Carbon containment in the spotlight
29/10/2007 The Norwegian continental shelf offers opportunities for storing carbon dioxide, according to a report for the Ministry of Petroleum and Energy (MPE) by Gassnova, Gassco and the NPD.
Tekst: Fridtjof Riis
In the North Sea, the most suitable options are the Utsira aquifer around the Sleipner fields and the Johansen formation in the Troll area.
The report is intended to recommend good solutions for carbon transport and storage, and NPD geologists drew on their know-how to identify various possibilities for further study.
The geological data and knowledge needed to find good underground storage sites are roughly the same required for mapping potential hydrocarbon traps.
Large volumes of sandstone or other type of reservoir rock have to be identified, and must be well sealed to ensure that gas and liquid do not leak out.
When the carbon dioxide is pumped down into such formations, it will displace the water held in the rock pores and be trapped without a chance of escape.
Knowledge of formation size and flow properties is crucial when assessing relevant sandstone structures for carbon storage from a geological perspective.
So is establishing whether the whole reservoir will be sufficiently sealed to hold the gas for thousands of years – which calls for knowledge of carbon dioxide’s physical and chemical properties.
Pressure and temperature influence the gas injected into a deep well. It will be gaseous at the surface and remain so for the topmost 500-800 metres.
At greater depths, the pressure causes it to transform into a liquid-like supercritical phase with a density somewhere between light oil and water.
Some carbon dioxide will dissolve in water as it flows through the reservoir. Since it also dissolves easily in oil, it can be used in a number of cases to improve oil and gas recovery.
Two different types of deposition are relevant when seeking to store large volumes of carbon dioxide underground without the aim of improving recovery from fields in production.
One involves the use of depleted fields, which have been emptied of commercially interesting hydrocarbons, while the other focuses on brine-filled aquifers.
In the latter case, sealed structures must be found which can hold injected carbon dioxide but contain no hydrocarbons which might be lost through such injection.
Options considered by the Gassnova/Gassco/NPD study do not include abandoned fields, since these contain small hydrocarbon residues.
The latter might become producible in future with new technology and high petroleum prices, so using them for storage would represent poor resource management.
A third possibility, which would really represent good resource management, involves carbon injection to improve recovery from fields where this approach is economic.
However, such projects would require carbon dioxide to be supplied in specified quantities and at specific times to harmonise with recovery from the field.
They are therefore never going to be able to absorb all the greenhouse gas released by industry and power stations, and have also been excluded from the study.
This map of the continental shelf off western Norway shows the two most suitable areas for carbon storage: In the north the Johanson formation near the Troll field and in the south the Utsira formation near the Sleipner fields.
The largest volumes of sand and sandstone which could be relevant for carbon storage on the NCS are found in the Utsira formation and the underlying Skade structure.
These sediments cover a large area of the central North Sea basin, are several hundred metres thick and lie about 800 metres down in the Sleipner area. They are much closer to the surface further north.
The Utsira formation is currently used to store carbon dioxide from Sleipner West and as a source of water for injection in the Oseberg field.
While it and the Skade structure derived from the Shetland platform to the west about five to 25 million years ago, the Sogne Fjord delta dates back 140-200 million years to the Jurassic.
Built up from the existing fjord of this name, the delta is best known among petroleum specialists for the Troll field. That lies 1 300-1 550 metres beneath the sea surface.
The sandstones which built up to create Troll are the youngest sediments in the delta, while the Johansen formation is one of the oldest in this area.
It lies more than 500 metres deeper than Troll, covered by several thick layers of shale. The NPD is fairly certain that the structure contains no hydrocarbons.
Large sandstone accumulations can also be found in the North Sea, dating from both the Jurassic and the Palaeocene. In addition come chalk formations around the Ekofisk field.
Several could be relevant for carbon injection, but have been excluded from the study because they lie further from the Kårstø and Mongstad power stations which would supply the gas.
The advantage of the Utsira and Skade formations is that they contain almost unlimited volumes of sandstone. No exploration for or production of hydrocarbons have occurred in them.
Utsira is well-sealed in the Sleipner area, and StatoilHydro has demonstrated that the formation functions as a store.
Overall, this structure lies so close to the surface in many places that any carbon dioxide injected will remain in the gas phase.
This occupies more space and is more volatile than if it had been in a dense phase – the condition recommended for carbon storage.
As a result, it remains uncertain whether Utsira is suitable for large-scale storage of Europe’s carbon emissions. Should it prove relevant to study this, however, it would create interesting jobs for geologists – and not only in the NPD.
Work so far indicates that storage capacity in the Sleipner area is more than sufficient to meet the needs both of these fields and of Kårstø, Mongstad and other Norwegian industry.
The advantage of the Johansen formation is that it provides a deep, sealed structure where carbon storage will almost certainly be possible without leakage to the surface.
A possible risk of carbon migration into Troll within a time frame of 100 years or more represents the biggest uncertainty, which geologists will need to work on.
The NPD’s study suggests that this would be very unlikely, but establishing an acceptable level of certainty always calls for an extra effort.