Probabilistic Analysis of Draupne Shale Caprock Reliability of the Alpha Prospect - A Potential Co2 Storage Site in the Smeaheia Area, Northern North Sea

Abstract

CO2 injection into a saline aquifer requires a viable caprock to arrest the vertical movement of the CO2 plume. Quantitative assessment of caprock integrity is often challenging because of uncertainties involved in the model input parameters. In this study, Draupne Formation's reliability as caprock is evaluated before CO2 injection by introducing a stochastic approach. We estimated both deterministic factors of safety and probabilistic failure values of different scenarios, and the results are compared. The probabilistic failure values are calculated using the First Order Reliability Method (FORM). Draupne formation shows a considerably low probability of failure with a high-reliability index in the initial stress condition. The sensitivity study reveals that the pore pressure and horizontal stress are the most crucial parameters and contribute two-thirds to failure probability. When the change of effective horizontal stresses in the reservoir is assumed considering the pore pressure change in the Troll field, this study shows that the field production may decrease the probability of shear failure. Moreover, the study indicates that the suggested probabilistic approach is critical in the presence of various uncertainties. However, the assumptions used in this study, especially the change in effective horizontal stresses within the reservoir, can be affected by other factors (e.g., stiffness contrast between reservoir and surroundings, geometrical effects, stress paths, etc.) and should be investigated further.