Implications of alternative post-injection regulatory guidance upon CO2 storage in dipping open aquifers

To transfer responsibility for a CO2 storage site to the competent authority, the EU Storage Directive requires it to be demonstrated that CO2 will be completely and permanently contained and that the storage site is evolving towards a situation of long-term stability. As outlined in current EU guidance, there may be room for interpretation by both the national competent authority and the storage operator in the implementation of this part of the Directive. Here we postulate that, for storage in dipping open aquifers, the requirements of the Directive can be met by modelling various combinations of a boundary constraint, a migration velocity constraint and a constraint on the minimum fraction of injected CO2 that is likely to be trapped at a defined time (1000 or 10,000 years) after injection ceases. A key outcome is that there is a strong trade-off between storage efficiency and storage security when storage is maximised under different combinations of constraints. To increase confidence in security under all scenarios, modelling longer post-injection periods is desirable; however there may be computational limits to this.