Assessing the Long-Term Risks in Subsurface Carbon Storage (SCS) Projects: The Quantitative Risking Scale #23

For potential events where insufficient empirical or analog data are available, an estimate of likelihood must be made based on expert judgment and experience. Individual events are likely to have very low annual frequency rates, therefore utilizing an expanded log scale is necessary. And because these low probabilities are outside of our usual experience, it is challenging to assign them to these events.

Since this is a very subjective exercise, a risking scale supported by verbal descriptions should be agreed during the project framing phase and consistently used by the project team (see Table in post 22). When combined with a structured process such as the Delphi Method or use of an Expert Panel (as recommended in the RISQUE methodology) an internally consistent estimation of likelihood can be derived.

An expanded log scale (discussed in the previous post) demonstrates that the annual frequencies calculated for a single event are often very low. However, when the potential for events is evaluated over very long timeframes, the unmitigated risk of an event occurring increases (see Table below). This table, built from a series of Monte Carlo simulations, illustrates the effect of a series of constant but low frequency events. Note, for example, in Case 2 that what is unlikely on an annual basis becomes almost certain over a 1000-year period.

In SCS projects, the frequency of events will vary across the long time frames involved. Hence a process to estimate these time-varying frequencies will be presented in subsequent posts.