Microscopic and macroscopic assessment of carbonate dissolution for geologic CO_2 storage

Carbon capture and storage in underground formations might be considered as a relevant technology to curb anthropogenic climate gas emissions. However, carbon dioxide ( CO_2 ) injection can lead to severe rock-fluid interactions depending on the thermodynamic conditions, rock and fluids composition. The progressive dissolution of CO_2 in the formation brine results in mineral dissolution/precipitation processes that may drastically change the properties of the reservoir. This study is an attempt to get a deeper understanding of the dissolution/precipitation processes in a heterogeneous limestone at microscopic and macroscopic levels by a synergy between Isothermal Titration Calorimetry (ITC) and core flooding experiments with in-situ imaging to quantify uneven displacement fronts and understand the influence of reactions on a larger scale. Rock-fluid and fluid-fluid interactions, evaluated by ITC experiments, indicate that the carbonate dissolution is unfavorable with respect to enthalpy change but thermodynamically favorable with respect to entropy change (cations and hydrogen carbonate increase in the brine). Core flooding experiments with in-situ imaging by PET/CT show that complex pore structures cause a variation in the availability and ratio of the reactive fluid throughout the porous medium, hence, non-uniform dissolution was confirmed at core scale. The synergy between microcalorimetry and core flooding provides relevant insights into the dissolution of heterogeneous carbonate rocks at both microscopic and macroscopic scales.