PESA ETSIG/CSIRO CCS Knowledge Transfer Series: Installment 6

Like any oil and gas operation, CCS projects need to assess environmental impacts, including potential impacts on groundwater. The identification of potential leakage pathways and approximation of leakage rates are a critical part of the site selection process and help to minimise material impacts on the environment. Compromised well or subsurface integrity issues may facilitate the migration or release of CO₂ out of its primary storage container into shallow groundwater. However, the probability of leakage via geological features when storing CO₂ at well-characterised sites is extremely low. While the probability of compromised well integrity occurrence based on petroleum industry experience is also extremely low, the probability of well leakage increases with the number of active and inactive wells within the area of the injected CO₂ plume. However, well leakage is relatively easily detected and well mitigation and remediation technologies are well-established. Generally, multiple barriers and partial migration into various intervening aquifers, as well as buffering, dissolution and residual saturation will reduce the total volume ending up in groundwater.

Carbon dioxide itself is not considered a pollutant or contaminant in groundwater and is not considered as a parameter for water quality in most groundwater jurisdictions. However, increased CO₂ concentrations could reduce the pH of groundwater, i.e., increase its acidity, and thereby enhance geochemical reactions between groundwater and aquifer sediments, potentially resulting in release and mobilization of toxic trace metals. Extremely high concentrations of CO₂ in groundwater would be required to cause material harm to the environment as demonstrated in exceptional cases of natural CO₂ leakage. Such conditions are improbable in the case of CO₂ geological storage. Also, the remobilisation of other harmful substance as result of CO₂ leakage has been investigated in many laboratory and field experiments, and material impacts are generally deemed unlikely under CO₂ leakage conditions from a well-characterised and monitored geological storage project. The already low risk of impacts on potable groundwater due to CO₂ leakage can be further minimised by early leakage detection before reaching groundwater wells; timely stop of injection and remediation.