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 five barrels of water co-produced. Once water is co-produced and brought to the surface, it must be separated from oil and gas, then treated either for re-injection, discharge or re-use. For offshore, the majority of this treated water is discharged, whilst for onshore this is mostly re-injected either for disposal or for reservoir pressure maintenance purposes.
As water production increases due to maturing oil fields, plus more stringent regulatory requirements introduced by regulatory bodies around the world for discharges, water production and handling often becomes a limiting economic factor for continuing oil production for a specific field. How to best manage produced water in oil and gas production will therefore have a significant impact economically, socially and environmentally.
“it is estimated that for every barrel of oil produced, there are roughly five barrels of water co-produced”
One of the important aspects of produced water management is the measurement and monitoring of oil in produced water. Regardless of which option (re-injection, discharge or reuse) is taken by operators, water quality in terms oil in produced water must be monitored and met against discharge, re-injection or re-use standards. Here, the use of online oil-in-water (OiW) monitors plays an increasingly important role in relation to produced water treatment process optimisation and control, as well as for reporting the discharge of oil in produced water overboard.
Benefits of using online OiW analysers
There are many benefits in using online OiW monitors. These may include:
- Providing continuous OiW concentration information for process control and optimisation
- Spotting process upsets quickly
- Reducing the usage of solvents required by laboratory OiW analysis methods
- Reducing the number of manual samples for laboratory analyses
- Potentially providing more accurate OiW measurement results
Online OiW 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 OiW analyses. Furthermore, deployment of online monitors can potentially lead to more accurate oil in produced water discharge data, when compared to taking two samples and analysing them daily. Currently, for manned platforms in the UK Continental Shelf (UKCS) that discharge more than two tonnes of “dispersed” oil in produced water per annum, two samples are required to be 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.
Indeed, preliminarily studies indicate that there is a large amount of uncertainty associated with measured oil in produced water results obtained by taking manual samples and then analysing them using an approved laboratory method such as OSPAR gas chromatography and flame ionisation detection (GC-FID). These uncertainties may be as high as ±50% (at 95% confidence level). The use of online OiW monitors, particularly those installed inline, could potentially reduce such uncertainties.
Also, there is an increasing interest in unmanned and subsea oil and gas production. If produced water were to be discharged from such operations, there must be a mechanism with which oil in discharged produced water can be accurately monitored and reported for compliance monitoring purposes. There is no alternative but to use online OiW analysers.
Online OiW measurement technologies
For online continuous OiW measurements, there are five main measurement techniques. These are:
- 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 OiW monitoring technologies. 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 Analysis-based monitors offer the advantage of providing both concentration and sizes 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.
A summary of the main types of online OiW measurement technologies is provided in the table below.
Selecting an online OiW analyser
Selecting a fit-for-purpose online OiW analyser for a specific application can be a challenge, even for the most experienced of engineers. This is due to a wide range of operating principles and instruments commercially available, different types of applications, complex nature of produced water properties and characteristics, as well as the hazardous environment with which the analysers will be installed.
When selecting an online OiW analyser, the following aspects should be carefully considered:
- Purpose of the application
- Produced water characteristics
- Measurement range
- Process conditions
- Operation environment
- Installation requirements
- Previous applications/experiences with similar produced water characteristics
- Field trial/lab testing consideration
- After-sales services
It is important to realise that OiW is a method defined parameter and, as a result, different online techniques will measure OiW differently and produce different values. Calibration and correlation to an existing lab method therefore becomes important, especially if the results from the online OiW analyser will be used for regulatory compliance monitoring purpose.
In general, one should always ask for advice early in the analyser selection process. This will ensure analyser requirements are correctly specified. Asking for advice too late in the process might mean having to re-design the system and risk failure in application. Identification of similar applications and experiences early is very useful.
Considerations for selecting an online OiW analyser
There are two main purposes for measuring OiW. One is for operations such as process optimisation and the other for oil in produced water discharge reporting. For regulatory compliance monitoring, accuracy is more important. For process optimisation, measurement range and repeatability become more important.
Produced water characteristics
Produced water is a complex mixture. Whilst it is mainly water, it also contains a range of compositions: dispersed oil, dissolved oil, solid particles, production chemicals, heavy metals, dissolved gas, even radioactive materials.
Dispersed oil is mainly comprised of hydrocarbons together with some organic acids such as carboxylic acid and naphthenic acids. The amount of these acids present in the dispersed oil depends on the level of pH of the produced water. Dissolved oil may include “BETX”, i.e. Benzene, Ethyl-benzene, Toluene and Xylene, and organic acids, as well as other aromatics such as “NPD”, i.e. Naphthalene, Phenanthrene and Dibenzothiophene and Polycyclic Aromatics or PAHs. Solids may include silt, scales and precipitates and dissolved gases may include CO2, H2S. These compositions vary considerably from field to field, even with the same field but at different time of the field production history. They are also affected and impacted by tie-ins and comingling of production fluids from different reservoirs. Each of the measurement techniques will respond to the produced water characteristics differently and therefore measure and produce OiW results differently.
“it is estimated that for every barrel of oil produced, there are roughly five barrels of water co-produced”
Fluorescence and LIF are based on measuring aromatics in the produced water, while microscopy image analysis, light scattering and ultrasonic acoustic are based on measuring the sizes of oil droplets and particles. Types of oils, presence of solids particles and gas bubbles can also impact on measurement. Some are more affected by the presence of these than others.
