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Statoil and partners to expand use of subsea gas compression to boost recovery

Since 2008, Statoil and its partners have worked with Framo Engineering to develop technology for compressing wet gas on the seabed. Statoil has now decided to take the concept a step further on the Gullfaks South subsea field.

This could increase production from the field by three billion cubic metres of gas, which means a six per cent increase in recovery.

—Ivar Aasheim, head of Field Development on the Norwegian shelf

The current field recovery rate is already 62%. The combination of subsea compression and conventional low pressure production in the later phases could lift the recovery rate to 74%.

Natural pressure declines as a field ages; this means that compression is necessary in order to extract more gas, and to get the gas to the platform. The standard solution is compression on a platform or from land; however, placing the compressor as close to the wellhead as possible enables the production of more gas from the well:

  • The compression unit comprises a gas cooler, liquid separator (in some applications) and compressor, powered from a production ship.
  • Electricity is converted to mechanical energy by a motor driving a compressor which consists of a number of vanes around an shaft.
  • The gas is introduced to the compressor at low pressure, is gradually compressed and flows out again at a substantially higher pressure.
  • Installing compressors between the reservoir and the receiving platform will reduce the pressure where the wellstream enters the seabed unit.
  • That in turn will boost production from the field by enhancing the pressure difference between reservoir and seabed receiving installation.
  • Through the compression process, the gas acquires sufficient additional pressure for it to be transported through the pipeline to the receiving platform.

The plan for the Åsgard and Ormen Lange fields calls for use of a conventional compressor. Here the moisture is removed from the gas before it is compressed on the seabed. On Gullfaks South, the wellstream flows directly into a wellstream mixing module before the total wellstream is compressed. This compression technology has been tested at Framo Engineering AS’ test facilities in Fusa outside Bergen, with a hydrocarbon blend corresponding to the Gullfaks satellites. The compressor test was concluded on 4 May 2011 with good results, Statoil says.

Statoil is currently working on three subsea compression projects in early stages. On the Åsgard field, the selected concept involves a subsea solution that will help increase pressure from the Mikkel and Midgard reservoirs. This could enhance recovery by 28 billion cubic metres of gas and 14 million barrels of condensate.

Together with operating company Shell, Statoil is are also working on a similar seabed solution on Ormen Lange.

And this is just the beginning. Subsea compression can increase recovery on several small and medium-sized fields in the time ahead. Statoil has already identified more candidates.

—Siri Espedal Kindem, Statoil’s vice president of technology

Today, almost 50% of Statoil’s production comes from a total of 488 subsea wells.

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