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Storage safety

10/02-2009

The most frequently asked questions about CCS are related to the safety of storing CO₂ underground.

Many people are concerned about whether the CO₂ injected into the ground will stay safely stored, or whether it will leak out again.

The answer is simple; CO₂ storage is safe. If storage sites are carefully selected and the CO₂ is injected according to standards, the CO₂ will remain stored without leakages.

« CO₂ storage experience	« Storage mechanisms
« Possibility of leakage	« Prerequisites for safe storage
« Myths	« References
« See also

CO₂ storage experience

The best evidence that CO₂ can be stored safely is the experience from CO₂ storage projects in operation. The best example is the CO₂ storage in the Utsira formation in the North Sea where about 1 million tonnes of CO₂ has been stored annually since 1996 without any indication of leakage.

The Utsira formation.

The reason for storing CO₂ in Utsira is simple. The natural gas produced from the nearby Sleipner field contains too much CO₂. The gas contains 9 percent CO₂, and customers will not buy natural gas with more than 2.5 percent CO₂. The CO₂ is therefore separated from the gas and injected into the Utsira formation.

The data from seismic surveys, which are performed to determine how the CO₂ behaves after it is injected, indicate that the CO₂ remains stored in the Utsira formation without leakages.

The experience from several other CO₂ storage projects confirms that CO₂ storage is safe. There are many CO₂ projects worldwide, and they all indicate that the injected CO₂ stays safely stored. The most well known projects are the Weyburn project in North America, the In Salah project in Algeria, and the Snøhvit projects in the Barents Sea where CO₂ is separated from natural gas and stored underground, just like the Utsira project.

Furthermore, there are many small-scale research projects where small volumes are injected, and none of them has reported unexpected leaks.

Storage mechanisms

A few CO₂ storage projects without leakage are an indication that CO₂ can be safely stored. Yet, there is no evidence that CO₂ storage will be safe under all circumstances. We need to take a closer look at the physics of CO₂ storage in order to get a better understanding of storage safety.

The effectiveness of geological storage depends on a combination of physical and geochemical trapping mechanisms. In general, these mechanisms can be summarised as physical CO₂ trapping, solubility trapping, and mineralisation.

Physical trapping

The most effective storage sites are those where CO₂ is injected into porous sediments below a thick solid rock, called caprock, that the CO₂ cannot penetrate. Sedimentary basins with such closed, physically bound traps are found all over the world, and they are occupied mainly by saline water, oil and gas.

CO₂ storage where the CO₂ is injected into a porous sediment (red) and trapped below a solid rock, also called caprock (dark grey), which the CO₂ cannot penetrate.

When CO₂is injected into a formation it displaces saline formation water and then migrates buoyantly upwards, because it is less dense than the water. When the CO₂ reaches the top of the formation and meets the caprock, which it cannot penetrate, further movement upwards is hindered.

The only way that CO₂ can escape and continue its upward movement is through fractures or faults in the caprock. Therefore, it is important to characterize the geology of storage sites and caprocks prior to CO₂ injection, and only inject CO₂ in sediments where there are no faults or fractures.

The shape of the caprock is also important. In the figure above the caprock has the shape of a cup turned upside down which traps the CO₂ inside. If the caprock had been completely flat, the CO₂ could have escaped at the ends of the caprock.

In addition to caprock, the CO₂ can also be physically trapped in the pores of the sediment where it is stored, as indicated in the figure below.

Physical trapping of CO₂ in a porous rock below a solid caprock. The CO₂ is visualised as red and black molecules. The CO₂ tries to move upwards, but is stopped by the caprock. The CO₂ will also be physically trapped inside pores in the sediment where it is stored.

Solubility trapping

In the long run, significant quantities of CO₂ dissolve in the formation water and then migrate with the water.

The primary benefit of solubility trapping is that once CO₂ is dissolved, it no longer exists as a separate entity, thereby eliminating the buoyant forces that drive it upwards.

CO₂ dissolved in water moves very slowly. It could take hundreds of years to move just a few meters.

