When people judge risk they usually want a numerical result, but paradoxically their risk judgment really boils down to their “degree of belief” that something is not risky (or risky), often represented using a numerical scale. Their belief is usually based on many types of information, some quantitative and some qualitative, but reflects their own perspectives.
Why could this be a problem? When planning a CO2 storage project we should try to reduce the impacts and / or probability of those phenomena that cause risks and increase the impacts and / or probability of those phenomena that reduce risks. We must therefore agree the important risks and how they should be judged using diverse information. In a large CO2 storage project having many stakeholders with different perspectives this could be particularly challenging. The best way to arrive at an outcome that is acceptable for all involved requires a structured discussion among the stakeholders that assigns equal relevance to subjective judgments and quantitative analysis.
In what ways are subjective and quantitative aspects important in risk assessment? According to the Society for Risk Analysis risk is “The potential for realization of unwanted, adverse consequences to human life, health, property, or the environment”. A practical interpretation of this statement is that risk = the probability of some phenomenon occurring x the adverse consequences of that phenomenon, but several factors complicate application of this equation. Insufficient site-specific information may increase reliance on expert judgments when estimating the future probability of some phenomenon. Moreover, the possible adverse consequences of that phenomenon may be hard to deduce. And finally, even when the probability and consequences can be estimated, the significance attributed to these consequences depends upon the concerns of a particular stakeholder. Therefore, subjective judgments and quantitative analyses are important in all aspects of risk assessment. Furthermore, both need to recognize that risk is not the same thing as uncertainty, because the probability of a phenomenon occurring and its potential consequences do not depend upon our knowledge.
Effective risk management must therefore: (1) account for varied subjective judgments of different stakeholders (it is not just numerical calculations); (2) integrate varied information from many sources; (3) treat uncertainties appropriately (and distinguish uncertainty from risk); and (4) be based on well-documented and transparent decisions about what the various kinds of information tell us about risks.
To meet these goals, CO2ReMoVe has developed a structured, documented and auditable approach for identifying and assessing risks. The risks associated with CO2 storage can be managed effectively only if it is understood how all the features, events and processes (often called FEPs) that occur within a storage site contribute to the risks. One tool that can be used for achieving this goal is building a decision tree, such as the one depicted below. By overcoming the paradox that stakeholders usually want quantitative estimates of risk, but subjective judgments and quantitative analyses are both necessary, CO2 storage projects can move forward based on well-founded risk management.
Part of a decision tree to structure information and hypotheses about a CO2 storage site, with green indicating confidence in a hypothesis, red indicating confidence against, and white space showing uncertainty. Green and red fields are assigned at the lowest level by expert judgment, based on qualitative and quantitative information, and propagated through the tree; white fields are calculated by difference.
Richard Metcalfe (Quintessa)
Richard is a geoscientist who manages activities in the field of CO2 storage at Quintessa Limited, a scientific and mathematical consulting company that specializes in developing and applying quantitative and qualitative techniques for integrated assessment and decision-making, particularly in the environmental science field. For more than 20 years Richard has worked to characterise varied underground environments in order to assess their suitability for a wide range of purposes, including natural gas storage and CO2 storage. He has contributed to various CO2 storage projects since the mid-1990’s, ranging from the development of databases of Features, Events and Processes, to coupled modelling of CO2 migration and CO2-water-rock reactions. During CO2ReMoVe he has managed Quintessa’s contribution to risk assessment and the development of performance assessment tools.