How do we ensure that captured CO2
is safely and permanently stored?
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.
Inside the layer of porous rock, there are three natural trapping methods which make the safety of CO2
storage generally increase over time. These are residual, dissolution and mineral trapping.
- 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.
- Dissolution trapping
Dissolution trapping is a process where a portion of the CO2 dissolves into the surrounding water.
- 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.
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.
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.
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.