This article was published in the Indian Institution of Engineers Magazine in August 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

Global water production is on the increase – so much so that today we are producing more water than oil.

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.

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)[1].

In the illustration, the y-axis is standard cubic meters in millions with the graph showing the current and estimated amount of produced water being discharged into the sea. With the current average oil-in-water content for all the installations on the Norwegian Continental Shelf, this translates into approximately 3000 tons of oil.

Taking a Closer Look at the Water

The increase in produced water, whether it is reinjection, discharged or processed water, has also led to a growing need from the operator for a better monitoring of produced water.

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

Optimising Production

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

There is the increase in revenue by separating the oil from the produced water. According to energy industry analysts Douglas-Westwood, 2.1 million barrels of oil are lost every day due to oil being lost through produced water discharge[2].

There are also other potential problems during the production phase that can be alleviated through produced water monitoring. 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.

Careful monitoring and quick preventative action can save quite literally millions of dollars.

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 also 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.

There are real dangers to production if produced water is not carefully monitored – not just during the separation phase but throughout production. 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[3] – 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 – especially produced water re-injection (PWRI) which is utilised to ensure pressures are sustained and to increase recovery rates.

It is essential that all oil and solid particles in the produced water re-injection 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.

Addressing the Environmental Challenge

Another key market driver in the development of reliable and accurate oil-in-water monitoring is the tightening 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[4] and a number of environmental regulations have emerged over the last few years to ensure the accurate measurement of oil in produced water.

Leading this is the 2000/2001 Oslo/Paris Convention (OSPAR) – also known as the Convention for the Protection of the Marine Environment of the North-East Atlantic.

OSPAR covers all the oil-producing coastal states of Western Europe with its goal being to ‘…prevent and eliminate pollution by oil and other substances caused by discharges of produced water into the sea.’  The key requirement 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.’

OSPAR means that operators must now demonstrate to regulators and government the effective monitoring of oil in water. As well as avoiding any financial penalties, accurate monitoring can also open up opportunities for participating in emission trading schemes.

The Weaknesses of Manual Sampling

So, with there being a clear demand from operators, are today’s oil in water monitoring technologies rising to the challenge in offshore production?

If this question had been posed a few years ago, the answer would have had to be ‘No’. That was when manual sampling was the predominant tool for oil in water monitoring.

According to what was previously the OSPAR defined reference method, manual sampling 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 methylene (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 tons 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, the IR method measured 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).’

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 analysing the spot samples with varying results.

Staff Productivity

The final, perhaps most obvious downside of manual sampling, is the labour intensive nature of the process and the negative impact on staff productivity which makes it unpopular with operators. A more automated form of monitoring would have a significant impact on freeing up resources and improving staff productivity.

The Emergence of Online Monitoring

Online, inline, real-time monitoring of oil in water, however, meets many of the requirements of today’s operator, providing 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 labour intensive sampling; and an avoidance of exposure to solvents, such as tetrachloroethylene.

In being an inline monitor with no need for sidestreams or sample extractions, the monitor acts as a flow instrument providing direct measurements at the dispersed and suspended phase. Here the monitor design is essentially like a flow instrument similar to a multiphase meter.

Online, inline monitoring also includes more detailed information on the size distribution and concentration of oil and sand in water. 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.

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 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?

Previous obstacles to online monitoring have included doubts as to its inability to effectively characterise complex water mixtures through to concerns about the accuracy, maintenance, calibration and its robustness in harsh environments.

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

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.

Through the insertion of an ultrasonic transducer directly into the produced water flow, ultrasonic technology takes individual acoustic pulse-echo measurements from solids, oil droplets and gas. These are then separated and analysed to provide accurate information to the operator on size distribution and concentration for oil and sand. The Roxar Oil-in-water monitor caters for concentrations of about 1000 parts per million (ppm).

An added benefit is that the technology can ‘sound penetrate’ material. If there is an issue of oil film or scaling, the ultrasonic technology can work just as effectively and accurately because the ultrasonic energy will penetrate the layer and still transmit a signal into the produced water flow. This is not the case with the majority of today’s oil-in-water monitors which are reliant on optical technology.

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

Through the ultrasonic technology, 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.

There is also 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.

The net result is increased accuracy and a positive impact on both optimising production and meeting environmental requirements.

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.

Changing the Way Oil in Water is Monitored

Today’s technologies are changing the way oil in water is being monitored, providing the operator with greater detail and accuracy in their water characterisation information as well as greater reliability and robustness. With the need to optimise production, meet environmental requirements and maximise returns from brownfields, the timing couldn’t have been better.

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. 

 

Case Study – Statoil

The Roxar Oil-in-water monitor was installed at Statoil’s Sleipner A platform on May 16^th 2006. Sleipner A is a fixed platform located in the North Sea in block 15/9, approximately 240 kilometers west of Stavanger, Norway and serving the Sleipner East, Sleipner West and Sigyn gas and condensate fields. The installation produces gas/condensate from different wells and the concentration and oil droplet size range is expected to be relatively low.

Statoil will use the Roxar Oil-in-water monitor to measure overboard water discharge from the platform and ensure that it meets environmental requirements for limited or zero oil emissions into seawater. The monitor will also act as an early warning detection system in the water treatment facility and will play a vital role in helping Statoil efficiently monitor the separation process.

Since installation, several tests have been completed to demonstrate the performance of the monitor. Data analyses clearly show that both in terms of accuracy and sensitivity, the monitor performs according to specifications. There is, as expected, a clear correlation between measured oil in water concentration and changes in the water level in the gravity separators.

The Statoil pilot to date has confirmed the Roxar Oil-in-water monitor’s ability to provide accurate information to Statoil on the size distribution and concentration of oil and is already playing a key role in monitoring Statoil’s overboard discharge and separation process.