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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.
In my previous article, I discussed how water production often becomes a limiting factor for continuing oil production economically. How to best manage produced water in oil and gas production will therefore have a significant impact economically, socially and environmentally. Consequently, the use of online oil-in-water monitors can play an important role in the management of produced water.
The needs
In oil and gas production, produced water brought to the surface is treated and then either discharged to the environment or re-injected back into a formation, either for disposal or pressure maintenance purposes. However, regardless of which option is taken, the quality of the treated produced water must be measured. Measurement of the water quality is also important for the control of the water treatment process itself.
Whilst laboratory bench top measurement methods are available, they require samples to be taken and brought into the laboratory for analysis. Sampling is well known to cause errors/uncertainties to the measured oil-in-water results. Also, laboratory methods tend to be laborious, time consuming, and usually require the use of solvents.
In addition, with a decline in oil and gas production in mature basins such as the North Sea, there is an increasing emphasis on maximising the recovery of the remaining oil and gas reserves. Subsea separation and produced water re-injection (PWRI) and / or discharge, alongside Normally Unattended Installations (NUI), have therefore all become increasingly considered by operators. However, without the availability of a reliable, accurate online oil-in-water monitor, the above becomes extremely difficult, if not impossible to implement.
“online oil-in-water monitors provide continuous information on a minute by minute basis and can also be used for process optimisation”
The benefits
There are many benefits in using online continuous oil-in-water monitors. These may include:
- Providing continuous oil-in-water concentration information for process control and optimisation
- Spotting process upsets quickly
- Reducing the usage of solvents required by laboratory oil-in-water analysis methods
- Reducing the number of manual samples for laboratory analyses
- Potentially providing more accurate oil-in-water measurement results
Online oil-in-water monitors provide continuous information on a minute by minute basis, if not more frequently. Thus, not only can one spot process upset conditions quickly and take actions to rectify the situation, but such monitors can also be used for process optimisation, such as chemical dosing.
The use of online monitors will also significantly reduce the number of samples taken for laboratory analyses, and therefore reduce the usage of solvents normally required for laboratory oil-in-water analyses. Furthermore, deployment of online monitors can potentially lead to more accurate oil-inproduced-water discharge data, when compared to taking two samples and analysing them daily. Currently, in the UK Continental Shelf (UKCS), two samples are taken and analysed for regulatory compliance monitoring purposes. One may argue against the representativeness of having two samples taken per day, as opposed to the actual daily average discharge concentration.
We have undertaken preliminarily studies, which indicate that there is a large uncertainty associated with oil-in-water data obtained using the existing practice, i.e. by taking samples and then analysing them using a gas chromatography and flame ionisation detection (GC-FID) method, or an alternative laboratory bench top method. This uncertainty may be as high as ±50% (at 95% confidence level). The use of online oil-in-water monitors, in particular, those installed inline, could potentially reduce the uncertainties associated with the current sampling and analysis practice.
The technologies
There are a significant number of technologies available on the market for online continuous oil-in-water measurements. They are mainly based on using the following techniques:
- Laser Induced Fluorescence (LIF)
- Light Scattering
- Microscopy Image Analysis
- Focused Ultrasonic Acoustics
- UV Fluorescence
UV fluorescence-based technologies are probably the most commonly used online oilin-water monitoring technologies of all. However, LIF technology is gaining acceptance and market share. However, LIF technology is gaining acceptance and market share due to the availability of probes which
can be inserted directly into the process pipeline. Additionally, LIF-based monitors from the leading suppliers are equipped with ultrasonic cleaning capability, which helps mitigate fouling on the sensor optical windows.
However, all fluorescence-based monitors are affected by oil droplet size and the ratio of aromatic to total hydrocarbons in the produced water. Microscopy Image Analysisbased monitors offer the advantage of providing both concentration and size of oil droplets and solid particles and are therefore popular for produced water re-injection operations. Since it is possible to view the particles/droplets present during measurement, the images produced in Microscopy Image Analysis are being increasingly used for process optimisation. Light scattering is a quick and robust process, and is well used in the shipping industry, but it is currently finding its way back for produced water applications.
