Executive Summary

Microorganisms are thought to be responsible for over half the biomass on the planet, with a substantial fraction of them in the subsurface. Some microorganisms are known to be able to exist in extreme conditions and have been found in the subsurface at depths greater than 3km and can therefore potentially exist at some locations considered for geological storage of CO2. In general the chemoautotrophic nature of subsurface ecosystems increases with depth, i.e. microbes in the deeper subsurface are more likely to be using CO2 to synthesise necessary organic compounds. Therefore these are what you may expect to find at the depth of a typical CO2 storage reservoir.

If CO2 is injected into the subsurface where microorganisms reside it is likely to cause an initial decrease in numbers, followed by a gradual increase as some microorganisms will be able to utilise the CO2 to produce necessary organic compounds. CO2 can also affect the pH, which will affect bacterial communities, but the extent to which this will occur is dependent on the microorganisms present as well as the lithology and mineralogy of the storage formation and the chemistry of the formation waters. Overall, studies show that microbial redox reactions are favourable improved with greater CO2 pressure.

Microorganisms can potentially have a positive or negative effect on the storage formation. Negative effects include corrosion or biofouling of wellbore materials as well as potentially affecting injectivity by causing pore blocking. Positive effects include increasing storage security by the use of biofilms or enhanced solubility and mineral trapping through ureolysis. The use of biofilms as a barrier has also been considered as a potential mitigation method to stop the flow of the CO2 plume if this becomes necessary.

This topic may become increasingly significant as more demonstration projects start to take place. There are several research projects happening, mostly at the stage of laboratory testing as well as monitoring for microbial activity at storage demonstration projects, such as the Ketzin and Otway sites. As knowledge and understanding of potential effects and uses of microbial activity increases, this may need to be taken account of at the site characterisation phase of a project as well as the risk assessment.