Influence of waterflooding history on supercritical CO2 injectivity, the rock, and storage performance in fine-grained chalk

Geological storage of CO 2 in depleted oil fields offers an immediate and scalable route to carbon sequestration; however, the effects of prior production and water flooding on reservoir performance remain poorly constrained. Here, it is experimentally investigated how the production history influences CO 2 flow and rock integrity during supercritical CO 2 flooding of fine-grained chalk from the Maastrichtian Tor Formation in the Danish North Sea. Two core-flooding experiments were conducted under reservoir-representative conditions (282 bar, 73 °C) using composite cores representing two different reservoir conditions: water flooded (WF), and non-water flooded (NWF). Despite the distinct initial conditions, both systems achieved nearly complete (>98%) oil removal. However, the apparent CO 2 end-point relative permeability, calculated from the final differential pressure measurements and corresponding flow rates, was significantly lower in the WF cores (∼0.09) than in the NWF case (∼0.49), indicating that production history and initial saturation conditions influence CO 2 mobility. Post-flooding analyses, conducted using petrophysical core analysis and scanning electron microscopy, revealed no apparent mineralogical alteration or porosity loss; however, a looser chalk fabric was noted after supercritical CO 2 injection. These results demonstrate that water flooding during production has a strong influence on CO 2 mobility but does not compromise the integrity of carbonate rock. The findings highlight that, despite contrasting production histories, Tor Formation chalk cores preserve their structural integrity, providing important insights for de-risking CO 2 sequestration in the Halfdan Field.