GEOLOGICAL STORAGE

CO2 six feet under

All hopes are pinned on the subsoil to combat global warming, in the guise of geological storage of CO2. This technique would enable a significant portion of our emissions to be absorbed... provided that continuing misgivings are dispelled before it is too late.

Diagram of CO2 capture and storage. © BLCom
Diagram of CO2 capture and storage.
© BLCom
Various types of geological storage of CO2
Various types of geological storage of CO2
© BRGM-im@gé

The latest report of the United Nations Intergovernmental Panel on Climate Change (IPCC) is categorical: it is imperative to limit global warming to 2°C. To do so, the human race must reduce its CO2 emissions to half 1990 levels between now and 2050. This means consuming less energy from hydrocarbons and using it more efficiently, developing "green" energies, and minimising our emissions into the atmosphere.

The foremost solution is carbon capture and storage (CCS). It is of concern to manufacturers and power plants, which are responsible for half the emissions. Scientists are currently working to reduce the still excessive costs of carbon capture. As storage is a relatively lowcost option, research efforts focus mainly on guaranteeing storage efficacy and security.

Artificial ocean storage of CO2 is now no longer envisaged, as it is too risky for marine biotopes: the acidification caused by naturally absorbed CO2 already poses an enormous threat to the environment. We are left with geological storage, which consists of re-burying the carbon from the coal or hydrocarbons that were originally extracted from the bowels of the earth.

The soil: a natural carbon sink

"A lot of progress has been made over the past 10 years with characterising and selecting suitable storage sites", explains Isabelle Czernichowski-Lauriol, geological engineer at the French Geological Survey (Bureau de Recherches Géologiques et Minières, BRGM) and Manager of CO2GeoNet, the European network of excellence on the storage of CO2. "These studies have allowed us to locate a string of potential reservoirs more than 800 metres deep dotted all over the planet. The sites are characterised chiefly by the presence of porous and permeable geological strata into which CO2 can be easily injected, as well as by the existence of an impermeable "cap rock" composed of clay or salts, which prevents any CO2 from resurfacing. Ideally, this cap rock should contain very few fractures or other irregularities that would allow the CO2 to escape."

Former oilfields and gas fields meet this profile. They are of enormous interest to scientists because their geological environment has already been studied extensively for exploiting the fields. All this data gathered over many years would allow an optimal assessment to be made of the behaviour of the CO2 after it has been injected into the field. In the case of oil reservoirs, injection also holds out the prospect of economic benefits. Injecting CO2 makes it possible to tap some of the oil still locked up in fields which cannot be extracted in the conventional way.

This principle of "enhanced oil recovery" has been extensively tried and tested by industry. It is currently being studied for the purpose of CCS at Canada's Weyburn oilfield, where the operating company, EnCana, is recovering and injecting the CO2 from an American synthetic fuel plant. "Once the CO2 has been injected into the oilfield, it mixes with the oil, making the oil less viscous and easier to channel into the extraction well," explains Isabelle Czernichowski- Lauriol. "First we bring the oil to the surface before removing and re-injecting the CO2 it contains."

However, hydrocarbon reservoirs are of little interest apart from these commercial interests. "The reservoirs have been bored many times during the course of exploitation, which could compromise the impermeability of storage," explains the project manager. "Special cements are being developed to guarantee optimal sealing of the well. That being said, they have a fairly modest storage capacity and are unevenly distributed throughout the world. On their own, they would not have the capacity to contain all human-induced CO2 emissions."

The same problem of limited capacity applies to unminable coal seams - another type of reservoir envisaged by researchers. These seams are situated at depths too great to be mined and the coal in them usually contains methane. The seams can be used to store CO2 as it adsorbs to the surface of coal. When CO2 is injected into the seams, the coal will release natural gas, which could then be recovered at the surface. "However, to date there have been few studies on CO2 storage in coal using ‘enhanced coal bed methane recovery'. It still poses technical problems owing to poor permeability of the coal, and many years of research will be required in order to confirm its viability," says Isabelle Czernichowski- Lauriol.

Deep aquifers - a promising solution

The chief option that remains is to use deep saline aquifers, huge porous and permeable geological strata situated at a depth of more than 800 metres. As the water they contain is much saltier than sea water, it is totally unsuitable for consumption. Saline aquifers sometimes contain hydrocarbon fields or even natural CO2 reservoirs. They hold enormous potential: while former hydrocarbons fields could contain one third of the human-induced carbon emissions produced in one century, the experts believe that saline aquifers have 10 times that capacity. In addition, as saline aquifers are well distributed throughout the world, they could potentially be exploited just about anywhere.

This type of geological formation has been used for geological storage at Norway's Sleipner site since 1996, where the company Statoil reinjects the CO2 from natural gas produced at Sleipner into the Utsira formation (a sandstone aquifer lying 800 metres under the bed of the North Sea). Sleipner is the world's first pilot project for geological CO2 storage and, up to now at least, it has been a confirmed success.

