Innovative technological approaches are being sought by Statoil specialists like Rune Mode Ramberg and Eric Ulland to develop small fields with a relatively high gas content. Maximising profitable recovery is a key aim.
Imagine an inflated balloon being emptied through a long straw. Resistance in the tube stops the flow once half the air has gone, and the rest must be sucked out. The same approach can be applied to gas fields, with a pipeline taking the place of the straw.
Boosting pressure as production emerges from the well will enhance recovery. "With the help of an extra "lung", a wellstream compressor in this case, we want to impart additional energy to the gas," explains chief engineer Ove Kallestad in Statoil's subsea technology sector.
"That raises pressure in the pipeline and helps us to recover 10-15 percent more of the resources in fields containing gas and condensate." He explains that the group is planning to apply this technology to new developments based on placing production equipment directly on the seabed rather than on expensive platforms. Statoil's process and subsea specialists are cooperating closely in this area.
Most of the small Norwegian fields so far developed with subsea solutions have contained oil, primarily because these discoveries used to outnumber gas finds. This is now changing, since the country's remaining resources largely consist of gas. Operators are preparing to tap a growing number of gas-rich small fields.
Increasing wellhead pressure from such reservoirs will not only allow more of the resources to be recovered but also let the wellstream be piped over long distances to existing platforms or directly to land.
Mr Kallestad envisages a future on the Norwegian continental shelf where production from small fields and new discoveries in deep water is transported for many kilometres to the nearest infrastructure.
But such approaches can only be made profitable by improving existing technology, and this is where the wellstream compressor represents one option.
But care must be taken to dimension the compressor to cope with the size of the fluid fraction in the gas/condensate mix, otherwise the machine could be damaged or its efficiency reduced.
"Success here will depend on a substantial technological commitment from Statoil, our suppliers and our partners in the relevant licences," says Mr Kallestad. He expects the growing proportion of gas in Norway's undeveloped resources to create a greater demand for wellstream compression than has been the case so far.
Operators must also cut development costs if they are to bring small fields on stream at a profit. The compressor will help here by making it possible to use existing pipelines and processing facilities.
And it can contribute to enhancing resource utilisation as well as to extending the producing life of fields.
Stepping up gas pressure forms a natural part of the process on a traditional offshore platform. The compressor is placed on the installation and imparts energy after wellstream separation.In other words, this device requires that all the fluid fractions – water, oil or natural gas liquids – have been removed before the gas reaches it.
Statoil has experienced several cases of compressors suffering a complete breakdown after being subjected to excessive fluid input. The wellstream compressor of the future must be able to raise both fluid and gas pressure simultaneously while preventing the former from separating out, and to work under water without maintenance.
Statoil is planning to develop a new generation of such compressors in cooperation with other oil companies and the Norwegian offshore supplies industry. Eric Ulland, an adviser in subsea processing, and Rune Mode Ramberg, staff engineer for rotating equipment, are involved in this work on behalf of the group.
Their job includes checking that this new piece of equipment is incorporated in subsea production systems with the highest possible operating efficiency. Subsea installations featuring a wellstream compressor will need a lot of power – as much as 10 megawatts.
By comparison, existing seabed systems with associated pumps for multiphase flow and water injection consume up to two megawatts. Mr Ulland adds that technological advances are also required for the electrical components, and notes that work on wet couplings and transformers for subsea frequencies is proceeding in parallel.
An umbilical from land will not only supply the wellstream compressor with power, but also with lubricants and control signals. Statoil is initially thinking of applying the "balloon" method to its Mikkel gas discovery in the Norwegian Sea, where production can begin in 2002-2003 at the earliest.
If this field gets the green light to supply gas under Norway's existing sales contracts with European buyers, the licensees want to develop it with subsea installations.
Mikkel could be tied back over a distance of 35 kilometres to the subsea installations on the Midgard section of Statoil's Åsgard project. But Mikkel is 70 kilometres from the start of the Åsgard Transport trunkline which would carry its gas to Statoil's treatment complex at Kårstø north of Stavanger.
So the compressor umbilical would have to be the same length. A subsea wellstream unit could also be an alternative to a new compressor platform on Statoil's Sleipner fields in the North Sea.
In any event, Mr Ramberg believes it would avoid the need for heavy capital spending on recompression in the short term.
Mr Kallestad reports that other operators off Norway, such as Shell and Norsk Hydro, have also expressed a growing need for this type of machine on new fields.
Candidates include Statoil's Snøhvit field on the Tromsø Patch in the Barents Sea and the Ormen Lange discovery in the Norwegian Sea, where Hydro will be operator for development and Shell for production.
Work on compressors is also being supported by the Norwegian government through its NOK 100 million Demo 2000 programme, which aims to increase the number of Norway's profitable offshore projects.
Messrs Ulland and Ramberg explain that work on the wellstream compressor is planned to run through various phases over eight years, with a subsea solution expected to be ready for use by 2009.
This timetable fits with existing field production profiles, since compression does not have to start until half their reserves have been recovered. The current schedule calls for Statoil to pursue a cost study until this summer, followed by the construction of a small-scale model in cooperation with suppliers for testing at Kårstø.
These extensive trials will check to see whether the compressor can operate with "water on the lung" before it is placed on an offshore installation. The next step will be to construct a full-scale unit to be positioned on the Sleipner T platform and then under water on Mikkel.
Mr Kallestad explains that the installation on Sleipner will be the first step towards new field developments incorporating the "balloon" philosophy.
© 2000 Mena Report (www.menareport.com)