Using natural storage mechanisms, CO2 is trapped between 700m and 5000m underground

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Using natural mechanisms

How do we ensure that captured CO2 is safely and permanently stored?

CCS Stroage

By storing CO2 underground, we are using a natural process that has trapped CO2, oil and gas for millions of years. Both oil and gas fields and deep saline aquifers have the same key geological features required for CO2 storage: a layer of porous rock to absorb the liquid CO2 and an impermeable layer of cap rock which seals the porous layer underneath, trapping the CO2.

Trapping

Inside the layer of porous rock, there are three natural trapping methods which make the safety of CO2 storage actually increase over time. These are residual, dissolution and mineral trapping.

Trapping CO2

1. Residual trapping

With residual trapping some of the injected CO2 is trapped in the tiny pores of the rocks and cannot move even under pressure.

2. Dissolution trapping

Dissolution trapping is a process where a portion of the CO2 dissolves into the surrounding water.

3. Mineral trapping

Over time, some of the heavy CO2-rich water sinks to the bottom of the reservoir where it may react to form minerals such as those found in limestone or sandstone. This is known as mineral trapping.

A rigorous monitoring process

Continuous monitoring

All areas of the CO2 reservoir are kept under close survey at all times: the well, cap rock and adjacent rock formations are monitored for changes in pressure and CO2 concentration levels – and this monitoring takes place during all phases of a CO2 reservoir's life: at the identification stage and the injection stage up to and after closure.

Predicting CO2 movement

Scientists follow the movement of CO2 in the reservoir by comparing the monitoring data they receive from simulated predictions which show them how they can expect the CO2 to move in the reservoir. In particular, monitoring teams look for possible migration out of the storage rock formation, or changes in storage capacity and any potential faults in the cap rock.

Monitoring methods

There are many monitoring systems available and the IEAGHG lists forty of these. Many of the companies involved in CCS monitoring use systems that have been developed and perfected over decades – principally for the oil and gas industries.

Monitoring systems include thermal sensors to track temperature changes and seismic monitoring instruments such as tiltmeters which measure the slightest movement in the ground. Trackers and wireline monitors are sent thousands of metres below the ground to check pressure and temperature changes near to and in the reservoir. Scientists also use nature to detect any CO2 leakage; hyperspectral imaging of vegetation highlights changes in the health of plants and insect behaviour, especially that of bees, is also monitored.

EU law requires close and effective monitoring

EU law demands that CO2 storage is closely monitored and the CCS Directive stipulates that CO2 storage schemes can only be admitted to the EU's Emissions Trading Scheme if the monitoring and verification of CO2 storage is carried out with complete satisfaction.

Swift detection and effective measures

In the highly unlikely event of a CO2 leak, companies with decades of experience in CCS already have detailed programmes to ensure that the leak is detected and swiftly stopped.

If a leak is identified, its source can be accurately determined by sensors that can detect CO2 flow. CO2-resistant polymers can be used to repair the leak and seal off problem zones. Further underground, diverter technology can be used to seek out low permeability zones, while areas with higher permeability can be treated.Once a leak has been treated, the effectiveness of the repair can be measured by using the similar sensors to those used in leak identification.

What’s next?

There is an urgent need for more technical and comprehensive characterisation of potential CO2 storage sites – especially deep saline aquifers – EU-wide. If storage is not sufficiently proven then investors, including state-owned entities, will not have the confidence to commit to an initial pipeline infrastructure and the possibility of a rapid transition from demonstration to wide-scale deployment will be compromised. Indeed, potential storage sites should be identified as early as possible as the most critical element of a CCS project in order to avoid any time lost.

A steep increase in support of storage characterisation at the pre-investment stage is urgently needed. This includes regional storage maps of relevant geological stages. One exciting opportunity is to develop a hub in the North Sea, which could store 100 million tonnes of CO2 a year by 2030.

A few examples on the storage side:

  • Large-scale storage projects Sleipner and Snøhvit in Norway, both separate CO2 from natural gas production and have stored about 1.7 MT CO2 a year - the equivalent of nearly 770 Olympic swimming pools full of CO2 per year.
  • The Quest CCS project in North America has already stored 1 million tonnes of CO2 – the equivalent emissions of around 384,600 passenger cars.
  • The Weyburn-Midale CO2 Storage and Monitoring Project - a geological storage project in Canada - has already stored the equivalent of 2,260 Olympic swimming pools of CO2.