Analysis of seismic potential in a depleted chalk reservoir subject to CO2 injection

This study presents a multi-scale modelling framework to evaluate fault reactivation risks and seismic potential during CO 2 injection into a highly depleted and deformable chalk reservoir, using the Harald East field in the northern part of the Danish North Sea as a case study. A robust multi-scale Thermo-Hydro-Mechanical (THM) modeling approach is developed to bridge field- and fault-scale processes, supporting fault stability and seismic risk assessment in CO 2 storage. A field-scale coupled flow-geomechanical model is used to screen for critically-stressed faults, while fault-scale simulations investigate slip behaviour using a Mohr-Coulomb frictional model, combined with a rate-dependent frictional model to assess specific potential seismic events. THM analysis under realistic CO 2 injection scenarios reveals that faults remain stable with friction coefficients of 0.6. However, simulations with reduced initial friction coefficients (e.g., 0.27 and 0.36) indicate localized slip risks during both production and injection phases along the plane of one single fault out of a total of 30 faults analysed. As the reservoir repressurizes, the stress regime transitions from normal to reverse faulting, accompanied by a significant reorientation in principal stress. This shift of stress regime causes a progressive rise in shear stress on the fault plane as repressurization continues, resulting in higher slip tendency values and a greater likelihood of seismic reactivation. Besides, the results demonstrate the benefit of a combined field- and fault-scale approach that enhances computational efficiency by restricting detailed analyses to critical faults and critical time throughout the injection period. This work provides a framework for fault stability and seismic risk assessments, offering key insights for the safe implementation of underground CO 2 storage projects.