Safe Storage

For CO2 Capture and Storage (CCS) to deliver on its promise of drastically reducing CO2 emissions, these must be stored safely and permanently underground (1).

We extract carbon as coal, gas and oil, which are then burnt to produce energy. With CCS, the resulting CO2 emissions from power plants and heavy industry can be captured, before being injected deep underground – in effect, returning carbon back to where it came from.

How do we store CO2 safely and permanently underground? By exploiting the same trapping mechanisms used by nature to ‘store’ CO2, gas and oil for millions of years; and by using existing technology to transport and inject the CO2. This technology has already been used for over 30 years by the oil industry to improve oil extraction.

Storage starts by pumping liquid CO2 into a carefully chosen reservoir. There are two main types: deep saline aquifers (which contain undrinkable salt water) and depleted oil and gas fields. Both have the same key geological features for storing CO2 safely and are usually sandstone or limestone.

A suitable CO2 storage reservoir needs:

  • a layer of porous rock at the correct depth to hold the CO2 (anywhere from 700 metres to 5,000 metres deep);
  • sufficient capacity;
  • and an impermeable layer of “cap” rock to seal the porous layer of rock underneath

Once the CO2 is trapped under the cap rock, three additional mechanisms contain the CO2 further: residual, dissolution and mineral trapping. It is precisely because of the complementary manner in which these trapping mechanisms take place that CO2GeoNet – a European Commission sponsored-network of 150 scientists from 13 public research institutes active in all aspects of CO2 geological storage research – has stated that “the safety of a CO2 storage site tends to increase with time.”(2) This is a truly unique - and completely natural - form of storage.

So, just how do these three trapping mechanisms work? Some of the CO2 injected into the smaller pores of the porous rock simply cannot move – even under pressure – this is residual trapping.

A portion of the CO2 also dissolves into the salt water and being heavier than the water around it, tends to sink to the bottom of the reservoir – this is dissolution trapping. Finally, in combination with the salt water, the CO2 can react to form minerals such as those found in sandstone or limestone – this is mineral trapping.

To ensure that a CO2 storage site functions as it should, a rigorous monitoring process begins at the reservoir selection stage and continues for as long as required. The well, cap rock and adjacent rock formations are monitored for changes in pressure and CO2 concentration levels. All of these systems ensure that response times are swift and corrective action can be taken should any abnormalities be detected. 

However, because the geological formations used to store CO2 actually diffuse it, massive releases are extremely unlikely. Because the CO2 becomes trapped in the tiny pores of the rocks, any leakage through the geological layers would be extremely slow, allowing plenty of time for it to be detected and dealt with.

Nevertheless, monitoring of a CO2 storage site will continue even after a well (through which the CO2 is injected into the storage reservoir) is closed – as required by EU law (3).

How much storage space do we have? Vast potential CO2 storage sites exist all over the world, given that the same formations which have trapped CO2, oil and gas for millions of years are entirely suited to this purpose. In Europe alone, we can store almost 60 years (4) worth of current annual CO2 emissions from power plants and heavy industry – time we will need to switch over to the sustainable energy systems of the future.


"We know that the safety of a CO2 storage site tends to increase with time."
- CO2GeoNet (scientific partnership focused on CO2 storage)


  1. The 2005 IPCC Special Report on CCS stated that the amount of CO2 that will remain trapped in properly selected and managed reservoirs is very likely to exceed 99% over 100 years and is likely to exceed 99% over 1,000 years.
  2. “What does geological storage really mean?” – CO2GeoNet
  3. Directive 2009/31/EC on the geological storage of carbon dioxide.
  4. Based on estimates by EU GeoCapacity across all CO2 storage reservoirs.