Accounting for chemical impacts on mechanical properties in coupled simulations of CO2 injection into a depleted chalk field

• Coupled HMC modelling of the first candidate worldwide to store CO 2 in chalk. • Minor impact of CO 2 on field deformation compared to depletion during production. • Plastic deformation during injection localized around the injectors. • This work reports encouraging results on the feasibility of storing CO 2 in chalk. The Bifrost project aims at repurposing the Harald West sandstone field as the primary target and the neighboring Harald East chalk field as a potential upside for CO 2 storage. One challenge in assessing the feasibility of CCS in chalk is the contradictory experimental results reported in literature and indicating both a softening and strengthening of rocks in contact with supercritical CO 2 . To account for this uncertainty, coupled hydro-mechanical-chemical simulations are conducted and the storage capacity and deformation behavior of the depleted Harald East field are assessed under three different scenarios. The supercritical CO 2 is assumed to have no effect on chalk's mechanical properties (scenario 1) as well as a strengthening (scenario 2) and softening effect (scenario 3). The results indicate that the ultimate storage capacity is primarily controlled by chalk compaction occurring during hydrocarbon production with a negligible impact of porosity reduction caused by CO 2 injection. Besides, assuming a strengthening or softening effect of CO 2 on rock strength has a minor impact on overall reservoir and overburden deformation. Only 4 % discrepancy is predicted in reservoir and seafloor subsidence between scenarios 2 and 3. Finally, irreversible deformation during CO 2 injection is spatially restricted around the injectors due to the difference in propagation velocities between the CO 2 and pressure fronts. Recalling that scenario 3 considers the most pessimistic experimental results reported in literature in terms of mechanical alteration of rock by supercritical CO 2 , this study reports encouraging results on the feasibility of re-purposing depleted chalk reservoirs as storage sites.