Geomechanical evaluation of a depleted chalk reservoirfor CO₂ storage

Carbon capture and storage (CCS) is the process of capturing carbon dioxide (CO₂) before it enters the atmosphere
and storing it for centuries. Depleted hydrocarbon fields represent an opportunity for CO₂ storage because of already
installed facilities and equipment and available subsurface data recoding during the lifetime of the reservoir. With
starting CO₂ injection in depleted chalk formations that have experienced elastic and visco-plastic deformation during
production, two main processes can occur simultaneously under visco-plastic deformation condition. First, reservoir
expansion is often expected as reservoir pressure builds up and effective stresses drops. The second is the reservoir
compaction that continues due to viscous deformation even after the cessation of hydrocarbon production. As a result,
the reservoir top tends to go up because of build-up pressure which may be limited at the same time with viscous
deformation that had induced from the production phase. Taking advantage of these two effects, this study investigates
the safest scenarios for CO₂ injection in depleted chalk formation to minimize geo-mechanical deformation. For this
purpose, a rate dependent constitutive model is utilized to calculate deformation in 1D simulation during depletion
and injection phases. The constitutive model is calibrated for pelagic chalk from the North Sea and takes into account
the water weakening effect. The simulation uses experimental and well log data obtained along two wells in Harald
East field. We evaluate the impact of various operational conditions for CO₂ injection in depleted reservoirs on longterm deformation of the storage sites.