Viscoplastic Deformation During CO₂ Storage in Danish Chalk Reservoirs: Role of Petrophysical Heterogeneity and Mechanical Alteration

Denmark aims at a 70% reduction in greenhouse gas emissions by 2030 compared to levels measured in 1990, with a long-term target of becoming carbon-neutral by 2050. As part of this national effort, the Bifrost project, aims at repurposing two depleted gas fields in the Danish North Sea for CO₂ storage, namely the Harald West sandstone field as the primary target and the neighboring Harald East chalk field as a potential upside. The Harald East chalk is the focus of this study. The storage potential and infrastructures available within the multiple chalk fields located in the Danish North Sea represent valuable assets to fulfill the national objectives enabling a time- and cost-efficient implementation of carbon storage activities. One of the main challenges for carbon storage in chalk is the contradictory experimental results reported in literature that indicate both a strengthening and a softening effect of supercritical CO₂ on the plastic and elastic properties of chalk. Such uncertainty hampers accurate prediction of the deformation response of storage sites. In this context, the study aims at assessing the impacts of two levels of uncertainty; the type of mechanical alteration induced by supercritical CO₂ and the petrophysical heterogeneity on the longterm deformation behaviour of chalk reservoirs. An in-house hydro-mechanical-chemical model calibrated against experimental data on chalk is applied in a reservoir model of the Harald East field. A 16 yearlong injection period is simulated assuming two scenarios. In scenario 1, supercritical CO₂ has no impact on the mechanical properties of the rock, whereas in scenario 2, a 30% and 25% lowering of the pore collapse stress and elastic modulus of chalk is assumed. A systematic comparison of the flow and mechanical behaviour of low and high porosity cells located in the vicinity of an injection well indicates that the impact of CO₂ on the mechanical properties of chalk, the distance of the cells from the injector, the local stress redistribution taking place in the reservoir between mechanically soft and strong cells, and the presence of natural gas in pore space before CO₂ injection are key factors controlling the amount and distribution of plastic deformation occurring in the storage site. The outcome of this work enables quantifying the main risks associated with rock compaction close to and further away from injectors during and after carbon storage in chalk fields.