Subsea gas compression to boost recovery rates

The queue of gas compression projects being formed by Statoil as it seeks to meet the Norwegian government's demands to further enhance production and recovery rates from the country's continental shelf is growing all the time.

The company is the first to admit that seabed compression represents a major technological leap into the future for the E&P industry. But it is a milestone that has long been a target for the operator as it stretches the limits in its efforts to improve already impressive recovery rates from its gas fields.

The reasons for developing subsea gas compression technologies are as follows, according to the company:

• Gas fields require boosting of the reservoir flow as reservoir pressure depletes;
• Subsea gas compression replaces the need for an offshore platform or onshore compression facility;
• It offers a cost-effective development solution (capex);
• It reduces operational costs (opex);
• It is advantageous to place the compressor close to the well;
• Production is increased and accelerated;
• COemissions are reduced through lower energy consumption;
• There are no emissions or disposals to sea; and
• The process is safer due to unmanned operation.

It is for the above reasons that Statoil has been at the forefront of subsea developments for decades, particularly in the area of subsea processing where it is acknowledged as a world-leader. Its historical track record is impressive, as the following milestones in subsea processing advances illustrate:

• Hydrocarbon boosting on the Lufeng field offshore China;
• Separation at the Troll pilot project and separation and boosting at the Tordis project, both offshore Norway;
• Raw seawater injection at the Norwegian Tyrihans development; and
• Compression at the Gullfaks, ?sgard, and Ormen Lange pilot projects offshore Norway.

This year the company has managed to achieve 10 years of operation on the seabed for its Troll pilot project, concept selection approved for Gullfaks subsea compression, final investment decision made on ?sgard subsea compression, and installation of the Ormen Lange field subsea compression pilot (at the operator's test pit in Nyhamna, Norway, for production operator Shell).

The Gullfaks project offshore Norway is Statoil’s second largest subsea gas compression project. According to the company, seabed gas compression is a key element toward establishing the reality of a true ‘subsea factory.’ (Images courtesy of Statoil ASA)

Esgard, Gulfaks, and Ormen Lange

Statoil's first subsea gas compression project to boost production when needed will be on the ?sgard field, where it will have two massive 11.5-MW compressors placed near the wells to produce enough pressure to push the gas to the field's platform as natural pressure declines. The official plan for development and operation was officially approved by the Norwegian government in April.

But the company's Gullfaks and Ormen Lange pilot projects also will see the technology used, with the company's Gullfaks South project in particular set to benefit most in the short-term.

This field has already achieved an extremely impressive recovery rate of 62%, but Statoil has said the combination of subsea wet gas compression and conventional low-pressure production in later phases could eventually lift the total recovery rate to 74%. This equates to what would be around 3 Bcm of increased gas production (an additional 6% of the field's reserves in the ground).

Statoil and partner Petoro will have to invest around US $490 million in this technology for Gullfaks South, with the aim of completing it in 2015 (the same year as planned for the completion of its ?sgard gas compression project) to extend the Gullfaks field's production plateau. The main Gullfaks field itself began producing 26 years ago.

The natural pressure in the Brent reservoir on the Gullfaks South field will be decreasing considerably by 2015, it is estimated, which is when the compression project will kick in. Although not quite on the same scale as ?sgard, the scale is still massive, involving two 5-megawatt (MW) gas compressors installed in a subsea template in 135 m (443 ft) water depth. Together they will handle a flow rate of 10 MMcm/d of gas. They will be tied in to existing templates and pipeline systems around 15 km (9 miles) from the Gullfaks C platform facility, where power and control modules will be integrated.

Schlumberger's Framo Engineering is carrying out the engineering, procurement, and construction of the compressor station, including the topsides power and control systems for Gullfaks C.

Margareth ?vrum, Statoil executive vice president, Technology, Projects, and Drilling, said in a press statement, "Innovation and technology development are essential to improved oil and gas recovery and extended life for the fields on the Norwegian Continental Shelf. The development of subsea compression and processing is a central part of Statoil's technology strategy for long-term production growth."

Each of the two Gullfaks South 5 MW wet gas compressors will be located by the ‘L’ and ‘M’ templates on the seabed and handle 10 MMcm/d of gas. The technology will help increase recovery from the field by 22 MMboe (a 4% rise).

The subsea factory

It is essentially also a central part of the company's long-stated ambitions to achieve what it dubs the "subsea factory" by 2020, just five years after it achieves the planned startup of its subsea gas compression technology on ?sgard and Gullfaks South. Statoil wants to achieve all the elements it needs, including subsea gas compression, to be able to install subsea factories on the seabed in challenging frontier areas such as the Arctic (quickly being opened up offshore Norway in the Barents Sea) as well as in remote deepwater regions around the world.

On top of this, the company also sees the application of subsea wet gas compression as a typical solution for small and medium-sized fields on its continental shelf, where it can downsize what is a flexible concept and apply it on both new and brownfield developments, with several such candidates already identified.

With seabed gas compression and the wider seabed processing spectrum of technologies becoming increasingly viable, Statoil's subsea factory vision is driving it inexorably toward the eventual application of more sophisticated gas processing on the seafloor (gas sweetening and gas dehydration) and also toward the development of local, potentially renewable power generation concepts.

As pumping and compression technologies continue to evolve, more robust and simplified systems also will become available that are capable of greater pressure boost while handling liquids without the need for upstream scrubbing. Separation systems will become more sophisticated, and new materials – including nanotechnology – will play their part.

All these are seen by Statoil as technologies that can enable it to achieve its goal of economically developing fields using its subsea factory concept by 2020, and pioneering wet gas compression projects like Gullfaks South will be a vital proving ground on the road toward that goal.