This article was published in Drilling & Exploration World (DEW) Journal in December 2011

LATEST DEVELOPMENTS IN SUBSEA SAMPLING & MONITORING

    BY EIVIND GRANSAETHER, CEO, MIRMORAX

As oil & gas demand continues to outstrip supply and as operators focus on managing costs and bolstering production, the effective subsea sampling and monitoring of oil & gas reservoirs has rarely been more important.

There are a number of reasons behind this renewed focus on sampling and monitoring relating to the following areas: multiphase meters and the need for these meters to operate at their full potential; the increased use of subsea tie-backs; the growth in brownfields and Enhanced Oil Recovery (EOR) programmes; and the rise in chemical inhibitors.

This article will examine these challenges in greater detail and examine how the latest developments in subsea sampling and monitoring (particularly oil in water monitoring) are addressing them.

The Growth & Importance of Multiphase Meters

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 provide early warnings if there is water breakthrough, for example. As well as being valuable to day-to-day operations, the meters are also a key element of long-term field development plans ensuring improved ultimate recovery over the lifetime of the field.

It comes as no surprise therefore that as of 2010, there were 3,314 multiphase meters and wet gas meters installed worldwide, according to Gioia Falcone of Texas A&M University and Bob Harrison from Soluzioni Idrocarburi Srl. The same authors estimate that this number will double over the next 10 years.

One of the biggest challenges with multiphase meters today, however, is ensuring that they continue to operate at peak performance as flow and field conditions change and as the verification of input data becomes both cumbersome to obtain and unreliable.

Here, subsea sampling and processing can play a key role in generating the fractional data on oil, gas, water, salinity, PvT (Pressure, Volume, and Temperature) and other information that the meters need to be calibrated for, in order that they can work at their full effectiveness.

The Growth in Subsea Tie-Backs

Another key driver today in subsea sampling and monitoring is the growth in subsea tie-backs.

The last few years have seen a growth in subsea tie-backs as operators look to tie in smaller fields to existing infrastructure and also manage costs. Some tie-backs today are as long as 150 kilometres and are prevalent in regions, such as the North Sea and Gulf of Mexico.

In the North Sea, for example, according to the UK trade association, the Energy Industries Council (EIC), 90% of all UK Continental Shelf offshore pipeline projects involve subsea tie-backs. Current North Sea tie-back examples include the Laggan to Shetland gas pipeline; the Sea Marten, Bright and Polecat oil & gas fields in the Central North Sea Block 20/3; and in Norway, the future Ormen Lange Phase 11 expansion project and the active Gjøa Oil & Gas field.

This growth of such tie-backs and longer horizontal production pipelines, however, brings with them a number of new challenges. The risk of longer tiebacks, for example, is that it takes longer to detect a water breakthrough in the well, which could lead to severe consequences and pipeline damage, before corrective action can be taken. The increase in carbon steel pipelines for cost saving purposes and their vulnerability to saline formation water has only increased the importance of real-time monitoring.

In such circumstances, real-time, subsea monitoring and sampling is crucial to track the fluids that are being transferred to support flow assurance and any threats to pipeline and production integrity. Potential threats to accuracy include changes in oil characteristics and varied flow conditions outside the calibration range.

Brownfields & EOR Programmes

According to the World Energy Organisation, 70% of the world’s oil and gas production comes from fields over 30 years old. This growth in brownfields has led to significant requirements in regard to subsea sampling and monitoring.

As opposed to newer fields, brownfields tend to have more flow assurance implications, with the increased danger of water and gas breakthrough in the wells – factors which can impact production capabilities significantly. The accompanying pipelines are also highly vulnerable to saline and unchecked water

The increase in brownfields has also seen an increase in enhanced oil recovery (EOR) programmes, such as the use of reinjection water to maintain field pressures. This has led to a corresponding increase in the need for water volumes to be treated in process facilities;  and the growth in chemical injection programmes.

The rise of Produced Water Re-Injection (PWRI) programmes, for example, has led to a rising  need for detailed information on the size and amount of sand and oil in produced water – whether it is reinjection, discharged or processed. The effective monitoring and control over the reinjection process can also help optimise the water flooding of the reservoir and ensure maximum production performance.

In addition, greater detail on the specific components of produced water can help improve the separation of oil and water taking place in separation process facilities, with there being real dangers to production optimisation if produced water is not carefully monitored.

Potential problems – during the separation process and production – 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.

Furthermore, in the case of chemical injection programmes, operators are looking for subsea sampling and monitoring information on how the chemicals propagate from an injection well into other wells. In this way, they can have a better understanding of their reservoirs and optimise their injection programs.

