Risks from geological storage of CO2 primarily result from the consequences of unintended leakage from the storage formation that might can range between short-term potentially large leakages and long-term, more diffuse leakages, onshore and offshore storage settings. Risk assessment for CO2 storage is the process that examines and evaluates the potential for adverse health, safety and environmental effects on human health, the environment, and potentially other receptors resulting from CO2 exposure and leakage of injected or displaced fluids via wells, faults, fractures, and seismic events. The identification of potential leakage pathways is integrated with a MMV (Measurement, Monitoring and Verification) plan. Risk assessment is used to ensure the safety and acceptability of geological storage. It involves determining both the consequences and likelihood of an event. Risk mitigation is the planning for and implementation of contingency plans, should the need arise, to remediate adverse impacts. A good monitoring and mitigation plan will decrease the risk and uncertainty associated with many potential consequences.


Risk is defined as a function of the probability of an event that causes harm and its consequence, i.e., "risk = probability × impact or consequences". In general, overall risk can be considered as the sum of the products of individual risk impacts and probabilities, although it is necessary to express the various risks in the same unit (e.g. financial) while risks can be various in nature (human life, leakage rate, financial loss, etc). In addition, considering overall risk might not be relevant and considering a series of risk levels might be more appropriate.For CO2 geological storage, the main issue is adverse impacts that might result from a potential loss ofstorage integrity leading to unplanned CO2 migration out of the confining zone. Other types of risk must also be considered such as geomechanical effects, water flow changes, etc. potential consequences are related to public safety and health, environmental (ecosystem) safety, greenhouse gas emissions to the atmosphere, interference with other uses of the subsoil (e.g. water and hydrocarbons), economic viability of the project (e.g. financial loss for investors or insurers) and public acceptance. Fig. 6-1 (EPA, 2008) shows a conceptual framework of vulnerability evaluation for geological storage of carbon dioxide.

Operators and regulators have to determine an acceptable level of risk for CCS. To establish a reference baseline for acceptable levels of risk, it may be useful to apply metrics which allow ranking the different risks, and compare for instance the health, safety and environmental (HSE) potential risks related to the CCS projects with potential risks arising from other large-scale public/private infrastructure developments (dams, railways, airports, etc.) or analogue activities (oil and gas exploration, natural gas storage, acid gas disposal, etc.). Of course, all these processes act on different time scales that have also to be accounted for. In particular, CO2 escape from the storage reservoir might be assumed to occur over an extended time scale (centuries to millennia) that has to be considered in each risk scenario (DNV, 2010). A first common risk criterion is that risks associated with an activity should not be disproportionate to the benefits. Although risks associated with properly managed CCS projects are expected to be very low, the risk perceived by the public may be higher and benefits regarding climate change impacts and energy security may be difficult for the public to relate to. A second basic principle for setting risk criteria is that an activity should not impose Risks that can "reasonably" be avoided. In general, the risk can always be reduced further by implementation of additional safeguards, but at the price of a higher cost. Defining an acceptable level of risk is, therefore, closely related to the viability of the project, the cost of implementation of preventive safeguards and the cost of possible corrective measures (DNV, 2010).

E. Fig . 6-1

Fig. 6-1: Vulnerability Evaluation Framework (VEF) for geological storage of carbon dioxide (EPA, 2008).


in depth

6.1 Health, safety and environmental risks and impacts

Health, safety and environmental (HSE) risks fall into two main categories....

6.2 Risk analysis

Risk analysis involves interactive exchange among risk assessors, risk managers, regulators, local community, news media...

6.3 Risk assessment tools for CGS projects in various field cases

A survey of various risk assessment tools that incorporate geologic CCS risk assessment methodologies was conducted and ...

6.4 Application of risk assessment activities in various field cases and countries

Risk assessment activities have been performed for several CGS pilot sites or projects in some countries. ...