This article was published in the Chemical Engineer in December 2007 by the previous owners of the Oil-in-water product line, Roxar. Mirmorax acquired the Oil-in-water (OiW) product line from Roxar in March 2011.

THE GROWING IMPORTANCE OF OIL-IN-WATER MONITORING

                  BY GEIR AANENSEN, ROXAR

The Rise in Produced Water

The last few years have seen a significant increase in global water production in the oil and gas industry.

Whereas today current oil production is 80 million of barrels per day approximately, current estimates of global water production are 250 million barrels per day – a three to one ratio. Today, the average water cut globally[1] is 75 per cent – a five per cent increase on water cuts ten years ago.

The increase in produced water is being seen on the Norwegian Continental Shelf where water/oil ratios have increased from 0.93 in 2005 to 1.13 in 2005 and annual emissions of oil into the sea are estimated at 3000 tons of oil (see figure 1)[2].

Taking a Closer Look at the Water

With the increase in produced water has come the increased need from E&P (Exploration & Production) operators for detailed information on the size and amount of sand and oil in produced water – whether it is reinjection, discharged or processed water.

There are a number of drivers for this – some economic, some environmental and some both.

Optimising Production 

There are a number of means in which increased oil in water monitoring can help optimise production.

Firstly, there is lost revenue due to oil being lost through produced water discharge. According to energy industry analysts Douglas-Westwood, 2.1 million barrels of oil are lost every day through water discharge[3].

Secondly, greater detail on the specific components of produced water can help optimise the separation of oil and water which takes place in separation process facilities and which has increased over the last few years with the maturing of fields.

The information on sand and oil size distributions and concentration will help the operator optimise the separation process and ensure that all separation equipment is designed to and working within its operating range with respect to particle size.

Accurate oil in water monitoring has a vital role to play in efficiently monitoring separators, hydro cyclones and chemical injection and accurate knowledge on size distributions will also aid the operator in optimising production through the enhanced design and use of separators and filters.

Finally, there are real dangers to production if produced water is not carefully monitored – not just during the separation phase but throughout production. Potential problems can include the plugging of disposal wells by solid particles and suspended oil droplets, the plugging of lines, pumps and valves due to inorganic scales, and corrosion due to the electrochemical reactions of the water with piping walls.

Real-time monitoring will enable the operator to make knowledge based decisions when it comes to water treatment facilities.

The Brownfield Challenge 

Linked to the challenge of optimising production through more effective oil in water monitoring is the growing challenge of brownfields.

Today more than 70 per cent of the world’s oil and gas production comes from fields that are over 30 years old[4] – fields which may well have started off producing very little water but are producing large volumes of water and increased water cuts today. In these cases, the ability to efficiently and economically dispose of this water is critical to success.

Another inevitable result in the growth of brownfields and the need to increase recovery rates (which currently tend to be between 35 and 40 per cent) is reinjection water to ensure pressures are sustained.

In this case, it is essential that all oil and solid particles in the produced water re-injection (PWRI) are detected to ensure higher recovery rates and longer lifetimes for existing oil fields. If not, surface sludge formation and oil saturation can cause significant problems.

Information on sand and oil size distributions and concentration will also minimise effects such as plugging and decline in formation permeability which can reduce reservoir pressure and injectivity in water flooding operations.

Effective monitoring and control over the reinjection process will optimise the water flooding of the reservoir and ensure maximum production performance.

The Environmental Challenge 

We have discussed some of the economic advantages of effective oil in water monitoring but probably the single biggest driver today in accurate oil in water monitoring is the environmental driver – the need to meet environmental requirements on produced water discharge.

Today, oil in produced water accounts for about 90 per cent of the total amount of oil discharged into the North Sea by the oil and gas industry[5].

Measurement of oil in produced water is now required by law. Regulations include the 2000/2001 Oslo/Paris Convention (OSPAR)  – also known as the Convention for the Protection of the Marine Environment of the North-East Atlantic; the UK’s Dispersed Oil in Produced Water Trading Scheme and The Norwegian State Pollution Control Authority (SFT)’s regulations, which call for zero harmful discharge into the sea.

OSPAR is the international regulation with its goal being to ‘…prevent and eliminate pollution by oil and other substances caused by discharges of produced water into the sea.’ 

The required performance for OSPAR in 2007 is that ‘no individual offshore installation should exceed a performance standard for dispersed oil of 30 mg/l for produced water discharged into the sea.’ 

Within this context, it is essential that E&P operators can demonstrate to regulators and government the effective monitoring of oil in water. And the need to adhere to legal requirements and avoid financial penalties is not the only driver.

An effective monitoring of discharges and attempts to reduce such discharges through accurate monitoring can open up opportunities for participating in emission trading schemes.

