6.4 Conclusions

There are sometimes large variations in the requirements concerning modelling, risk assessment, monitoring and safety demonstration between the different regulations. However the standard is that these requirements are met already during application in an after-injection plan, approved by the competent authorities. There are few prescriptions of the requirements during the closure and post-closure stadiums, as there are no projects within this time-frame yet, but better definitions of necessary tasks would lead to better understanding on the operator's part.

In general not requiring any particular monitoring techniques allows for an application of updated technologies and methodologies. OSPAR FRAM, which provides a really good basis for modelling and risk assessment, specifies instead the outcomes of the risk assessment which will lead to desired results in a flexible environment.

CO2 safety can be split up in three main parts: demonstration of no leakage, demonstration of conformity with prediction models, demonstration of long-term stability. An additional requirement may also be the demonstration of no possible environmental endangerment. Demonstrating no leakage is dependent on the used monitoring system, which should be suited for the site-specific requirements. So far no regulation permits leakage, but this condition is severely discussed as it may be necessary to allow minor leakage in some cases. This discussion is also linked to the definition of leakage, which also varies within the regulations. Furthermore there is an obvious limitation of demonstrating the safety by comparing model predictions and monitoring. Diffuse leakage may be found but not detected with the used monitoring time interval. There are also a number of different geological models or scenarios matching the observed data but with different risk factor, therefore any conformity with the observed data may not demonstrate safety.

And there are still certain points which will need to be addressed during the development of the regulations for closure and post-closure of CO2 storage. Is any leakage acceptable? What about the environmental impact and the leakage rates? If diffuse leakage occurs with a small amount of CO2, remediation may be unfeasible, but what if the environmental effects were minor? Is the requirement of no leakage too restrictive?