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

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 CO₂ 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 CO₂. 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 CO₂ 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 CO₂ injection. Besides, assuming a strengthening or softening effect of CO₂ 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 CO₂ injection is spatially restricted around the injectors due to the difference in propagation velocities between the CO₂ 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 CO₂, this study reports encouraging results on the feasibility of re-purposing depleted chalk reservoirs as storage sites.