Geological storage of gases and liquids is a well-known technology applied mainly to natural gas. From exploration for oil and gas and for geothermal energy, we know of many different types of reservoirs suitable for storage of CO2.

Storage sites may consist of highly porous geological layers with a large CO2 storage capacity and high permeability, allowing CO2 to be pumped down quickly, for instance sandstone or limestone beds. To make sure that CO2 will not migrate to the surface and escape into the atmosphere, the storage site must be tightly sealed, for instance by claystone.

Pressure and temperature are higher the deeper into the subsurface we get, and, at a depth of about 800 m, the CO2 gas starts behaving like a supercritical fluid.

As a fluid, CO2 takes up less space, and by storing it under pressure at depth ranges of 800 m or more, much larger CO2 volumes can be stored. Moreover, by locating the storage plants in layers this deep, with mostly saline reservoirs, we avoid conflicts with aquifers supplying drinking water.

The technical aspects of CO2 storage in deep reservoirs of saline pore water – saline aquifers – are very much the same as for natural gas storage.

However, there are a number of important differences: CO2 dissolved in water becomes a weak acid (carbonic acid) that may affect the rock in the reservoir and the cap rock. Measures must be taken to make sure that CO2 remains in the reservoir for several hundred years and longer. The time span required remains an open question.

If CO2 storage is to gain wider application as a generally accepted method for reduction of CO2 emissions in the future, we must be certain that the geological storage facilities are impermeable and secure, and that methods are designed to monitor the facilities.

Experience and studies show that CO2 can be stored safely below the surface if a number of prerequisites are fulfilled. To store CO2, the gas is compressed and is pumped down into porous sandstone layers or exhausted oil/gas reservoirs. Storage takes place at a depth of more than 800 m where CO2 behaves like a liquid. Impermeable claystone layers stop the CO2 from escaping to the atmosphere.

In water CO2 becomes corrosive, as already noted. But dry CO2 can be transported in ordinary pipelines, like those used for natural gas. Transport of dry CO2 in pipelines and ships is a well known technology, and there are more than 3000 km of CO2 pipelines in the USA and Canada.

Geological storage of CO2 at commercial scale is already taking place in the North Sea, in Algeria and in North America.

Sleipner, SACS and In Salah

Since the 1990s Statoil has been producing natural gas from Jurassic sandstone beds in the North Sea Sleipner Field. The natural gas in this field contains 5-10 per cent CO2 and for sale to the consumers in continental Europe the limit is 2.5 per cent CO2. Therefore CO2 is separated from the natural gas on the production platform. To begin with the CO2 gases were emitted directly to the atmosphere, as is done worldwide at most other rigs. However, in 1996 Statoil and its partner group decided to be less traditional, and to try to inject CO2 back into the subsurface – a more environmentally sound method.

CO2 injection started in 1996, making the Sleipner Field the world’s first commercial scale site for geological storage of CO2.

Statoil and their partner group also launched a European research project – SACS (Saline Aquifer CO2 Storage) – aiming at studying all aspects of CO2 storage, on the basis of data from Sleipner.

In the past eight years a special well has been used to inject about 8 million tonnes of CO2 into the saline sandstone, and Statoil expects to inject a total of 20 million tonnes into the field within the next 20 years. The storage reservoir is the North Sea Utsira Formation at a depth of 1000 m, where the temperature is about 37°C and the pressure around 110 bar.

The Utsira sandstone forms a very large reservoir, 400 km from north to south, and 5-100 km from east to west. The thickness of the sandstones ranges from 50 to 250 m and the porosity is high (35-40 per cent of the rock volume is pores filled with water) as is also the permeability. The Utsira Formation consists of several sand units separated by thin claystone layers. A claystone unit, several hundred meters thick (Nordland Shale Formation) covers the reservoir, forming an impermeable top seal preventing CO2 from migrating to the seabed and further into the atmosphere.

Over the years, the SACS project has studied the effects of CO2 on the reservoir sandstone and the impermeable top seal, and it has closely monitored whether CO2 remains in the subsurface reservoir.

