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What is CO2 sequestration?
CO2 sequestration is sometimes used to describe what is generally referred to as CO2 storage. The word sequestration has specific legal and chemical meanings but in the context of CO2 concerns fixing, binding or taking out of circulation. The CO2SINK project concerns deep underground geological storage, which involves injecting CO2 at high pressure into underground reservoirs. It is essentially the reverse of oil or gas production.

At the pressures encountered deep underground, CO2 is quite dense and behaves more like a liquid than a gas. This means that large quantities can be stored in a relatively small volume. Most of the CO2 stored in this way occupies pore spaces in porous rock. However, some of the CO2 reacts slowly with some rock types to form carbonate minerals: this process provides permanent storage. The earth's crust already contains vast deposits of carbonate rocks; the most common forms are limestones and dolomites. Permanent storage for the unreactive portion of the CO2 occurs when the formation into which it has been injected is covered by an impervious cap rock, similar to those that seal gas and oil deposits into reservoirs. Knowledge gained about the age of containment for oil and gas deposits indicates that CO2 can be permanently stored in underground reservoirs, and only released if future generations chose to drill into the storage formations.

Why should CO2 be captured and stored?
Approximately one-third of all the CO2 emissions associated with human activity come from fossil fuels burned to generate electricity. Every fossil-fuel power plant can emit several million tonnes of CO2 each year. Several other industrial processes emit large amounts of CO2 from their production plants, including oil refineries, cement works, and iron and steel works. These emissions could be reduced substantially, without major changes to these industrial processes, by capturing and storing the CO2. Other CO2 emission sources, such as transport systems and domestic buildings, are too small and numerous to be tackled in the same way.

What is the status of CO2 storage?
Once the CO2 has been captured, it would have to be stored securely for hundreds or even thousands of years in order to avoid it reaching the atmosphere. Major reservoirs suitable for storage, have been identified under the earth's surface and in the oceans. Several scientific projects are currently exploring how to develop these options.

In the oil and gas industry, underground storage of CO2 has been used for many years, as injection of CO2 into oil fields can enhance oil recovery. Now, for the first time, CO2 is being deliberately stored in a salt-water reservoir under the North Sea for environmental reasons. The potential capacity for underground storage is large but not well documented. Other geological storage schemes are under development, and plans to monitor them are well advanced.

How much does it cost?
Most of the cost for CO2 capture and storage lies in the capture and transport of the CO2. Costs are variable but currently lie in the range of $30-$50 per ton of CO2 captured and stored. This might add 1-2cents/kwh to the cost of generating electricity and would make alternative sources of power considerably more competitive. Nevertheless, in the medium term CO2 storage is one of the lower cost options for reducing emissions.

Where are the storage sites?
There are thousands of possible storage sites for CO2. The sites likely to be used first are existing oil and gas reservoirs, which are located in sedimentary basins around the world. Many of these basins are in unpopulated or sparsely populated areas. However, since CO2 is emitted in populated areas, basins that are close to cities and industrial centres are more likely to be useful for CO2 storage. Local populations in these regions are already familiar with many of the implications of utilising underground oil and gas resources, storing CO2 would have minimal additional impact. The main issue would be a delay in abandonment of the producing facilities during which, some of the reservoirs would be refilled, not with gas or oil but with CO2.

Saline aquifers suitable for permanent storage are also found in sedimentary basins but their distribution includes areas that are not hydrocarbon-bearing. The structure and extent of these basins is reasonably well known but the positions of suitable deep, saline aquifers are is still very uncertain. Obviously those that are close to points of major CO2 emissions will be the first to be considered for geological storage.

What happens when the storage sites are full?
When CO2 is injected into a reservoir it will displace the fluids, water, oil, or gas that are already there. CO2 is lighter than oil and water and thus accumulates at the high points beneath the cap rock. Some reservoirs are effectively sealed above below and on all sides. Injection or withdrawal of any fluid will permanently raise or lower the pressure. Others reservoirs are sealed only above and lighter fluids such as oil, gas or CO2 can be trapped in any high points below a sealing layer of rocks because of their buoyancy. In sealed reservoirs, injection must stop at a point determined by the strength of the cap rock. In reservoirs which rely on a topographical seal an accumulation of CO2 will form at the highest point and the saline water will be displaced downwards and outwards under the low points of the seal. the CO2 is contained only by the lower contours of of the cap rock, which in effect form a dome-shaped structure in the rock sequence. Injection is stopped before the CO2 can spill under the edges of the dome The point at which this happens can be measured by a combination of seismic survey, observation wells and special instrumentation (FIGURE).

Once the reservoir is full, the injection wells will be monitored and the stability of the CO2 in storage will be observed for a number of years. Once it can be proved that the CO2 is held safely in the reservoir, the wells will be decommissioned and cemented shut. A few wells may be retained for long-term observation, but eventually these would also be resealed with cement.

The capacity of potential underground CO2 storage reservoirs is almost certainly less than the volume of emissions which could be generated by burning all the world's fossil fuel reserves of coal, oil and gas.,. The storage of CO2 is not, therefore, a permanent solution to the greenhouse-gas issue. However, it may provide relief for as much as a century, during which time there would be opportunities for alternative energy sources and technologies to replace fossil fuels. Long before the storage sites are full alternatives will need to be found, but by that time the stored CO2 may in itself become a valuable resource.

Is it safe?
Much of the technology for transportation and storage of gases is established and in use today. Large quantities of CO2 are routinely transported in pipelines and tankers and CO2 is injected underground in many enhanced oil recovery projects. Underground storage of natural gas, an analogous technique, is widely practised. This indicates that the underground storage of CO2 could be achieved in a safe and reliable manner. Nevertheless, because CO2 is an asphyxiant and heavier than air, there may be concerns about the safety of underground storage. Problems might arise from possible slow leakage or sudden large-scale emissions resulting from seismic activity.

Slow leakage of CO2 is unlikely to give cause for safety concerns unless the gas is inadvertently trapped in surface structures. The risk of a sudden, large-scale release of CO2 would be avoided using the same precautions that are applied to handling other gases, such as avoiding unsuitable or geologically unstable sites. The geology in the area surrounding Ketzin is very stable.

It is vitally important that the CO2 remains in the underground storage system long enough to help minimise climate change. Oil and gas fields have remained secure for millions of years but there is a possibility that drilling and extraction of oil and gas may have disrupted the integrity of the cap rock. Chemical interactions between injected CO2 and underground minerals would have the beneficial effect of permanently storing the CO2, but there is a possibility that some chemical interactions could have an adverse effect on the integrity of the cap rock. Unlike oil and gas reservoirs, deep saline reservoirs have until now, had no commercial value and are generally less well characterised than their hydrocarbon-bearing counterparts. More information is needed to assess whether deep saline reservoirs can contain CO2 for the necessary timescales. A detailed risk assessment will be carried out as part of the CO2SINK project.

What is the storage reservoir like?
It is a common misconception that underground storage reservoirs are like caverns. Nothing could be further from the truth. The picture shows some samples of the reservoir material which at first sight is just solid rock. However this type of rock contains many microscopic pores which are filled with salty water. As much as 10-15% of the volume of the rock may consist of these water filled pores. Storage of CO2 occurs when the water in the pores is displaced or when the CO2 dissolves in the water. Pushing the water aside and replacing it with CO2 requires pressure and time but once displaced the gas is not easily released, particularly any which has dissolved in residual water.


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