BY EIVIND GRANSAETHER, CEO, MIRMORAX

While the last few years has seen a host of new subsea technology innovations, few technologies have developed quite as fast as that of subsea sampling.

Originally characterised as a manual-focused process with little consideration for flow dynamics and fluctuating reservoir conditions, today subsea sampling is providing operators with real-time information on oil, gas and water fractions as well as fluid properties and PVT data from closer to the wellhead than ever before.

The Importance of Subsea Sampling

There’s no doubt as to the importance of subsea sampling today. As operators look to generate a more complete picture of the reservoir, increase recovery rates and extend field life, subsea sampling is playing a crucial role – particularly in supporting multiphase meters.

Such meters can only operate to their full potential if they are precisely calibrated and sensitively aligned with the changing fluid, process conditions and flow rates of the reservoir. Such changing conditions might include an increased amount of liquid and water in the gas flow, growing water cuts, or varying well characteristics. And such calibration can only be achieved through the true volumetric sampling of oil, gas and water in the well.

Accurate PVT (Pressure, Volume, and Temperature) data is also vital. If operators can generate an accurate understanding of the PVT properties of their reservoir fluids and changes in fluid properties, such as density and viscosity (often as the reservoir is depleted), the more they will be able to predict reservoir behavior, ensure production optimisation and calibrate multiphase meters for maximum performance.

Such data is also particularly important as fields start to age, as is the case in many North Sea fields. In such cases, as figure 1 illustrates, the uncertainty of metering systems tend to increase and up to date volumetric sampling information is required to put the meters back on track. With the cost of purchasing and integrating just one multiphase or wet gas meter approximately US$400,000, subsea sampling can have a significant impact on the field’s economics as well.

Figure 1

Overcoming Technology Limitations

So given the importance of subsea sampling, what are the technology limitations preventing accurate and volumetric sampling?

The limitations of previous and even current sampling technologies include samples being taken randomly and topside; little consideration for flow dynamics; an inability to track and react to fluctuating reservoir conditions, such as high Gas Volume Fractions (GVF) and increased salinity; and a failure to maintain original pressure conditions in the laboratory.

The latter point is particularly important when it comes to water conductivity. While oil is relatively stable, water conductivity may change significantly during the time between the sample being taken and the results received by the laboratory. An inabilty to maintain the exact conditions subsea can jeopardise the validity of these results.

Furthermore, there are also the dangers of fluid contamination in cases where samples are exposed to oil-based drillling or other reservoir production fluids, for example, and the inevitable cost implications and risks surrounding subsea intervention.

The result is no volumetric representation in the samples, low repeatability and an incomplete and potentially inaccurate sample with a lack of PVT data and other information the operators require.

New Subsea Sampling Developments

It’s with these restrictions in mind that Norwegian-based Miromorax has committed to developing a subsea sampling system that delivers true volumetric sampling on oil, gas and water in the well – below the flowline, in controlled conditions and without interrupting production. In this way, the operator can accurately capture fluid properties, calibrate multiphase and wet gas meters, and ensure that the wells are performing at the peak of their production limits to deliver flow assurance

Mirmorax is delivering on this through a new Subsea Multiphase Sampling System (SMSS) that delivers accurate sampling through phase representative samples and in-situ analysis capabilities that are integrated to allow both fractional and salinity data.

At the centre of the new system is a permanently installed Analyzer system module that, mounted to the multiphase sampler, provides online, on-demand fractions of oil, gas, water, salinity and density without the need for subsea intervention.

The Analyzer uses a unique acoustic-based technology where online quantitative values can be obtained by just a push of a button and then made available through standard communications. Based on the functionalities of the in-line multiphase sampling system, the operator can then read the oil, gas and water fractions directly from the sampling bottles taken at a specific time window, providing a phase fractional set of data that can be directly compared to the metering data for the same time period.

By calibrating this fixed point at given pressure, temperature and volumetric fractions, the operator can provide the multiphase meter with a fixed point and increase the meter’s accuracy significantly in relation to the pressure and temperature conditions the sample was taken under.

Figure 2, for example, illustrates the correlation between variables in a multiphase meter. The landscape 3D plot illustrates the correlation curve between two core data inputs and the output value. The red point places the 3D plot of the meter in reference at a fixed point, with PVT data then needed to ensure that all values correlate.

Figure 2

The online fractional data generated through the Analyzer also provides crucial, high-value information to the operator when quality checking samples before they are extracted and transported to surface and can also help determine when a full PVT compositional analysis is required. Knowledge of the sample quality and representation before extracting and transporting for laboratory analysis also removes any risk of faulty samples and reduces transport and analysis costs.

Sometimes the knowledge generated through the system prior to extraction may even be enough to support EOR and other multiphase meter activities without the need for further laboratory analysis.

The System has also undergone rigorous testing at the Christian Michelsen Research Multiphase Flow Loop in Bergen.Here, the subsea sampling system delivered excellent Water Liquid Ratio (WLR) results with an absolute error of less than 1% within a 90% confidence interval; and GVF results that more than met expectations.

In addition, the system was able to accurately quantify the salinity of water. With salinity affecting water conductivity and adding uncertainty to the fraction measurements of the flow, such information is crucial to the operator in calibrating multiphase meters and optimising production.

It is the Miromorax Subsea Multiphase Sampling System’s ability to take measurements directly from the flowline under measurable and controlled conditions with fluid properties and PVT data taken closer to real-time and closer to the wellhead that is revolutionising subsea sampling today. For the first time, operators, through just a push of a button, can generate online, on-demand fractions of oil, gas, water, salinity and density without subsea intervention.

Other Technology Innovations

Yet, it is not just in subsea sampling that Miromorax is making an impact subsea. The Mirmorax oil in water monitoring solution has also been taken subsea.

With increased water production and discharges in the North Sea, the growth in brownfields, and stricter environmental regulations, the detailed information on the size and amount of sand and oil in produced water – whether it be reinjection or discharged – is crucial for operators today. And it is with these issues in mind that Mirmorax has developed a highly effective, and robust oil in water monitoring solution.

The system is based on an ultrasonic measurement technique in which individual acoustic echoes from both solids and oil droplets are analysed. Recent new developments also include new temperature and salinity measurements; an auto calibration feature to compensate for scale build-up and that allows for several millimeters of scale and oil to grow on the ultrasound transducer without affecting accuracy; and self-cleaning mechanisms that prevent the danger of thick oil clogging the system.

Through developments such as this, the monitor can cover a wider measurement range and compensate for layers of scaling and possible contamination. Salinity measurement is also a valuable source of data when determining the origin of water coming from multiple wells. Information about the salinity and density mix also provides the operator with verification of how much of the separation capacity is being used for each of the fields.

Rising to the Challenge

With operators needing greater control and insight into their subsea operations than ever before, the spotlight has fallen on subsea technology development.

It is encouraging to see that companies, such as Mirmorax, are rising to the challenge.

Eivind Gransaether is CEO of Mirmorax AS, an industry leader in sampling, monitoring and processing solutions to the oil & gas industry which he founded in 2009. Prior to founding Mirmorax, Eivind had a number of roles at Roxar (now Emerson Process Management) including Commercialisaton Manager and Global Subsea Sales Manager. Eivind is a graduate of the Norwegian University of Science & Technology (NTNU)