Shear and Vertical Deformation Behaviour at the Field Scale During CO2 Storage in a Depleted Chalk Reservoir (Danish North Sea)

ABSTRACT CO2 storage represents one of the recommended remediation methods to reduce the emission of anthropogenic CO2 in the atmosphere. As part of the Bifrost project that aims at developing CO2 storage activities in Denmark, a series of coupled hydro-mechanical-chemical simulations is carried out in the Harald East chalk field to assess the flow and mechanical response of the storage site, assuming both continuous and intermittent injection. A total of two continuous injection scenarios during 16 years (scenario 1) and 32 years (scenario 2) and scenario 3 which assumes intermittent injection are considered. The injection rate in scenario 2 and 3 is half of that of scenario 1, and the duration of the injection in scenario 3 is increased compared to scenario 1. The rate and duration of injection are adjusted to ensure a similar volume of stored gas in each simulation run. The outcomes show that fast injection prevents significant compaction in the reservoir. Besides, the calculated shear deformation in the overburden is utilised to study wellbore stability during storage. The results indicate that the accurate characterisation of the porosity distribution in the reservoir as well as the potential mechanical weakening of chalk by CO2 are essential for reliable wellbore integrity analyses. Further studies will be conducted to assess wellbore stability in the Harald East area during CO2 storage activities. INTRODUCTION The capture and underground storage of anthropogenic CO2 (CCS) represent a valid technology that helps tackling climate change by reducing the amount of greenhouse gas emitted by our societies (Al Amer, 2022; ICPP, 2005). Using pre-existing infrastructures present in depleted hydrocarbon fields drastically reduces the cost associated with CCS activities compared to other sites e.g., onshore and offshore aquifers (Schmelz et al., 2020). Besides, the proven efficiency of the cap rock that contained hydrocarbon for millions of years and the current low formation pressure below hydrostatic conditions are limiting risks of leakage. Located offshore, CO2 can be either shipped to the storage site, such as planned for the Greensand project or transported by pipeline as planned for the Northern Lights project. Taking into account that the floating units used to temporary store CO2 transported by boats prior to injection have limited storage capacities, shipping gas can compel the stakeholders to carry out intermittent CO2 injection in case of bad weather conditions, workovers, and potential shortages in CO2 supplied by the industry.