Subsea Sampling Delivers Value

?Getting reliable well data has become a serious challenge. Deepwater production facilities require 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.

Identifying the problem

The deepwater measurement challenge was recognized in 2006 by the US Department of Energy, which established the Research Partnership to Secure Energy for America (RPSEA) in the same year. The partnership recognized deepwater measurement as a critical need in developing reserves and set a task of developing and standardizing deepwater sampling as well as looking at the means of installing measurement systems on deepwater wells via ROV.

A key way of making subsea measurements today is to use multiphase and wet gas meters that 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, which 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.

System limitations

There are several subsea sampling techniques in use. The hot stab method – a technique that 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, also are used. These sampling technologies primarily are developed and used topside with multiple samples taken.

Such techniques have limitations. Samples often are 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. One set of differential pressures is used to sample, and then other differential pressures are used to transport the samples, which introduces inaccuracy.

The uncertainty of metering systems tends to grow over time. Above a certain threshold, the values that are represented are so uncertain that they deliver little or no value in terms of production optimization. (Images courtesy of Mirmorax)

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 also is little means of achieving a volumetric representative sample or being assured that the sample contains fluids from all of the phases. The result is low-quality samples, no volumetric representation, and low repeatability.

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 deliver little or no value in terms of 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, meter calibration often requires production to be stopped, costing the operator hundreds of thousands of dollars.

By adding a subsea process sampling system, operators can generate fractional data on oil; gas; water; salinity; pressure, volume, and temperature (PVT); and other information for which meters need to be calibrated.

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 old data to be re-processed by applying the updated parameters to the metering systems data processing software.

Subsea sampling needs to deliver true volumetric sampling on oil, gas, and water in the well without interrupting production so the operator can 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 recovery from the reservoir.

Developing a new system

These limitations needed to be addressed to achieve true volumetric sampling without interrupting production.

It was clear that any system developed needed to work subsea. It is only through sampling at or near the wellhead that samples that are representative of the fluid flowing through the meter can be generated, yielding more accurate fluid properties and more accurate multiphase measurements.

The system is shown in sampling mode after the DSU has been docked onto the SSI. The DSU is inserted into the funnel. When it is fully inserted and connected, the operator can extract representative samples without interrupting production. The operation can be repeated for multiple sample points on a single ROV operation.

Mirmorax decided that an important means of achieving this would be through a design that is compatible with subsea ROV operations. The system developed also needed to represent a seamless process from sample collection to final analysis topside – from extracting a representative sample, taking it to the surface, and storing and transporting it to the laboratory facility. And all of this needed to take place while maintaining the sample at its original pressure through to the laboratory. Maintaining the pressure condition and the true representation of the process is vital to providing accurate PVT analyses.

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(s) into sampling bottles under isobaric conditions and 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 connected with a connector and barriers tested to verify pressure integrity.

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

The system has been designed for HP/HT applications of up to 1,000 bar/15,000 psi and 350°F (180°C) and a design depth of 11,500 ft (3,500 m). Testing also has shown the system to be in compliance with design codes even when it is tested at 22,500 psi.

The result is a system that not only provides a high-quality representation of the hydrocarbons, but also an accurate “split” and representation of all phases, solids, and chemical content.

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