M. Loizzo, SPE, O.A.P. Akemu, SPE, L. Jammes, SPE, J. Desroches, SPE,
Schlumberger; S. Lombardi, A. Annunziatellis, Sapienza Unviersity of Rome
SPE International Conference on CO2 Capture, Storage, and Utilization,
10-12 November 2010,
New Orleans, Louisiana, USA
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
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.
Leakage through wellbores is a major source of concern for the large-scale
deployment of geological carbon storage. Reliable estimates of both the
quantity of CO2 that can migrate through wells and the consequences of this
migration are needed in order to properly manage the leakage risk and show that
CO2 can be safely stored underground for very long periods of time.
These two tasks are subtly different: “risk” implies that the leaked CO2
affects a target (or stake) and causes a loss, to people (health and safety),
environment, assets or infrastructure. Large quantities of CO2 could leak to
the atmosphere, for instance in a desert, without causing any damage;
conversely small leaks, if allowed to accumulate, could result in substantial
losses. Estimating occurrences of leakage rate is thus solely dependent on the
leakage mechanism, whereas risk depends on the additional uncertainty of
whether targets are affected and what is the possible level of loss. On the
other hand data on losses, thus on risk, are easier to come by since the
general public and regulators are involved. Furthermore, even if CO2 leaks to
the atmosphere without causing immediate damage, it will still defeat the
purpose of storing it in the ground. We can then speak of “global risk”, as
opposed to the local risk of losses at or near the storage site.