Subsea multiphase sampling

Eivind Gransaether

December 4, 2013

Subsea sampling can help increase PVT accuracy and improve subsea multiphase meter performance.  Test lines for subsea well testing cost as much as US$60 million and come with logistical and well intervention challenges. Eivind Gransaether explains.

The deployment of permanent subsea multiphase meters as an alternative to well testing and as a way to increase recovery has become a priority for many operators.

Subsea multiphase meters are viewed as a more flexible, cost-effective and accurate option, compared to large and maintenance-intensive test separators.

To be effective and accurate, subsea multiphase meters, which can cost up to US$400,000 to buy and integrate, need to be calibrated through quality, volumetric sampling over the field’s lifetime.

This is particularly important as fields age and more reservoir variables come into play, leading to an increase in the uncertainty of metering systems. The majority of multiphase meters, for example, use a gamma source that must be configured with the fluid properties of oil, water and gas, and ideally must reflect changing reservoir data over time.

Fluctuations in PVT Data

One of the most important sources for evaluating fluid properties and predicting reservoir performance is pressure, volume and temperature (PVT) data, which is likely to change significantly over the lifetime of the reservoir as fluid and process conditions change.

Changes can include increased liquid and water in the gas flow, higher water cuts, commingled well streams from subsea tie backs, changes in water properties from sea- or freshwater-flooded wells, or differences in salinity between injected and reservoir water.

These changes are likely to result in significant variations in PVT properties. Effective volumetric subsea sampling and the accurate tracking of PVT data can have a positive effect on the meters’ performance and operators’ production strategies in these cases.

The dangers of inaccurate PVT analysis were outlined in an SPE paper by Adel M. Elsharkawy of Kuwait University (SPE 37441). It illustrated how inaccurate PVT analysis resulted in an underestimation of the ultimate oil recovery from a particular field by as much as 40%.

Well test separators play an important role in capturing PVT data by determining the volumetric behavior of reservoir fluids as they pass through the separator. However, production often has to be ceased in order to conduct a well test, which has a negative impact on the field’s economics.

The process can also lead to potential inaccuracies on longer pipelines, and where lower pressures decrease PVT accuracy.

Subsea sampling, the alternative to well testing, has also come with limitations. These 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), oil in water content and increased salinity; and a failure to maintain original pressure conditions in the laboratory.

Conventional PVT analysis can take weeks to be delivered and can be based on a limited number of samples, which are retrieved through wireline sampling or flow tests. While oil is relatively stable, water conductivity may change between the sample being taken and results received by the laboratory, resulting in questionable accuracy.

Subsea sampling can only be effective if it meets four key criteria:

1. The sample needs to be maintained at its original pressure condition from extraction to delivery to the laboratory.

2. Sampling needs to take place regularly, and repeated on the same well.

3. Sampling must take place as close to the wellhead as possible, so that samples captured are representative of the fluid flowing through the meter.

4. Sampling must take place without interruption to production, and must generate PVT data and online, on-demand, fractions of oil, gas, water, salinity and density, without the need for subsea intervention.

These criteria have formed the basis for the Mirmorax Subsea Multiphase Sampling System.

It delivers sampling through phase-representative samples and in-situ analysis, which are integrated to allow both fractional and salinity data.

The system is built around a remotelyoperated vehicle (ROV) operated docking sampling unit, containing a hydraulic sample extraction system and sampling bottles. Via an ROV, the subsea sampling system extracts and transports the sample into sampling bottles under isobaric conditions and transports them to surface. The operation takes place a number of times on the same well. The operator is able to obtain multiple sample points on one single ROV operation in order to have a fully representative sample over a set period, and to provide the accumulated volume needed to perform analysis topside.

At the center of the system is a permanently installed analyzer module, which consists of a sampler, an analyzer, which reviews the content of the sample, and an electronics control module. If installed subsea, the analyser should be positioned near the subsea choke.

Oil, gas and water fractions can be read directly from the sampling bottles taken at a specific time window, via a BUS communications protocol to a customer control system. This enables the operator to access a phase fractional set of data that can be 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 in relation to the pressure and temperature conditions the sample was taken under.

The data generated provides information to the operator when quality checking samples, before they are extracted and transported to surface, and can determine when full PVT compositional analysis is required.

Recent Testing

The system was recently tested at the Christian Michelsen Research (CMR) Multiphase Flow Loop in Bergen, and is now being trialled at Statoil’s K-Lab facilities. During CMR testing, the system delivered water liquid ratio results in an absolute error of less than 1% and within a 90% confidence interval. Two salinity levels were also tested on the sampler, with the system able to quantify water salinity successfully. OE

Eivind Gransaether is CEO of Mirmorax AS, which he founded in 2009. Before founding Mirmorax, Gransaether had a number of roles at Roxar (now Emerson Process Management), including commercialization manager and global subsea sales manager. Eivind has a Master’s Degree in Marine Science & Subsea Engineering from the Norwegian University of Science & Technology (NTNU).