Putting On The Pressure Under Water

Statoil and its partners on Åsgard have opted for subsea gas compression to help recover the big remaining reserves in this Norwegian Sea field. Similar projects elsewhere are in the pipeline.

Many years of development work underpin this choice.

Subsea gas compression has also been assessed on Gullfaks and for the Ormen Lange field together with operator Shell.

Leap

"Compressing gas on subsea installations represents a considerable technological leap for the industry," says Margareth Øvrum, executive vice president for Technology & New Energy.

"With this technology in place, the recovery factor and producing life can be substantially increased for a number of gas fields."

Preparations for seabed compression have come furthest on Åsgard, where the Midgard and Mikkel gas deposits are tied back to the Åsgard B platform.

These two reservoirs have been developed with seabed installations, and their wellstreams are piped to the gas production floater about 40-50 kilometres away.

Analyses show that pressure in Midgard and Mikkel will become too low to sustain their ability to produce to the B platform towards the end of 2014.

"Installing compressors will sustain the gas flow and ensure a long producing life for the platform," explains Ståle Tungesvik, head of reserves and business development in Statoil.

"This makes it possible to recover large volumes – we're adding 28 billion cubic metres of gas and 14 million barrels of condensate after a possible investment decision."

The latter is due to be reached in the first quarter of 2011.

Alternative

The alternative to subsea compression on Åsgard would be to build a new platform with conventional surface compressors.

Detailed assessments underlie the decision by Statoil and its partners, and these show that the chosen option represents the best concept in commercial terms.

"Moving compression to the seabed gives both improved energy efficiency and lower costs compared with keeping it on a platform or on land," explains Øvrum.

"The closer to the well we compress the gas, the higher the efficiency and the production rates."

Interest

Developing subsea gas compression also opens exciting prospects for the supplies industry, and the work is being followed with great interest by Norwegian and foreign technology companies.

Partners at the Åsgard field: Petoro AS (35.69 per cent), Statoil (operator, 34.57 per cent), Eni Norge AS (14.82 per cent), Total E&P Norge AS (7.68 per cent) and Mobil Development Norway (7.24 per cent)

How gas compression works

• Compressing gas on seabed installations will help to boost the recovery factor from the reservoir. This process is needed to boost pressure so that the gas can be piped to Åsgard B.
• The compression unit comprises a gas cooler, liquid separator and compressor, with the latter due to be powered from the Åsgard A oil production ship.
• This electricity will be 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 latter unit 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.