What if chalk becomes mechanically weaker during supercritical CO₂ injection in a depleted gas reservoir?

Even though most past endeavors concluded no significant mechanical weakening of chalk flooded by supercritical CO₂, here, we comprehensively assess the mechanical response of a depleted chalk reservoir by considering the most pessimistic experimental results reported in literature in terms of mechanical alteration of chalk by supercritical CO₂. The primary aim stems from the lack of simulation studies documenting the deformation behaviour of CO₂ storage site in chalk. We perform a series of hydro-mechanical simulations on the Harald East gas field, the first candidate worldwide to store CO₂ in chalk formations located in the Danish North Sea. Different scenarios are considered by modifying the injection schemes (intermittent vs. continuous, low vs. high injection rate) and assumptions regarding the CO₂-induced alteration of the rock's properties. The subsidence trends of the reservoir top and the distribution of stress and elasto-visco-plastic strain across the reservoir, underburden, and along the wells are compared between scenarios. Besides, the mechanical responses of subsurface chalk upon pressure depletion during gas production and repressurisation by CO₂ injection are reviewed. The results highlight the effects of stress arching occurring during production as well as the injection rate, distance of porous chalk from injectors, timing between repressurisation and CO₂ propagation, and elastic strain rebound on the deformation response of the storage site. The present study also indicates that the field exhibits an overall negligible to minor deformation during the injection phase compared to the production phase, thereby providing encouraging results on the feasibility of re-purposing depleted chalk reservoirs as storage sites.