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 are almost
nonexistent, so wells that exhibit a similar risk profile (such as gas storage,
gas production, and steam injection) should be used as a 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 to 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 upon 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 two 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. Because 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 shutdowns.
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.
© 2011. Society of Petroleum Engineers
View full textPDF
- Original manuscript received:
8 September 2010
- Meeting paper published:
11 November 2010
- Revised manuscript received:
1 January 2011
- Manuscript approved:
28 June 2011
- Published online:
1 September 2011
- Version of record:
15 September 2011