The study of CO2 natural reservoirs to develop criteria for risk assessment and safety strategy.

Due to the large volumes produced, carbon dioxide (CO2) is the main anthropogenic compound identified as affecting the stability of the Earth’s climate. The injection and storage of anthropogenic CO2 in deep geologic formations (i.e. saline aquifers, hydrocarbon reservoirs, and unmineable coal seams) is a feasible strategy for rapidly reducing CO2 emissions to the atmosphere. However, a lack of public support, due to concerns over risks, could potentially block the widespread implementation of this promising technology. Only by facing these legitimate concerns will we be able to assure the public that (i) the scientific knowledge exists to select the best and safest sites; (ii) the techniques and approaches exist to monitor the safety of these sites during operation and post-injection; and (iii) the technologies exist if it is necessary to remedy a leak. There is the potential to examine some of the many naturally occurring CO2 reservoirs that occur throughout the world, some of which have trapped gas over geologic time periods while others leak due to gas migration along faults and fracture networks. These natural reservoirs occur in volcanic, geothermal, and sedimentary basin settings, and the unique geological and structural characteristics of each site can give important information regarding CO2 migration mechanisms at the required timescales and within complex, heterogeneous geological settings. While well-sealed, natural CO2 reservoirs can help us to understand the processes that isolate CO2, and other gases in the subsurface, leaking sites can be used to study gas migration processes along unsealed fault and fracture zones. In this work we present results from gas migration studies conducted at sites which represent the two extremes of gas leakage found in nature. These two sites are used to highlight how faulting style and near surface geology/hydrogeology can influence the migration of deep origin gases towards the ground surface.