The impact of CO2 was studied in the laboratory by means of well core samples. Core samples were saturated with CO2 and saltwater, thus simulating the conditions found in the reservoir. The cores were then exposed to pressure and temperature identical with the Utsira Formation.

From the studies it appears that very few reactions have taken place between the pore liquid and the rock, possibly indicating that the impact of CO2 is insignificant, or that the rate of reaction is very low. However, over a period of several thousand years the very slow reactions might have an appreciable impact on the rock. Efforts will therefore be made to assess the possible reactions in the reservoir during the long time span.

The movement of CO2 in the reservoir is monitored seismically. Prior to storing the CO2, detailed seismic measurements (3D seismic) were made in the Sleipner field.

Four years later, the same kind of seismic measurements across the same area were made. Comparison of the results of the two studies (4D seismic) gives an impression of the distribution and size of the CO2 bubble. CO2 is injected into the base of the sandstone and rises like a bubble in the reservoir, somewhat like the coloured bubbles rising vertically in a lava lamp, although much more slowly. When reaching the impermeable claystone top seal, the CO2 spreads out laterally, and, currently, the CO2 body extends 2 km from north to south, and 0.5 km from east to west.

Model calculations show that roughly 18 per cent of the CO2 injected will be dissolved in the pore water when migrating vertically to the top of the sandstone bed. Some 5000 thousand years after injection ceases, all the CO2 will have been dissolved in the formation water and will be distributed over a large part of the reservoir. CO2 dissolved in the pore water will, however, be released when flowing to areas with less pressure, exactly like gas that fizzes up when a bottle of carbonated water is opened.

Based on the experience from the SACS project, it can be concluded that geological storage of CO2 in saline aquifers is technically and economically feasible.

Continued studies of the movement of the CO2 bubble will provide information that can be used in risk assessment of future geological storage sites.

Meanwhile, in Algeria, onshore geological carbon dioxide storage is underway at In Salah, a large field in the Sahara Desert, from which BP, together with partners Sonatrach and Statoil, produce natural gas for the European market.

The storage project started up in 2004, with an expected capacity of 1.2 million tCO2/y.

The situation is rather similar to the Sleipner field, but captured CO2 is injected into the flanks of the same reservoir from which the natural gas is produced. Monitoring and verification of the storage of CO2 will be a part of the EU R&D project CO2 ReMoVe, which was initiated in the spring of 2006.

North American experience

In Northern America CO2 is injected into the subsurface in order to enhance recovery of oil from existing oil fields. By diluting the oil, CO2 makes it flow more easily to the production wells. Since the 1970s, this concept – CO2 EOR (enhanced oil recovery) – has been applied in the USA, primarily in small fields where traditional methods have not allowed production of more oil. Experience gained by the largest CO2 companies specialising in this type of work shows that CO2 injection enhances recovery by between 8 and 16 per cent of the oil originally found in the oil fields.

However, as a means of reducing emissions of CO2 to the atmosphere, EOR operations in the USA have not yet been effective since the CO2 employed has generally been pumped up from natural underground CO2 accumulations of volcanic origin.

An EOR project in Saskatchewan, Western Canada, has been more successful in this regard. Near the town of Weyburn, CO2 is injected into a producing oil field using CO2 from a chemical plant manufacturing synthetic fuel and fertilisers by gasification of coal. The CO2 comes from North Dakota, USA, via pipeline.

In the Weyburn EOR project, the chimney stack has been “turned upside down” and has for a number of years been providing CO2 for “flooding” of an oil field through a 350 km long pipeline. The oil field, which is the same size as the North Sea Dan Field, started production in the mid 1950s. It is expected that, in the 20 years of operation of the EOR project, the rate of recovery from the field can be increased from 25% to at least 35% of the oil originally in-place.

The primary purpose of the operation is to increase the volumes recovered, but the spin-off effect of the project is the benefit to the environment gained from geological CO2 storage. Quite large volumes are at stake. Calculations show that a total of 20 million tonnes of CO2 will be stored when the project is completed, ie the same as the North Sea Sleipner project.

