Oilfield Technology (June 2011)



Subsea Sampling and Oil in Water Monitoring

The increased challenges operators are facing as they look to maximize production, squeeze more oil & gas from their older fields, and meet environmental requirements, are seeing a renewed focus on two vital but often under-reported technologies – subsea sampling and oil in water monitoring.

While tending to focus on different elements of the reservoir – in the case of subsea sampling below the reservoir surface and in the case of oil in water monitoring normally on an offshore platform  – both technologies share one key challenge.

That is the need to introduce greater intelligence, automation and subsea engineering principles into their operations and move away from the manual focus that has too often dominated both technologies in the past.

Let’s take a look at subsea sampling and oil in water monitoring in greater detail.

The Rise in Subsea Sampling

One of the key means of generating accurate and reliable information from oil & gas wells is through multiphase and wet gas meters. Such meters provide crucial real-time information on flow conditions in the reservoir. They can be used to determine maximum oil production and gas handling capacity and can provide early warning signs if there is water breakthrough, for example.

Aligned to this and just as important, however, is the process of subsea sampling. It is accurate subsea sampling that leads to the precise calibration, accuracy over time, and effectiveness of these metering systems. By adding a subsea process sampling system, for example, operators can generate fractional data on oil, gas, water, salinity, PvT (Pressure, Volume, and Temperature) and other information that the meters need to be calibrated for.

Subsea sampling, that can provide high quality, accurate volumetric sampling for the lifetime of the field, support such multiphase meters in the areas of reservoir simulation, field economics, system integrity, revenue allocation, and production optimization – to name just a few. In summary, subsea sampling is central to reservoir management and the monitoring of reservoir operations.

Yet is subsea sampling rising to the challenge?

Subsea sampling techniques on the market today include the hot stab method, extracting  the samples by differential pressure, or flowing the well to a surface test facility that captures samples.

All these techniques, however, share a number of limitations. Firstly, the techniques are often used topside and are manual-driven; samples are taken randomly without due consideration to the flow dynamics of the fluids being sampled; and the original pressure conditions are not maintained during the fluid sample’s journey to the laboratory.

The focus on the manual and the risk of human error means that there is therefore little way of achieving a truly volumetric representative sample or being assured that the sample contains fluids from all the phases. The result is low quality samples, no volumetric representation, and low repeatability.

Oil in Water Monitoring

As with subsea sampling, oil in water monitoring is an equally important technique where the technologies don’t seem to keeping up.

The last few years have seen a significant increase in global water production. One of the main reasons for this is the growth of brownfields and Produced Water Re-Injection (PWRI) to ensure higher recovery rates and a longer lifetime for existing oil fields.

This increase in produced water has led to a growing need for detailed information on the size and amount of sand and oil in produced water – whether it is reinjection, discharged or processed water. Effective monitoring and control over the reinjection process will optimize the water flooding of the reservoir and ensure maximum production performance.

There are a number of other drivers behind the increased focus on oil in water monitoring.

There is the lost revenue due to oil being lost through produced water discharge; greater detail on the specific components of produced water can help optimize the separation of oil and water, taking place in separation process facilities; and there are real dangers to production optimization if produced water is not carefully monitored.

This is 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.

And then there are environmental regulations. 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. Within this context, it is essential that E&P operators can demonstrate to regulators and governments the effective monitoring of oil in water.

So are oil in water monitoring technologies doing better than their subsea sampling counterparts in meeting these increased operator demands?

Again, there are a number of flaws to traditional techniques.

Traditionally, oil in water monitoring tends to be manual, with samples taken from the produced water discharge, acidified to a low PH and then extracted with a chemical known as tetrachloroethylene.

Once the solvent is extracted, infrared quantification then takes place with oil content determined by the infrared absorbance of the sample extract and the total methylene (CH2) that is present. 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.

There is also potential confusion as to what constitutes ‘dissolved’ and ‘dispersed’ oil with both extracted by the extracting solvent. 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 environmental requirements.

The result is inconsistent ways of analyzing the spot samples with varying results and, in terms of employee productivity, a highly labour intensive process.

A Focus on Subsea Engineering – Subsea Sampling and Oil in Water Monitoring

Against this backdrop, we at Mirmorax are focusing on providing operators with a more automated and intelligent subsea sampling and oil in water monitoring system built on strong subsea engineering principles.

Taking subsea sampling first, we have developed an ROV-based subsea sampling system that collects samples subsea. It is only through sampling at or near the wellhead that samples, representative of the fluid flowing through the meter, can be generated, yielding more accurate fluid properties and more accurate multiphase measurements.

The sampling system via its ROV extracts and transports the sample into sampling bottles under isobaric conditions and then transports them to the surface. Key components of the new system is an ROV operated docking sampling unit (DSU), consisting of a docking unit, a hydraulic sample extraction system and sampling bottles. The sampling unit itself is based on standard subsea engineering principles and is a combination of field proven technologies, such as the hydraulic actuator, collet connector and system for testing sealing integrity.

The second key element – essential in taking samples subsea and isolating the sample from the process – is a stationary subsea sampling interface (SSI). The ROV transports the sampling device from the surface vessel and docks onto the stationary SSI through a standard hydraulics and manipulator system. The two parts are then connected with a robust connector and barriers which are tested to verify pressure integrity.

The operation described is repeated multiple times on the same well in order to secure a number of samples over a certain time period. This ensures accuracy on the sample in case of unstable flow and provides the accumulated volume needed to perform analysis topside.

The result is a seamless process from sample collection to final analysis topside – from extracting a representative sample, taking to the surface and then storing and transporting to the laboratory facility.  And all this takes place while maintaining the sample at its original pressure conditon all the way through to the lab.

In oil in water monitoring, we have recently acquired the Oil-in-water (OiW) product line, an online and inline oil-in-water monitor for topside oil and gas applications, from Roxar Flow Measurement, a division of Emerson Process Management.

As part of the acquisition, Mirmorax has also signed an agreement to secure all Intellectual Property (IP) rights for the product with Dutch technology company TNO Science and Industry. TNO was part of the original Joint Industry Project (JIP) with Roxar in developing the monitor along with Statoil, Eni SpA, and Shell and Petroleum Development Oman (PDO).

The Oil-in-water monitor and its ultrasonic pulse-echo technology provide accurate, real-time information on the amount of sand and oil dispersed in water and is an important alternative to previous manual-dominated operations.

The monitor is based on an ultrasonic measurement principle. 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.  Each detected echo is analyzed and classified as coming from an oil droplet, a sand particle or a gas bubble. Concentration levels can then be calculated based on the size distribution.

The monitor caters for concentrations of about 1000 parts per million (ppm) and by separating and analyzing individual acoustic pulse-echo measurements, the monitor can provide complete size distributions ranging from two to three micrometers. Calculations can be made simultaneously for oil and sand.

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.

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). Furthermore, by using advanced auto diagnostics functionality, the monitor is also able to overcome challenges, such as equipment degradation, scaling and temperature or chemical changes.

We have plans to develop the monitor for subsea applications, allowing for water characterization at an earlier stage of the process and enabling the monitor to become an important tool in subsea processing.

In this way, the acquisition will help us come closer to attaining our goal of delivering innovative, high quality subsea processing solutions that help operators optimize flow assurance, meet environmental requirements, and generate the best possible returns from their reservoir assets.

Together, with our subsea sampling system, we are injecting much-needed subsea engineering principles into subsea processing operations, ensuring that operators are finally able to meet their production optimization and environmental challenges.

The manual-focus of the past is now very much behind us!