Reading Between the Lines

28 February 2006 (Last Updated February 28th, 2006 18:30)

Statoil's Subsea Improved Oil Recovery (SIOR) project makes an important step forward for subsea production. It does however, draw attention to the need for a number of technological developments. Matt Willey considers the company's progress to date.

Reading Between the Lines

Statoil's corporate statement of 12 December 2005, 'Improving recovery from subsea fields' outlines the company's Subsea Improved Oil Recovery (SIOR) project. It reads:

"The ambition of the SIOR project in Statoil is to achieve an average recovery factor of 55% from subsea-completed fields operated by the group in 2008."

This corresponds to an increase of 1,200 million barrels in recoverable oil reserves. New technology will be applied to raise the average recovery factor for mature fields while contributing to smart solutions for new projects.

This multidisciplinary project has initiated a number of R&D activities in order to fulfil its ambition. These cover such areas as:

  • Reservoir management
  • Cost-effective drainage points
  • Light well intervention
  • Increased subsea handling of liquids
  • Reduced subsea wellhead pressure
  • Integrated operations

The following important milestones have been passed in efforts to meet the 55% ambition:

  • The world's first subsea Through-Tubing Rotary Drilling (TTRD) operation was performed from a mobile unit on the Norne field in the Norwegian Sea
  • Tordis in the North Sea will be the world's first commercial field with subsea processing
  • Subsea injection of raw (untreated) seawater has been included in the Plan for Development and Operation (PDO) of the Tyrihans field in the Norwegian Sea
  • A project has been established to prepare for subsea gas compression on the Åsgard field in the Norwegian Sea from 2011

"A full-scale subsea separation facility on Tordis could improve oil recovery from this Statoil-operated development in the North Sea from 49% to 55%."

"Statoil wishes to raise the oil recovery factor on the subsea-completed fields it operates to 55%."

Statoil also plans to develop Tordis, the world's first commercial field with seabed processing. Water and sand will be separated from the wellstream and injected into a sub-surface formation for storage. In addition, a multiphase pump will help to drive the remaining oil and gas through the existing pipeline to Gullfaks C.

This solution is also environmentally friendly because it reduces the amount of produced water being discharged into the sea. New technology could yield some 35 million barrels of additional oil, of which 18 million barrels will derive from subsea processing.

These solutions could also improve recovery from small fields, discoveries in deeper water and fields remote from existing installations. In addition, they could help maintain tail production and developments in environmentally sensitive parts of the continental shelf.

Located in the Tampen area, Tordis has been on stream since 1994. All its production installations are subsea and controlled from Gullfaks C. oil from the field is piped to the latter platform through a 10km pipeline for processing, storage and export.

SMART SOLUTIONS IMPROVE TYRIHANS RECOVERY

"Multilateral and smart wells with downhole control equipment and injection pumps for seawater standing directly on the seabed could yield some 18 million barrels of additional oil from Tyrihans."

This oil and gas project is being pursued by Statoil in the Norwegian Sea, about 35km east of Kristin and a similar distance southeast of Åsgard. It will be developed as a satellite to Kristin, with pressure support in the form of gas injection from the Åsgard B platform. Production is due to begin in the summer of 2009.

Tyrihans is the largest Statoil-operated field off Norway still to be developed. It holds 176 million barrels of oil and 30 billion cubic metres of gas. While Tyrihans South is an oil reservoir with a gas cap, Tyrihans North is a gas reservoir with a thin underlying oil zone.

These two formations are connected via a water-filled saddle. Reservoir properties are normal. Tyrihans will be developed for subsea production with five templates – four for production and one for seawater injection. A total of 12 wells are planned.

Since the Tyrihans reservoirs are elongated and cover a fairly large area, plans call for seven multilateral wells drilled to a depth of 3,700m beneath the seabed. This will improve recovery, while drilling one sidetrack per well will also save some NOK1.5bn. Statoil has not developed a field with so many multilaterals
before.

"Statoil is one of the world's largest operators of subsea fields."

The aim is also to use 'smart' wells, including a Downhole Instrumentation And Control System (DIACS). This permits remote operation of valves in the well to boost oil output and reduce water production.

Should a multilateral well experience a gas surge, for example, it will be possible to shut down one branch while the other continues to produce. Pressure can also be regulated. The result is increased oil and gas production and a reduction in intervention costs.

"Raw (untreated) seawater will be injected on a daily basis into the intermediate water 'saddle' to maintain reservoir pressure in both Tyrihans North and South."

This injection will be handled directly by pumps installed on the saddle at a rate of 87,500 barrels (14,000 standard cubic metres) per day with a pump pressure of 206bar.

One injection well is due to be drilled, but the opportunity to add an additional well will be provided. That would raise the injection rate to almost 120,000 barrels (19,000m³) per day.

Water injection is intended to:

  • Increase pressure support in order to improve oil and condensate (light oil) recovery from Tyrihans South
  • Stabilise the oil zone in Tyrihans South and delay gas breakthrough
  • Equalise pressure differences between the two reservoirs by injecting water into the saddle between them
  • Overcome limited gas injection capacity and avoid using expensive gas

Injection of raw seawater is controlled via the main umbilical from the Kristin platform, with electricity for the subsea pumps supplied by a dedicated transmission cable from the same source.

The injection station is based on a standard four-slot subsea template. Pumps, subsea transformers, piping and electrical connections will be delivered as a package.

"Statoil will apply new technology to raise the average recovery factor for mature fields."

LIGHT WELL INTERVENTION – A TOOL FOR IMPROVED RECOVERY

Light well intervention methods are needed if Statoil is to achieve its ambition of raising the oil recovery factor on the subsea-completed fields it operates to 55%.

This work will be done safely, effectively and cost-efficiently with specially designed vessels which can replace conventional drilling rigs. A ship of this type will have to work for more than 180 days per annum over the coming years.

Island Offshore Management AS has secured a six-year contract to provide Statoil with these well intervention services from 2006. Work is scheduled to begin in 2006 with Island Frontier, but this vessel is due to be replaced during 2008 by the new build, Island Wellserver.

On the basis of newly developed technology, the aim is to introduce a wider range of well intervention services on the latter vessel. That will in turn increase opportunities for meeting the group's target on improved oil recovery from subsea-completed fields.