Statfjord late life

Harald Tønnesen and Finn Harald Sandberg, Norwegian Petroleum Museum

Statfjord came on stream in 1979, peak annual production was achieved in 1985 and this level was maintained for eight years until output began to decline. After 2000, however, oil production declined rapidly and it was difficult to find new drilling targets with good commerciality – despite large remaining oil volumes.

— Obvious signs of "late life" on Statfjord C. When the gas pressure is lowered, the pipes must be dimensioned accordingly. Photo: Shadé Barka Martins/Norwegian Petroleum Museum

A new drainage strategy for the field was approved in 2005 on the basis of an expanded plan for development and operation (PDO). This involved reducing reservoir pressure so that gas could be liberated from the remaining oil. To achieve such a reduction, a high level of liquids production would be maintained while ceasing all injection of water and gas. This new strategy was called the Statfjord late life (SFLL) project. Plans call for an increase of more than 20 per cent in gas output while the producing life of the field is extended by at least 10 years.

Production stratergy

Producing as much liquid as possible is important in order to reduce reservoir pressure. This will be ensured by a number of special gas lift wells [REMOVE]

Fotnote: Gas lift When gas bubbles are injected immediately above the producing zone, viscosity and friction are reduced and production thereby increased. Gas lift is an effective production method down to a reservoir pressure of 130 bar. and by drilling some wells to accommodate electrical submersible pumps (ESPs). The gas lift wells are located at the top of the reservoir, and will also function as receiving wells for the produced gas. To achieve the desired gas production rate, pressure will be reduced by about 20-30 bar annually. Pressure sensors have been installed in the reservoir to monitor the rate of decline. Gas production from SFLL was around six million standard cubic metres per day in 2007, and has since been in decline.

The project

The SFLL project was initiated in 2005 and comprises three main activities:

Drillling and recompletion of a large number of wells as describe above
Modifications on the platforms to handle larger volumes of gas under lower pressure
Installation of the new Tampen Link pipeline between Statfjord and Britain’s Far North Liquids and Associated Gas System (Flags), which carries gas to St Fergus in Scotland.

Drilling

Almost 60 new wells are due to be drilled and recompleted, mostly on the Statfjord B and C platforms. Gas lift solutions need to be installed in these wells to permit production under lower reservoir pressure. Plans call for eight dedicated wells producing water with the aid of ESPs. Due to be drilled from Statfjord C towards the end of the drilling programme, these will be able to produce even more water than the gas lift wells and are accordingly important for pressure reduction in the reservoir during its final years on stream.

Platform modifications

Statfjord A was extensively upgraded in 2004. It is due to be shut down a few years after reservoir pressure has been reduced, and no major modifications are accordingly planned for this installation in the SFLL project. Equipment for gas lift and operating the downhole ESPs has been installed on the other platforms. That includes modifications to process plant for handling larger gas volumes under lower pressure than before. The gas compressors have been modified and partly replaced, for instance.

Drilling equipment has been replaced on all three platforms (before the SFLL project on Statfjord A). The main changes include new hoisting and rotation systems, a new pipehandling machine and a new drilling mud module.

The platforms were designed in their time with a limited lifetime for much of the equipment on them. Since SFLL will extend the producing life of Statfjord B and C until 2016-17, part of their production facilities have been modernised – including the installation of new equipment. (Note! The article was written in 2010. Since then, several life of field extensions on Statfjord have been approved, and the current plan (as of January 2020) allow for production from Statfjord A until 2027 and Statfjord B and C beyond 2035.)

New gas pipeline

All gas produced on Statfjord has flowed since 2007 through the new Tampen Link pipeline from Statfjord B, via the Northern Leg Gas Pipeline (NLGP) and Flags, to St Fergus. A longer producing life on Statfjord also has positive consequences for the Statfjord North, Statfjord East and Sygna satellites, which can therefore be kept on stream for longer.

Milestones in the SFLL project

Plans approved 2005
Start of offshore installation 2005
Installation of gas lift 2006
Halt to water and gas injection 2007
Start of gas exports through Tampen Link 2007
Start of low-pressure gas production from Statfjord C 2010
Start of low-pressure gas production from Statfjord B 2011
Planned implementation of ESPs 2012

Sources:
Konsekvensutredning Statfjord Senfase – 2004-11 (Environmental Impact Assessment Statfjord Late Life).
Boge, R, SPE, SK Lien, A Gjesdal and A G Hansen, SPE, Statoil. Turning a North Sea Oil Giant into a Gas Field – Depressurization of the Statfjord Field . SPE 96403 2005.
Rosland, Bjørn, lead planner. One project, four layers, one common interface . Statoil, 2005.
Stangeland, Egil, lead reservoir engineer. Statfjord– challenges and opportunities by converting from oil field to gas field. PowerPoint presentation, Statoil.
Statfjord late life – a new spring for Statfjord . Statoil 2009.
Tengström, Martin. Statfjord late life (SFLL) –status September 2011 . PowerPoint presentation, Statoil.