"No leaks have been recorded in more than 10 years of injecting CO2," says Andrew Chadwick with satisfaction. He is a geophysicist at the British Geological Survey (BGS) and Head of Monitoring Technologies at the European CO2Remove project. The aim of the project is to develop procedures for the surveillance of CO2 storage based on a number of pilot sites. To do this, the researchers are studying not only the Sleipner site but also the In Salah site in Algeria, where the CO2 is injected into a deep onshore aquifer, and Snohvit, another offshore geological storage initiative in Norway.

"The results of six three-dimensional seismic surveys conducted at Sleipner are key to understanding the movements of the CO2 plume," adds Chadwick. "The plume behaves exactly as we had predicted. It rises to the top of the reservoir and, as it is blocked by the cap rock, it is displaced horizontally to either side of the injection well."

A safer process in the long term

A number of mechanisms in the reservoirs act to retain the CO2. Over time and to varying degrees, these mechanisms work in either combination or succession, depending on the type of reservoir. "Before being injected, the CO2 is pressurised to turn it into a supercritical gas, which reduces its volume and facilitates its diffusion," explains Andrew Chadwick. The storage reservoir must be situated at a depth of at least 800 metres to guarantee these pressure conditions.

"In supercritical form, the CO2 is less dense than the saline water of the aquifer, which is why it migrates to the top of the reservoir. This phenomenon is called ‘structural trapping'. Over time, the CO2 is expected to gradually dissolve in the water and migrate to the bottom because carbon-laden water is heavier. According to our estimates, in 7 000 years time, all the CO2 at Sleipner will have undergone this ‘solubility trapping'. Over a much longer timescale, the CO2 could also react with the surrounding minerals to form carbonates. At Sleipner, where the Utsira formation contains a lot of quartz that reacts very little when it comes into contact with CO2, this ‘mineral trapping' is likely to be limited."

In short, if the experts' projections are right, the longer CO2 is stored, the lower the risk of leakage. In fact, once CO2 has dissolved, it will be harder for it to escape from the reservoir. And if it acquires solid form, the risk of leakage becomes virtually zero, although mineral trapping remains of limited importance, except where there are special geological characteristics, as in the case of basalt aquifers.

Closer surveillance

These theories must be carefully tested before any commercial deployment of geological storage can be envisaged. The stakes are momentous, as the timescale is enormous and the process takes place in an environment that is basically out of sight. "It's a difficult task," says CO2Remove Coordinator Ton Wildenborg, a geologist at the Netherlands Organisation for Applied Scientific Research (Nederlandse Organisatie voor Toegepast Natuurweten - schappelijk Onderzoek, TNO). "Mostly we use acoustic imaging instruments, which give us a fairly accurate picture of the way CO2 behaves inside the reservoir and in the adjoining geological strata. These analyses are conducted at intervals of around two years in order to keep a close watch on how the process is evolving. As geological characteristics differ widely from one reservoir to another, the surveillance strategy must be tailored specifically to the type of formation concerned, its depth and the characteristics of the surrounding rocks."

"That is the crux of the CO2Remove project. Based on existing demonstration sites, we are endeavouring to develop the technological bases required for surveillance and to define the procedures for establishing it. At last we have succeeded in developing an operational surveillance plan for In Salah, which was no mean task, as this gas production plant is located right in the middle of the desert and, as with all the other pilot sites, our timetable of on-site measures must be drawn up to take into account the other research teams' timetables."

The CO2GeoNet project faces the same battle. The only difference is that CO2GeoNet concentrates on surveillance techniques for detecting leaks. "Since all the demonstration sites are currently leak-proof, we are instead studying natural reservoirs and areas of natural CO2 emanations above ground", explains Nik Riley, geologist at the British Geological Survey and Coordinator of CO2Geonet. "This gives us an idea of the potential behaviour of CO2 over the long term and allows us to test the efficacy of the leak detection instruments. At the Latera site in Italy, we recently tested a technique using a helicopter to detect anomalies in the plant cover that might indicate a CO2 leak."

Although the CCS concept was still in its infancy only 15 years ago, since then it has developed at lightning speed. Europe has taken the lead in this race against the clock to beat global warming. It has been investing in CCS research since the early 1990s, when nobody else was considering it as an option. The EU's current plans for geological CO2 storage involve setting up a dozen demonstration sites between now and 2015. The main aim is to deploy the technique on a commercial scale from 2020, provided that the research results prove that it is safe.

Julie Van Rossom


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Solidifying CO2

Above-ground mineral carbonation is another option for CO2 storage, although scientific research on carbonation is much less advanced than for geological storage. The reason is that, in its natural state, atmospheric CO2 reacts with silicate rocks to form carbonate minerals. Some researchers are planning to make the CO2 react directly with olivine or serpentine - two silicate rocks that are available in large quantities. Others plan to kill two birds with one stone by carbonating industrial residues, such as alkaline brine or steel plant slag. "While carbonation is technically feasible, it is of low impact in terms of human-induced CO2 emissions. However, it might become an interesting niche option for use on a factory scale," says Isabelle Czernichowski-Lauriol.



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  • CO2Geonet
    13 partners - 7 countries (DE- DK- FR- IT- NL- NO- UK) www.co2geonet.com

  • CO2Remove
    27 partners - 12 countries (AR- DE- DK- FR- IN- IT- NL- NO- PL- SE- UK- ZA) www.co2remove.eu