Chalk as a Strategic CO₂ Storage Medium: Experimental Insights into Reactivity and Flow Dynamics

Summary This study examines the suitability of North Sea chalk reservoirs, which have historically been utilised for oil production, as potential sites for geological carbon dioxide (CO2) storage. While sandstone formations in Denmark have been the primary focus due to their favourable properties, chalk offers vast but underutilised storage potential despite its low permeability and high geochemical reactivity. The research addresses critical gaps in understanding chalk-CO2 interactions, focusing on continuous versus intermittent supercritical CO2 (scCO2) injection, and the influence of residual hydrocarbons on flow behaviour and mineral reactions. Four core flooding experiments were conducted using Tor Formation chalk samples from original and waterflooded reservoir conditions. Brine-saturated cores exhibited increased permeability under continuous injection, attributed to matrix dissolution, whereas intermittent injection resulted in reduced permeability due to carbonate precipitation. In hydrocarbon-bearing cores, increased permeability was observed in conjunction with modest changes in porosity, suggesting that residual oil moderated reactivity and influenced fluid-fluid interactions. The results highlight the strong influence of reservoir history and injection protocol on injectivity and reactivity in chalk formations. These insights are vital for optimising CO2 injection strategies in fine-grained carbonate reservoirs with significant, yet untapped, storage potential.