Manual Sampling and Its Flaws 

So what technologies are available for oil in water monitoring today?

Traditionally, oil in water monitoring consisted of manual sampling. According to what was previously the OSPAR defined reference method, this would consist of taking one litre samples from the produced water discharge, acidifying to a low PH and then extracting with tetrachloroethylene (also known as perchloroethylene, perc, PCE, and tetrachloroethene).

Once the solvent is extracted, infrared quantification would then take place with oil content determined by the infrared absorbance of the sample extract and the total -CH₂   that is present (as defined in the OSPAR Agreement 1997-16).

According to OSPAR regulations, at least 16 samples must be taken each month for installations that discharge more than two tonnes of dispersed oil per year.

There are a number of down-sides to manual sampling, however.

Firstly, as they are spot samples and as the concentration of the oil in water often vary over time, operators are not getting the full, accurate picture. The use of spot data to calculate a continuous flow is only valid if the measured component is consistent with time. Figure 2 provides a good illustration of the dangers of manual sampling.

There is also potential confusion as to what constitutes ‘dissolved’ and ‘dispersed’ oil with both extracted by the extracting solvent. Whereas dispersed oil tends to refer to small droplets in produced water (containing aliphatics, some aromatics (PAHs) and acids), dissolved oil can also take the form of soluble hydrocarbon compounds, such as benzene, ethyl benzene, toluene, and xylene (BTEX) which are only partially soluble in water.

When the calibration takes place after solvent extraction, it is the total absorbance of -CH₂ measured that is plotted against the known concentration of the crude oil (total hydrocarbons) in the solvent.

As a result what is measured using the IR method are the total hydrocarbons including both the dispersed and dissolved oil. The result is that dissolved oil is often included in the dispersed oil content, making it more difficult for operators to effectively and accurately meet the OSPAR target of ‘dispersed oil not exceeding “30 milligrams per litre (mg/l).’ (italix)

Health & Safety 

There are also concerns about the health and safety implications of tetrachloroethylene – so much so that OSPAR today recommends a new reference method involving Gas Chromatography and Flame Ionisation Detection (modified ISO 9377-2 GC-FID).

While this is to be applauded, there is a real danger that this will lead to even greater inconsistencies in manual sampling due to the inherent differences between ISO 9377-2 GC-FID and the previous method of infrared quantification.

Whereas in countries, such as the Netherlands, there is no legal requirement to avoid tetrachloroethylene, in countries, such as Norway and Denmark, an alternative method has become a priority. In the UK, the new OSPAR reference method as detailed above came into force on 1 January 2007, although in the words of the Department of Trade & Industry ‘it is anticipated that some offshore facilities will continue to use the IR method.’ 

The result is inconsistent ways of analyzing the spot samples with varying results.

Staff Productivity 

And the final, perhaps most obvious downside of manual sampling, is the labour intensive nature of the process. A more automated form of monitoring would have a significant impact on freeing up resources and improving staff productivity.

The Need to Monitor the Separation Process 

Another weak link in oil in water monitoring is during the separation process where chemicals, such as biocides, emulsion breakers or corrosion inhibitors, are often used to improve oil/water separation.

How the chemicals are used can influence the final result. If, however, you have information on the amount of oil in water, and especially the droplet size distribution during different stages of the separation process, you have more empirical information to go on when introducing the chemicals. The oil droplet size distribution found at different stages in the process may influence your separation efficiency significantly.

The Emergence of Online Monitoring 

A constant theme throughout this article is the gap in information available to the operator. While the latest in multiphase metering technology is enabling operators to have accurate, real-time information on flow rates, water and sand in the well stream, the same doesn’t appear to be the case for oil in water monitoring.

The situation is changing, however, with the emergence of online, inline oil in water monitoring technologies. The move towards ‘inline’ monitors, where there is no need for sidestreams or sample extractions and where the monitor design is essentially like a flow instrument similar to a multiphase meter, is an important development.

Online, inline monitoring and its ability to provide direct measurements at the dispersed and suspended phase provides clear benefits to the operator with more detailed information on the size distribution and concentration of oil and sand in water and, as a result, more accurate discharge figures; a reduction in labor intensive sampling; and an avoidance of exposure to solvents, such as tetrachloroethylene.

The fact that the monitoring is able to take place in real-time also provides a highly effective early warning system. When the water sample analysis comes back from the laboratory showing that something is wrong, the damage may already be done. With online monitoring, if something happens, such as the identification of a process upset, you know about it and can react accordingly (as a result reducing oil pollution).

Real-time monitoring also optimises the entire ongoing separation process. With any deviation, one can quickly step in so that production can continue and be optimised. Separators, hydro cyclones and the type and regularity of chemical injection can all be run accordingly. The environmental and economic impact is obvious.