The challenges
The main challenges for using online oil-inwater monitors are thought to be related to the following:
- Overcoming the perception of poor performance
- Improving the reliability of online oil-in- water monitors
- Applying online monitors for discharge reporting
- Development of sensors for subsea applications
Historically, online continuous oil-in-water monitors are perceived to be unreliable and have poor performance. When it comes to reliability, like all online measurement devices, oil-in-water monitors require a good calibration and scheduled regular maintenance to ensure high quality performance. These instruments are not a type that one can fit and forget! However, one of the key issues related to reliable operations of these devices is fouling of the optical window. Consequently, new technologies have been developed to mitigate this problem, e.g. ultrasonic cleaning being incorporated into Laser Induced Fluorescence devices, or a hydrodynamic mechanism being built into a Light Scattering sensor. Some of the instruments also use a high-pressure jetting mechanism to mitigate fouling.
As highlighted previously, evidently there is a large uncertainty associated with oil-in-water figures obtained by sampling and laboratory analyses. It is quite possible that the perception of poor performance of online oil-in-water monitors might be linked to the way in which we assess their performance, by comparing results from the online monitors to those from using sampling and laboratory methods. With a relatively large uncertainty related to laboratory results, it is entirely possible that the performance of the online monitors might have been wrongly judged in the past!
Online oil-in-water monitors have been predominately used for process trending and optimisation purposes for surface installations, but there is now a strong need by the oil and gas industry to utilise them for produced water discharge reporting purposes. To achieve this, we may need to develop new guidelines and / or new approaches to be confident in their use for this application.
For subsea applications, without a reliable and accurate online oil-inwater measurement instrument, discharge and/or re-injection of subsea separated produced water would be extremely difficult, if not impossible
The future trends
With the advancement of online oil-in-water monitoring technologies (including fouling mitigations), together with an improved perception of the performance of online monitors and a more pragmatic approach in accepting online devices for discharge reporting, we are likely to see the following trends:
- Increasing use of online oil-in-water monitors for process control and optimisation
- Increasing use of online oil-in-water monitors for reporting the discharge of produced water from manned installations
- Emerging use of online oil-in-water monitors for the management of produced water from unmanned installations
- Ongoing efforts in developing online oil-in-water monitors for subsea applications
- Exploring new ways in defining oil-in-water and in assessing the performance of online oil-in-water monitors
New JIP initiative
To fill the knowledge gaps, improve existing oil-in-water measurement guidelines, and make use of online oil-in-water monitors for produced water discharge reporting a common practice, TÜV SÜD National Engineering Laboratory has launched a new Joint Industry Project (JIP) entitled “Making Online (OiW) Analysers for Reporting (MOAR). Five operators and a government regulator have already committed to support the project. The project was formally initiated in October 2019 and would last over nine months.
Specific objectives of the JIP include the following:
- To provide a status report on online oil-in-water monitoring technologies and also on the uses of online analysers for reporting purposes
- To understand and establish uncertainties associated with oil-inwater sampling and measurements
- To refine acceptance criteria for using online oil-in-water analysers for reporting purposes
- To develop regulatory requirements for subsea discharges
- To develop guidance for accepting online oil-in-water analysers for unmanned and subsea applications
- To propose a set of changes to be made for the existing Oslo-Paris (OSPAR), Department of Business, Energy and Industrial Strategy (BEIS) and Norsk Olje&Gas O85 guidance in relation to using online oil-in-water analysers
Summary
Management of produced water is critically important for the safe and economical production of oil and gas. The uses of online oil-in-water monitors can play a significant role in production process control and optimisation, injection water quality monitoring, and discharge reporting. With technology advancements; an improved perception; a better understanding of the uncertainties associated with the exiting sampling and measurement practice; plus continued efforts from industry, we will see an increased use of online oil-in-water monitors for the management of produced water. They will become increasingly common on manned, unmanned and subsea installations, not only for operations, but also for the purposes of produced water discharge reporting.