CO₂ storage by physical and solubility trapping. Left: The storage mechanism known as physical trapping of CO₂ in a porous rock below low-permeability seals (caprocks). Middle: In the long run, significant quantities of CO₂ dissolve in the formation water, which is visualized by fewer CO₂ molecules as separate entities, and a color change in the water indicating that the water contains CO₂.

Mineralisation

CO₂ in the subsurface can react with the rocks in the sediment where it is stored. This is a trapping mechanism known as geochemical trapping, mineralisation, or mineral trapping. CO₂ dissolved in water will then form solid and stable carbonate minerals that cannot move. This is the most permanent form of geological storage. Mineral trapping is believed to be comparatively slow, potentially taking thousands of years or even longer. Nevertheless, the permanence of mineral storage makes this a desirable feature of long term storage.

CO₂ storage mechanisms. Left: The storage mechanism known as physical trapping of CO₂ in a porous rock below low-permeability seals (caprocks). Middle: In the longer term, significant quantities of CO₂ dissolve in the formation water. Right: CO₂ can undergo a sequence of geochemical interactions with the rock and formation water, which is visualised by the dark brown area representing solid carbonates.

Possibility of leakage

The possibility of CO₂ leakage is very low. The likelihood of stored CO₂ release is less than one percent in thousands of years.

One example is the Weyburn CO₂ storage project in Canada where a scientific study concluded that less than one percent of the CO₂ will leak in the next 5000 years [1].

This type of scientific result often raises a new question. If one percent leaks in thousands of years, does that mean that all the CO₂ will leak out again in millions of years?

This is not necessarily the case because mineral trapping will become more prominent as time goes by. As mineral trapping forms solid rock that cannot be moved, the possibility of leakage will probably be reduced with time.

Furthermore, scientists says that combating climate change means keeping the CO₂ out of the atmosphere for tens of thousands of years, not necessarily for an indefinite time. Within millions of years natural processes will probably have larger effects on atmospheric greenhouse gas levels than the leaks we can expect from CO₂ storage sites.

Prerequisites for safe storage

As shown above, safe CO₂ storage is possible. However, there are some important prerequisites.

First, the geology of potential storage sites must be carefully characterized so that only the sediments with optimal geology are chosen as CO₂ storage sites.

Second, the injection of CO₂ must be properly monitored. The operators must take specific precautions like ensuring that injection wells are built by materials that can withstand the acidic CO₂ and ensuring that the CO₂ injection pressure does not exceed the pressure that the ground can withstand.

Myths

There are some myths related to CO₂ storage. Some of the most common misunderstandings are clarified below.

Myth: CO₂ stored underground is dangerous because the CO₂ can burn and explode.
This is not true. CO₂ cannot burn under any circumstances, and it can not explode.

Myth: Earthquakes can cause CO₂ leakages.
This will probably never happen because we know where earthquakes are likely to ocurr, and CO₂ will never be stored in earthquake-prone areas. And even if an earthquake should happen in a area where CO₂ is stored it does not necessarily mean that CO₂ will leak. When earthquakes ocurr, we do not see large volumes of underground water being pumped up to the surface. Furthermore, oil and gas have been stored naturally below the ground for millions of years without leaks, even though there have been earthquakes at all time.

Myth: CO₂ injected for enhanced oil recovery (EOR) leaks out.
When the CO₂ is injected into an oilfield to increase the oil production, the CO₂ will mix with the oil. When the oil is pumped up it will contain some CO₂. But this CO₂ cannot be caracterised as a leak. Long before CO₂ injection began it has been well known that part of the CO₂ would come out again together with the oil.

Myth: CO₂ leaked from the Utsira project during the Tordis accident.
In 2008 there was an accident at the Tordis oil field. As a part of oil production water is injected underground, and this water injection accidentally cracked the Utsira formation. There was no leakage of CO₂, but questions were raised about whether CO₂ storage was safe or not. The reason for the accident was that the operator had not consulted a geologist prior to water injection. Poorly operated oil production projects should not be used as arguments against the safety of storage.

References

Allan Casey, Carbon Cemetery, Canadian Geographic Magazine. Jan/Feb 2008, p. 61