The operator of the Weyburn field, the Canadian oil company EnCana Corp, was very early in its initiation of a large international research project, co-operating with the International Energy Agency (IEA). Later, the EU joined in, contributing financially via the European Commission research programme. The aim is, first, to describe the chemical reactions in the oil field when supercritical CO2 is injected into a carbonate field consisting mainly of dolomite and calcite, and, second, to initiate a monitoring programme to ensure that CO2 does not migrate to the groundwater and further up to the surface.

Estimating storage capacity

A summary of current estimates for CO2 storage capacities and emissions in European countries is given in Table 1. On the basis of current information storage capacity looks adequate.

Similar conclusions have also been reached for the USA.

In 1999, a mapping project was started on the initiative of GEUS, called GESTCO (Geological Storage of CO2 from Combustion of Fossil Fuel). The project aimed to determine the extent of geological storage capacity in eight European countries, Belgium, Denmark, France, Germany, Greece, Netherlands, Norway and the UK. The GESTCO project was funded by EU research programmes and was carried out as a collaborative effort between the geological survey organisations of the eight countries.

The idea was to integrate information on large stationary CO2 point sources, such as power plants, with information on the geological structure of the subsurface to identify the largest and economically most favourable options for storage of CO2 in Europe. The work also included an assessment of the magnitude of costs associated with the separation, transport and storage in each individual case.

GESTCO showed that there is indeed great potential for geological storage in Europe, notably in Denmark, northern Germany, the United Kingdom, Norway and the Netherlands.

The enlargement of the EU to the east in 2004 brought new areas of heavy industry into the European Community, and new areas have therefore been included in subsequent surveys of geological storage capacity. Further mapping of geological storage capacity in eight new countries is included as part of the EU funded research project CASTOR (CO2 from Capture to Storage): Bulgaria, Croatia, Czech Republic, Hungary, Poland, Romania, Slovakia, and Slovenia. Other objectives of CASTOR include developing and applying a methodology for selection and secure management of storage sites as well as well as improving the “best practice manual” started with SACS/Sleipner by adding four real site projects (Casablanca, Snøhvit, Atzbach, K12b), discussed briefly later. The four new sites cover a range of geologies: clastics (sandstones) vs carbonates; onshore vs offshore (with consequences for monitoring); depleted oil field; depleted gas field; aquifer.

The work on mapping emissions and assessing European geological storage capacity is also continuing under the EU GeoCapacity project, funded for three years, from January 2006, under the EU’s FP6 funding for R&D. The project will include:

• mapping of major CO2 sources in 13 European countries;

• full geological assessment of Bulgaria, Croatia, Czech Republic, Estonia, Hungary, Italy, Latvia, Lithuania, Poland, Romania, Slovakia, Slovenia, and Spain;

• review of four neighbouring states, Albania, Macedonia, Bosnia-Herzegovina, Luxembourg;

• updates on five of the eight GESTCO countries, Germany, Denmark, UK, France and Greece;

• provision of consistent and clear guidelines for assessment of geological capacity in Europe and elsewhere; and, last but not least

• initiation of international collaboration with countries of the CSLF (Carbon Sequestration Leadership Forum) – China first.

Demonstrating geological CO2 storage

Further demonstration projects will be a key part of efforts to develop geological CO2 storage. Some key schemes are summarised below.

Snøhvit project (part of CASTOR). Due to start up in 2007, the plan is to inject around 0.7 million tCO2/y into a subsea well (offshore sandstone aquifer) at a depth of 2500m over a period of 20 years. The source of the carbon dioxide is removal from natural gas prior to cooling for LNG. The field is operated by Statoil and located in the Barents Sea.

Ketzin. Launched in January 2004, the aim is to construct and run the world’s first plant for storage of CO2 in a densely populated area, to be located at Ketzin, near Berlin in Germany.

It is part of the CO2SINK (In-situ R&D Laboratory for Geological Storage of CO2) project and aims to demonstrate that it is technically possible and safe to store CO2 from conventional power generation in underground storage sites. It is the first CO2 project that is to both analyse the technical and economic aspects, and at the same time ensure the public accept and feel safe in relation to storage in the vicinity of a densely populated area. The project is to run for five years, with several European research institutions and industrial enterprises participating.

