THE IMPORTANCE OF SUBSEA SAMPLING

BY EIVIND GRANSAETHER, MIRMORAX

The Importance of Subsea Sampling

With operators facing increased challenges in maximizing production from geologically complex and often remote and inhospitable fields, it has never been more important to generate accurate and reliable information from their wells. It is only then that these wells can operate at their maximum potential

These challenges have only increased through the growth in deepwater production facilities around the world, requiring measurement capabilities beyond what current technologies can provide. Furthermore, with many deepwater fields consisting of complex commingled streams and royalty allocations, inefficient measurement and allocation could end up costing operators significant sums of money.

This deepwater measurement challenge was recognised in 2006 by the US Department of Energy, who established the Research Partnership to Secure Energy for America (RPSEA) in 2006. The partnership recognized that deepwater measurement is a critical

need in the development of reserves and set a task (among other goals) of developing and standardizing deepwater sampling as well as looking at the means of installing measurement systems on deepwater wells via ROV.

A key means of subsea measurement today is that of multiphase and wet gas meters. Such meters provide crucial real-time information on flow conditions in the reservoir. 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.

Subsea sampling, that can provide high quality, volumetric sampling for the lifetime of the field, is key to the role multiphase meters play in many important reservoir management areas today. These include reservoir simulation and field economics, virtual metering systems, system integrity such as erosion and corrosion, the allocation of revenue from tied in fields, flow assurance (scaling and hydrate clogging), and production optimization.

Yet, today’s subsea sampling technologies are falling short of growing operator requirements.

The Limitations of Subsea Sampling Systems

There are a number of subsea sampling techniques today. The hot stab method – a technique which is used to move fluid from one device to another – tends to be the most popular although other sampling methods, such as extraction by differential pressure or flowing the well to a surface test facility that captures samples, are also used. These sampling technologies are primarily developed and used topside with multiple samples taken.

Such techniques come with limitations, however. Samples are often taken randomly without consideration to the flow dynamics of the fluids being sampled and fail to maintain the original pressure conditions of the fluid sample when in the laboratory. That differential pressures are used to sample and then differential pressures are used to transport the samples are a main source of inaccuracy.

The hot stab technique, for example, tends to be very sensitive to the specific flow regime and is incapable of making the phases of the sample more representative of the phases of the process flow. There is also little means of achieving a 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.

As figure 1 illustrates, the uncertainty of metering systems tends to grow over time – so much so that above a certain threshold, the values that are represented are so uncertain that they bring little or no value to the customer in respect to production optimization. Confidence in such real-time production data often tends to diminish over time, as field conditions change and the verification of input data becomes both cumbersome to obtain and unreliable. In addition to this, calibration of the meter often requires production to be stopped, costing the operator hundreds of thousands of dollars.

Figure 1

By adding a subsea process sampling system, 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.

This not only allows operators to calibrate the fractional values of the meters by adding new property and fractional input data, but it also allows the operator to re-process old data by applying the updated parameters to the metering systems data processing software and then running the desired timeframe again.

It’s up to subsea sampling to deliver true volumetric sampling on oil, gas and water in the well without interrupting production. In this way, the operator will be able to accurately capture fluid properties throughout the lifetime of the field, conduct comprehensive PVT and chemical analysis, calibrate multiphase and wet gas meters, deliver optimized well production, and increase oil & gas recovery from the reservoir.

Developing a New Subsea Process Sampling System

So how can these limitations be addressed? How can true volumetric sampling on oil, gas and water in the well be delivered without interrupting production? The rest of the article looks at the criteria we went through in meeting these challenging and developing an effective subsea sampling system.

What was clear to us was that any system we developed needed to go subsea. It’s 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.

We decided that an important means of achieving this is through a design that is compatible with subsea ROV operations.

The system we developed also needed to represent 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 needed to take place while maintaining the sample at its original pressure conditon all the way through to the lab. Maintaining the pressure condition and the true representation of the process is key to providing accurate PVT analyses.

It is against this criteria that one of the two main 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 tool extracts and transports the sample or samples into sampling bottles under isobaric conditions and then transports the samples to the surface. 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 will provide the accumulated volume needed to perform analysis topside.

The system has been designed for HP/HT applications of up to 1000 bar/15,000 PSI and 180 °C / 350°F and a design depth of 3,500 meters. Testing has also shown the system to be in compliance with design codes even when it is tested at up to 22,500 PSI.

The end result is that the system not only provides a high quality representation of the hydrocarbons, but also an accurate ‘split’ and representation of all phases, solids and chemical content

What Next?

So what next subsea sampling?

The new system is currently undergoing qualification testing. Once operational, it wil provide a vital source for calibrating multiphase meters and ensuring that they operate at their full potential.

And with areas, such as the North Sea having ever more complex ownership structures, it is hoped that effective subsea sampling can help meters meet the fiscal metering requirements so many operators and government authorities are requiring.

What is clear is that accurate subsea sampling is going to have a crucial role to play in future offshore operations.