Is a Depleted Chalk Reservoir Safe for CO2 Storage?

Abstract Chalk formations, composed predominantly of calcite, are highly reactive under CO2-rich conditions. This study assesses the suitability of depleted chalk reservoirs for long-term CO2 storage by evaluating CO2-brine-rock interactions, their impact on fluid flow behavior, and their potential implications for reservoir integrity and storage safety. Laboratory-scale experiments were conducted on brine-saturated chalk core samples exposed to both pure and impure CO2 under reservoir-relevant pressure and temperature conditions. Coreflooding experiments were used to characterize CO2 transport behavior, while geochemical exposure tests assessed calcite dissolution induced by CO2-acidified brine. Capillary pressure and capillary entry pressure measurements were performed on both reservoir and caprock samples before and after CO2 exposure to quantify changes in sealing capacity associated with geochemical alteration. The experimental results show that exposure to CO2-saturated brine induced measurable calcite dissolution in the chalk samples, with the effect further amplified in the presence of impurities. However, this geochemical alteration did not translate into detectable changes in the key petrophysical properties or inferred indicators of rock integrity. Capillary pressure curves and capillary entry pressure measurements for CO2-exposed samples were comparable to those obtained from fresh samples, indicating that CO2 exposure did not compromise the sealing capacity of either the reservoir or caprock materials under the investigated conditions. Furthermore, no significant changes in pore structure were observed between fresh and exposed samples, suggesting that the extent of dissolution was insufficient to alter the pore network or induce mechanical weakening. Overall, the results indicate that, despite ongoing calcite dissolution, short- to medium-term exposure to both pure and impure CO2 does not adversely affect capillary sealing behavior or inferred rock-integrity indicators under the investigated conditions. These findings support the potential viability of depleted chalk reservoirs for CO2 storage, while underscoring the importance of evaluating dissolution kinetics and exposure duration when assessing long-term storage safety. This study provides integrated experimental insight into the coupled effects of CO2 composition, geochemical reactivity, and capillary sealing behavior in chalk formations. The results offer practical guidance for engineers assessing depleted chalk reservoirs for CO2 storage, particularly by linking geochemical alteration to flow performance and containment risk under realistic storage conditions.