Leakage through new or existing wellbores is considered a major risk for carbon dioxide (CO2) geological storage. Long-term effective containment of CO2 is required and the presence of millions of suspended or abandoned wells exacerbates the potential risk in mature hydrocarbon provinces. Accurate estimates of risk profiles can support the acceptance of geological storage and the adoption of economically effective risk prevention and mitigation measures.
Reliable data about long-term containment of CO2 is almost non-existent, so wells that exhibit a similar risk profile (such as gas storage, gas production and steam injection) should be used as proxy to assess failure rates and consequences for cemented wellbores.
Statistical data about occurrence of leaks and their consequences are analyzed to determine the risk profile of CO2 leaks. A smaller sample of data about leak rates is also analyzed to provide their statistical distribution. Rates and consequences are then compared to try and assess the order of magnitude of major and catastrophic leaks.
Hydrothermal CO2 leaks in natural analogs are also reviewed to compare the distribution of leak rates and the consequences on health, safety and environment of CO2 releases to soil and atmosphere.
Analysis of existing data will show that major leaks are likely to occur in less than 2 wells per 1,000, with the overwhelming majority of CO2 leaks being small and with limited or negligible consequences.
Given their risk profile, CO2 wellbore leaks should be addressed through a routine risk management approach. Their frequent occurrence requires effective prevention measures, such as understanding leaks and adapting and deploying practices to minimize their occurrence. On the other hand, their low impact ensures maximum effectiveness of mitigation measures, such as monitoring: since leaks can be detected long before damage ensues, they can be observed to predict their long-term consequences and to plan the most effective intervention without unnecessary immediate operation shut-downs.
In conclusion, the recommended course of action is to focus on risk prevention and early detection. This implies the evolution from a "no leaks" attitude (even for negligible leak consequences) to one that seeks no damage and relies on tight surveillance.