The storage is to take place under a disused natural gas storage site near Ketzin. The old natural gas storage site is being closed down, but the existing infrastructure around the old plant remains intact and some of it will be used in connection with the project. The geological conditions around the structure are well documented through numerous boreholes and seismological studies. The CO2 gas for the trial geological storage will be supplied by a local energy plant.

One injection well and two observation wells are planned near the top of the Ketzin structure. Monitoring of possible escape of CO2 to the surface will be carried out using seismological studies and sensitive instruments at the surface.

Prior to the injection of CO2, a number of geological studies will be carried out in order to document the base-line condition of the Ketzin structure. Geologists and engineers will thoroughly analyse factors that may lead to migration of CO2 from the underground storage site.

In general, it is possible to differentiate between technical and geological risk factors. Technical risks can for example be poorly sealed wells, badly cemented injection wells, and chemical reactions between water containing CO2 and the cement. Geological risks may include leaks through the cap rock of the CO2 storage reservoir caused by cracks/faulting in the seal or the presence of a porous layer that leads the injected CO2 away from the storage structure.

The complete preliminary investigations will be used for contingency plans to monitor and reduce or eliminate the risks of the project. CO2 is a non-toxic gas under normal pressure and temperature, however CO2 is heavier than atmospheric air. Therefore escaping CO2 can accumulate in depressions in the landscape or in cellars, where it will displace the oxygen in the air bringing the risk of suffocation.

Maximum safety as well as public acceptance and security in relation to CO2 storage in the Ketzin structure are crucial to the success of the project, and thereby also for future possibilities of geological storage of CO2 in densely populated areas. In recognition of this, two project activities have been designated with the responsibility for safety and communication with the public. On the communication side, a co-ordinated effort is being undertaken to ensure good communication of information about the project’s activities, results and safety levels to local authorities, residents, NGO’s, industry, the media and so on.

Five years is a very short time horizon for a research project such as CO2SINK. At least two years will pass before the first CO2 can be stored, and there will therefore be no opportunity for studying the spread of the CO2 gas in the reservoir on a long term basis. It is therefore also hoped that the CO2 storage site at Ketzin can be used by future research projects as a research laboratory for the geological storage of CO2 many years after the completion of the present project.

Casablanca (part of CASTOR). The plan is to inject 0.5 t CO2/y into a depleted Repsol oilfield off the coast of Spain (in carbonates) at a depth of 2500 m. The CO2 would come from the Tarragona refinery.

K12b (part of CASTOR). Envisages injection into Gaz de France’s K12b offshore gas field in the Netherlands, in clastics at a depth of 3500-4000 m. The aim here is to enhance gas production by injection of CO2 – a fairly new concept.

Atzbach (part of CASTOR). The plan is to inject CO2 into a Rohoel gas field in Austria, in Sandstone at a depth of 1600 m, with opportunities for enhanced gas recovery.

FP7 and Technology Platforms. Further CO2 storage demonstration projects are likely to be included in the EU’s seventh framework programme for R&D, with various reservoir and cap rock types, different geological settings, and aquifers or combinations with EOR or ECBM.

In order to involve European industry further, the EU has also defined a number of Technology Platforms, one of which is Zero Emission Power generation. The ZEP Research and Deployment Strategies, which will include projects on capture and storage, are to be officially launched in Brussels at a conference scheduled for 12-13 September 2006, to be opened by the Commission President, José-Manuel Barroso.

Recent major initiatives

The momentum behind capture and storage is growing and in recent months a number of major initiatives have been announced by some of the key European energy companies. These initiatives include:

• Elsam’s pilot plant for CO2 capture at the Esbjerg coal-fired power plant in Denmark – about 8kt/y CO2 captured from March 2006 (CASTOR project under FP6).

• Vattenfall’s 30MW coal-fired pilot boiler with CO2 capture under construction at Schwarze Pumpe in Germany.