Chemical Inhibitors

Finally there is the rise in chemical inhibitors. With operators facing increased threats to flow assurance from hydrates, the injection of chemical inhibitors, such as Methanol and Ethylene Glycol (MEG) and low dose hydrate inhibitors (LDHIs), has never been more widely used. Such inhibitors are playing an important role in combating scaling and corrosion, with chemicals often used to break up surface tension and facilitate the oil & gas flow.

At the same time, however, operators also need to establish greater control over the measuring and injection of hydrate inhibitors to ensure the correct inhibitor amounts are injected and that injection rates are changed when conditions change. Subsea sampling can play a key role in guaranteeing this control.

The Rise in Subsea Sampling

We have seen some of the challenges that necessitate effective subsea sampling and monitoring, yet are today’s technologies rising to the challenge? The rest of the article will address this question – firstly, subsea sampling.

There are a wide variety of subsea sampling techniques on the market today. These include the hot stab method, extracting the samples by differential pressure, or flowing the well to a surface test facility that captures samples.

Such techniques, however, share a number of limitations. They are often used just topside and are manual-driven; samples are taken randomly without consideration to the flow dynamics of the fluids being sampled; and the original pressure conditions are rarely maintained.

The result is an inability to generate a truly volumetric representative sample that contains fluids from all the phases and that can play a key role in areas such as multiphase meter calibration, tracking the injection of chemical hydrate inhibitors, and monitoring subsea tie-backs.

It’s against this backdrop that the Mirmorax Subsea Process Sampling System (SPSS) delivers true volumetric sampling of oil, gas and water in the well as well as high quality PVT analysis, salinity and chemical content.

Via its ROV (Remotely Operated Vehicle), the subsea sampling system extracts and transports the sample into sampling bottles under isobaric conditions and then transports them to the surface.  Key components of the new system are an ROV operated docking sampling unit (DSU), consisting of a docking unit, a hydraulic sample extraction system and sampling bottles.

The ROV transports the sampling device from the surface vessel and docks onto a stationary subsea sampling interface (SSI) through a standard hydraulics and manipulator system. The two parts are then connected with a robust connector and barriers which are then 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. 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.

As well as the true volumetric representation it generates, the subsea sampling system is the ideal solution for measuring fluid composition within subsea tie-backs over the lifetime of the field, negating the high costs of subsea interventions and periodic fluid sampling.

The system also supports EOR programmes, such as chemical injection, by tracking the flow of injection fluid into the well, measuring its effects, and providing an accurate sample where chemical content can be extracted.

Finally, the validation of reservoir models and reservoir simulations depend on accurate sources of field information being generated over the lifetime of field. The subsea sampling system generates this data, irrespective of production rates or how long the field has been in production.

Oil in Water Monitoring

Again, like subsea sampling, traditional oil in water monitoring techniques have their limitations. They have tended to be manual and highly labour intensive and reliant on spot data to calculate a continuous flow, with the results often varied and inconsistent.

The Mirmorax Oil-in-water (OiW) monitor, however, counteracts this by being a highly effective processing and monitoring solution that can track water production and ensure that both production and separation facilities are performing optimally.

Traditionally designed to operate topside, we are currently developing a subsea application of the meter, which can allow for water characterisation at an earlier stage of the process and enable the monitor to become an important tool in subsea monitoring and processing.

The monitor is based on an ultrasonic measurement technique in which individual acoustic echoes from both solids and oil droplets are analysed. Each detected echo is analysed and classified as coming from an oil droplet, a sand particle or a gas bubble and concentration levels can then be calculated based on the size distribution. The monitor caters for concentrations of up to 1000 parts per million (ppm) and can provide complete size distributions ranging from two to three micrometers.

The information the monitor provides on the specific components of produced water brings with it a number of benefits.

The accurate monitoring of water during production, for example, prevents obstacles to production and plays a key role in production optimisation with ensuring maximum production performance.

For example, the monitor can detect oil and solid particles in produced water re-injection (PWRI), preventing surface sludge formation and oil saturation, facilitating wastewater disposal, and ensuring that pressures are maintained for enhanced oil recovery.

The information the monitor generates on sand and oil size distributions and concentrations will also minimise effects such as plugging and any decline in formation permeability which can reduce reservoir pressure and injectivity in water flooding operations. In addition, too much oil lost in production and a combination of fine sands and small oil droplets can also clog injection wells.

Finally, the monitor can also result in the

Rising to the Challenge

Whether it is multiphase meter calibration, enhanced oil recovery, chemical injection or subsea tie-backs, what this article has demonstrated is how crucial it is for operators today to have effective subsea sampling and monitoring capabilities in place.

While the challenges and demands are continuing to increase, it’s encouraging to see that a number of technologies are now keeping up.