Production chemicals present in produced water can also impact on the measurement. It is known that UV and LIF based can be affected by certain types of production chemicals. Also, oil droplets size and size distribution can affect OiW measurement, e.g. using UV, LIF or light scattering techniques. Certain techniques are more affected than others. A change in the ratio of aromatic hydrocarbons to total hydrocarbons in produced water could have a significant impact on certain types of measurement techniques, e.g. UV and LIF.
Measurement range and accuracy level
For discharge reporting, a narrower measurement range but a higher accuracy is required, while for process optimisation a bigger range is perhaps more important. Whilst suppliers’ brochures are useful, other references should also be sought to check the claims made by the vendors, e.g. previous independent flow loop testing and/or field experiences. Experiences gained from applications with similar produced water characteristics can be invaluable.
Process conditions and operating environment
Process conditions may include OiW concentration range, operating pressure, temperature, flow rate, and their variations. Operating environment may include humidity, high and low ambient environmental temperature, exposure to electromagnetic interference, etc.
OiW analysers may be fitted directly inline or on a by-pass line. If it is fitted inline, one needs to consider if the retrieval of the instrument sensor would require the stopping of the production process. Some OiW analysers are equipped with retrievable sensors. If an analyser is fitted on a by-pass line, one needs to make sure that oily water in the by-pass line will be representative of the oily water in the main pipeline. Points to consider will include how the by-pass line is arranged (i.e. connection to the main pipeline, sample point, use of quill, etc.) and what is the piping size, as well as the length.
Single point or multiple point measurement
Some analysers have the capability of multiple point measurements. These analysers usually have a single process unit that is connected to a number of sensors that can be installed at different locations of the produced water treatment process. Installation requirements will obviously be different for single or multiple point measurement arrangements. Also, the concentration range at different locations will likely be different.
For regulatory compliance monitoring or operations, multiple sensors may be installed in series as part of contingency provision. If this happens, then data from which sensors to use will need to be decided.
These may include power consumption, voltage, use of compressed air, etc.
Sample conditioning/upstream installation effect
Sample conditioning can be useful in terms of creating an oily water mixture with a consistent oil droplet size. The use of sample conditioning systems is often associated with an analyser fitted on a by-pass line. UV or light scattering-based technologies often have such sample conditioning units built in. However, the inclusion of a sample conditioning unit adds to the complexity of the overall online OiW analyser system. These sample conditioning units will also need maintenance.
Pipe fittings are well known to provide mixing, which is good for creating an oily water with a more consistent oil droplet size. However, they can also result in swirl and a change of oil droplet size. Installing analysers or taking samples for a by-pass from a vertical line is usually a better choice than from a horizontal pipe. This is due to a better mixing and more homogenous condition being achieved on a vertical pipe.
“one may also need to consider the use of multiple analysers/sensors to offer contingency”
Signal output and analyser remote access
There are various output signals available from a particular online OiW analyser. Choosing what signal to use will depend upon individual applications. Remote access is becoming increasingly important for diagnostics, checking the status of the online analyser, maintenance, and data downloading. Therefore, it is important to get the requirement right for remote access of the instrument.
Previous applications and experiences obtained (both good and bad) from fields with similar produced water characteristics are invaluable. Both vendors and operators should be approached to provide information on these. It is good practice to check internally first to see if there have been any similar applications previously. Vendors would also usually keep a record of customers and the applications, and it should be easy for them to short list similar applications. Literature is the other place to find online OiW analyser applications and experiences. There is also the possibility of checking with an independent subject matter expert.
If field applications are hard to find, then results from previous flow loop testing can also be of use. There have been a good number of flow loop tests on online OiW analysers that have been carried out on behalf of individual operators, or a group of operators, as part of Joint Industry Projects (JIPs). Not all the test results/information will be available in the public domain, but it is worth checking.
Field trial/lab flow loop testing
Before an online OiW analyser is purchased, a field trial may be arranged. A field trial is the ultimate way of finding out if the technology can work. Some vendors may also offer good terms for such a trial before a purchase. However, field trials can be logistically difficult and are always expensive. Independent lab-based flow loop testing offers an alternative option. Whist a complete replication of field produced water conditions is not possible, these flow loop tests offer a way to assess the performance and suitability of an online analyser. In a flow loop setup, various process conditions can be easily simulated. Using flow loop testing, one can also look into the correlation between the online analyser and an approved (reference) method, as well as calibration and maintenance requirements.
There is a huge variation in the costs of having an online OiW analyser supplied/fitted. The difference in cost between different vendors and technologies can run into tens of thousands of pounds. Some of LIF-based online analysers can cost well over £100,000. In addition to the capital costs, there are also running costs, i.e. for spare parts and maintenance, which should also be taken into consideration.
After-sales service is another important aspect for consideration, which may include repair work, routine maintenance, periodical calibration and validation, etc. As an online OiW analyser is not a “fit-and-forget” type of instrument, understanding what after-sales services are available and included in the purchase, and making a good use of them, will become important.
Selecting a fit-for-purpose online OiW analyser is not an easy task even for an experienced engineer. As it is usually costly and logistically complex, one should always ask if an alternative method, e.g. a field bench-top method, can be used.
If the decision is to go for an online OiW analyser because of the benefits that it may offer, then one needs to fully understand the different types of technologies, how they work and what advantages and disadvantages they have. In addition, there are many other aspects that one needs to consider and understand. These may include the characteristics of the produced water, the purpose of the applications, costs, whether to have a field trial or lab flow loop tests, installation, after-sales service, etc. If the application is critically important, e.g. subsea, unmanned, and for regulatory compliance monitoring, one may also need to consider the use of multiple analysers/sensors to offer contingency.