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.

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 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.