2.5 Conclusions

The preliminary geological characterisation of potential CO2 storage sites includes site screening, preliminary storage capacity assessment and then ranking leading to selection of optimal storage site for a CCS project.

The whole process cannot be carried out without taking into consideration legal and regulatory, environmental, technical and even to some extent economic aspects of CO2 geological storage.

The most significant knowledge gaps and uncertainties may exist in the case of CO2 storage in saline aquifers, where often insufficient information is available to evaluate the sites in question against principal screening criteria. For onshore saline aquifers, criteria related to safety of underground potable water resources are considered the most important. Although offshore storage does not relate to such problems, prevention of significant leaks from reservoir to the sea bottom through natural or man-made pathways is the main safety concern.

In the case of saline aquifers, it is often difficult to make a reliable site ranking and selection without acquiring new data, including drilling new exploratory wells, carrying out new seismic surveys and other characterisation surveys.

There are more possible conflicts of use of the subsurface in the case of onshore than offshore saline aquifer CO2 storage and fewer conflicts for depleted hydrocarbon fields and un-mineable coal beds.