There are two dimensions of risk and uncertainty to consider in SCS projects. Epistemic probabilities and uncertainties are those that can be determined and refined by increasing knowledge of the physical properties of the system. In contrast, aleatory probabilities and uncertainties are inherently random and cannot be reduced by technical work.
Earthquake prediction provides an example of aleatory probability. For the San Francisco area, Aagaard et al. (2016) state that there is a “72% percent probability of one or more M >= 6.7 earthquakes from 2014 to 2043”. Similarly, for SCS projects, a team may forecast an annual frequency that faults may slip as injection proceeds. This would be an aleatory probability forecast. Frequency data from relevant analogues can inform the estimation of aleatory probabilities and uncertainties for SCS projects.
Epistemic probabilities and uncertainties for SCS projects can be evaluated using system data and models. Uncertainty can be reduced by additional data gathering and improved analyses. For example, the range of the average porosity for a target reservoir can be narrowed by the careful analysis of additional well logs.
This duality means that reservoir models will have parameter variability that arises from both natural heterogeneity (aleatory) and from a lack of knowledge (epistemic). These two types of uncertainty should be considered separately and clearly documented. For example, the combination of aleatory and epistemic uncertainty was applied to a model of CO2 plume extension by Bellenfantet al. (2009). They proposed weighting optimistic and pessimistic forecasts to describe the overall uncertainty to decision-makers.
Reference: Aagaard, B.T., Blair, J.L., Boatwright, J., Garcia, S.H., Harris, R.A., Michael, A.J., Schwartz, D.P., DiLeo,
J.S. 2016. Earthquake Outlook for the San Francisco Bay Region 2014–2043. USGS.
Reference: Bellenfant, G., Guyonnet, D., Dubois, D. and Bouc, O. 2009. Uncertainty theories applied to the analysis of CO2 plume extension during geological storage. Energy Procedia, 1, 2447-2454.