurement practice is therefore crucially important in minimising this potential harm, and must involve multi disciplines and use an integrated or holistic approach. With an increasing water production from mature fields; and also the move toward deep-water, chemical EORs (enhanced oil recovery), heavy oil and unconventional oil and gas production; the oil and gas industry is facing a significant number of challenges in order to minimise the potential harm that can be caused by the discharge of produced water.
References
- OSPAR Agreement 2005-16 “Oil in produced water analysis – Guideline on criteria for alternative method acceptance and general guideline on sample taking and handling”, 2006.
- BEIS (formerly DECC), “Methodology for the sampling and analysis of produced water and other hydrocarbon discharges”, 2014.
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