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

Geological storage of CO2 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 CO2 flow and rock integrity during supercritical CO2 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 CO2 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 CO2 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 CO2 injection. These results demonstrate that water flooding during production has a strong influence on CO2 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 CO2 sequestration in the Halfdan Field.