• Plans for CO2 capture at a new natural gas fired combined cycle power plant at Kårstø, Norway, with the CO2 used for EOR.

• BP/SSE plans for an industrial-scale (500MW) hydrogen power plant at Peterhead, Scotland with storage of 1.8Mt/y of CO2 in the Miller oil field – starting 2010.

• Shell/Statoil plans for a new natural gas fired power plant at Tjeldbergodden, Norway, with CO2 capture. The CO2 will be transported to the Draugen and Heidrun oil field and used for EOR.

• RWE’s proposal to build a large-scale (450 MWe) integrated gasification combined cycle (IGCC) coal fuelled power plant in Germany with the CO2 captured and stored onshore.

• E.On’s proposal for an IGCC power plant in the UK, with CO2 captured and stored in the southern North Sea.

Decision making, economics, law

To profit from experience among the EU countries, the CO2STORE (On-land and long-term Saline Aquifer CO2 storage) project was established to develop a common European platform for decision making on the establishment of actual storage projects.

The project analysed four storage scenarios, in Norway, the UK, Germany and Denmark, of which each comprises one or more CO2 sources and a geological storage site.

The Danish part of the project is investigating the technical aspects of the possible storage of CO2 from two point sources in the vicinity of Kalundborg. These sources are the Asnæsværket coal plant, which is owned by Energi E2 and Statoil’s oil refinery. The study assumes that in the future CO2 will be separated from the flue gases and stored in a deep-lying sandstone formation to the northwest of the town.

In Norway the possibilities for storing CO2 from a projected paper mill close to Trondheim were studied, but results are so far not encouraging.

In the Valleys project in Wales, UK, studies have been done on a possible new IGCC plant with capture and CO2 storage offshore under the Irish Sea.

In Germany a promising site for storage of large amounts of CO2 from the Schwarze Pumpe power plant was investigated.

The latter two studies indicate good prospects for storage of CO2.

Economics is clearly an important prerequisite for enabling geological storage of CO2 to become a reality in the climate policy of the future. Therefore, as part of the GESTCO programme, some estimates were done of how much it would cost to store CO2 at 17 potential European geological storage sites. The storage examples included coal and natural gas fired power plants as well as various types of industrial enterprises. The total cost of reducing CO2 emissions to the atmosphere by 1 tonne varied between €105 and 32.

The most expensive part of the process is capture. This requires energy, just as compressing CO2 into a liquid also does. Transport and storage of CO2 do not cost as much. If CO2 is used for enhanced oil recovery (EOR), there may even be positive revenues.

Current expectations are that capture and storage costs can be brought down to 20-25 €/t CO2 – equivalent to a few eurocents per kWh. Already the cost – although an important factor – is so low that CO2 capture and storage looks feasible and the cost impact would be less than that due to fluctuations in petrol prices.

There are a number of legal and regulatory aspects of geological carbon dioxide storage that need to be resolved. The OSPAR and London conventions are unclear on offshore CO2 use/storage and amendments have been proposed by Australia, the UK and Norway and supported by a number of other countries.

Probably no national European legislation (with the possible exception of Denmark, which needs further study) covers onshore CO2 storage/disposal and there is obviously a need for clear legal and technical guidelines – nationally as well as for the EU as a whole.

Concepts for geological storage of carbon dioxide. Possibilities include saline aquifers, enhancing recovery in oil and gas fields, exhausted petroleum structures, enhanced coal bed methane and mineral sequestration Aquifer storage at the Sleipner field (Source: STATOIL)
Seismic image before CO2 injection
Seismic image after CO2 injection European CO2 emissions, 1990 (million t of CO2), showing countries participating in CASTOR and in GESTCO The Ketzin project: the first of a kind. The aim is to establish and operate a new fossil/biomass fired CHP plant with CO2 capture and storage in an existing natural gas storage infrastructure (deep saline aquifer) close to Berlin (Source: GFZ, Potsdam) Geological map of Western Europe. The ovals show major onshore and offshore sedimentary basins. Such basins, containing, for example, porous sandstones, are potential carbon dioxide storage sites