Remote Management 

And with the rise in remotely managed operations and increase in subsea tiebacks, online, inline oil in water monitoring provides effective knowledge-based maintenance for remote operations with information distributed and assessed by both offshore and onshore personnel.

Ultrasonic Pulse Echo Technology 

Yet, if online monitoring offers such clear benefits over manual analysis, why isn’t it more prevalent and a regulatory requirement today?

There have been a number of previous obstacles to online monitoring from the complex mixture of produced water through to concerns about the accuracy, maintenance, calibration and its robustness in harsh environments.

The result has been that, in the past, the furthest online monitoring has developed is as a tool for process monitoring rather than for regulatory compliance monitoring.

Today’s technologies and in particular ultrasonic pulse echo technology, however, are overcoming these concerns.

Ultrasonic pulse echo technology provides enhanced robustness in produced water environments. Exploiting the full range of properties of a propagating scalar wave field (diffraction, attenuation, time-of-flight, etc), ultrasonic measurement techniques are commonly found in a wide range of industrial applications such as medical ultrasound, non-destructive material testing and oil and gas.

The Roxar Oil-in-water monitor (see Figure 3), which is based on a patented solution with TNO Science and Industry, is built on an advanced ultrasonic pulse-echo technology.

A highly focused ultrasonic transducer is inserted directly into the produced water flow, enabling direct measurements on the suspended particles and dispersed oil phase. In the transducer focus, particles passing through the measurement volume will scatter the transducer beam and generate reflected waves or acoustic echoes. These acoustic signatures contain particle specific information.

The peak amplitude of the scattered signals from each passing particle is then used to characterise the suspension.  A large number of peak amplitude measurements are performed to generate a distribution of peak amplitudes.  From the distribution of these peak amplitudes, the particle size distribution and particle concentration can be calculated from accurate acoustic scattering models.

Higher Concentrations and Simultaneous Results 

With an increasing focus on oil in water monitoring at higher pressures, there is also a need for oil in water monitors to operate at higher concentrations.

And to optimise separation and water treatment processes, there is also a need to distinguish between gas, oil and sand and evaluate different particles simultaneously.  For re-injection applications in particular, the sand is a major concern when it comes to water flooding of the reservoir.

The Roxar Oil-in-water monitor can cater for concentrations of about 1000 parts per million (ppm). And by separating and analysing individual acoustic pulse-echo measurements, the monitor can provide complete size distributions ranging from the extremely low two to three micrometers.

Simultaneous calculations can be made using the generalised scattering model where scattering curves for oil and sand respectively are implemented in the model, and using feature extraction and classification of echoes, the correct forward model is used for each individual response.

One of the additional benefits of ultrasonic technology over more traditional technology is that it can ‘sound penetrate’ material. If there is an issue of oil film or scaling, the ultrasonic technology can work just as effectively and accurately simply because the ultrasonic energy will penetrate the layer and still transmit a signal into the produced water flow.

There is no need for detergents or other, separate cleaning mechanisms. And with a reference signal being continuously extracted from the system, the operator can make knowledge based decisions when it comes to maintenance intervals.

Online Monitoring – Taking on the Concerns 

A number of other traditional concerns on online monitoring are also being allayed. Take calibration and recalibration, for example, which is often required when chemical compositions change. Since the measurements are performed directly on the dispersed phase, this reduces the need for recalibration when the chemical composition changes.

The challenge surrounding reliability and robustness are also met head-on. With many of today’s oil in water monitors unable to work properly over long periods in harsh environments, the Roxar Oil-in-water monitor has been designed to be reliable, easy to maintain and have a long lifespan with the ultrasonic technology enhancing robustness.

By using advanced auto diagnostics functionality, the Oil-in-water monitor is also able to detect and overcome challenges, such as equipment degradation, scaling and temperature or salinity changes. In addition, the monitor has a ‘one size fits all’ that can be fitted on all pipe dimensions and is suitable for installation in hazardous conditions.

Meeting Expectations

With the increasing focus on subsea and downhole processing, the increase in water volumes and upstream separation and the rise in remote operations, as well as, of course, the growing environmental pressures, there is a real need in today’s oil and gas industry for accurate, online, inline monitoring of oil in water.

With the development of truly inline monitors for permanent installation, ultrasonic technology for enhanced robustness, sizing and classification capabilities and the ability to provide knowledge based maintenance for remote operations, oil in water monitoring technologies are now finally beginning to meet the E&P industry’s expectations.

Geir Aanensen is Business Unit Manager, Oil-in-water at Roxar Flow Measurement and can be contacted at Geir.Aanensen@roxar.com. Roxar is a leading international technology solutions provider to the upstream oil and gas industry.