Geochemical Barriers in $$\hbox {CO}_{2}$$ CO 2 Capture and Storage Feasibility Studies

$$\hbox {CO}_{2}$$ sequestration in geological formations requires specific conditions to safely store this greenhouse gas underground. Different geological reservoirs can be used for this purpose, although saline aquifers are one of the most promising targets due to both their worldwide availability and storing capacity. Nevertheless, geochemical processes and fluid flow properties are to be assessed pre-, during, and post-injection of $$\hbox {CO}_{2}$$ . Theoretical calculations carried out by numerical geochemical modeling play an important role to understand the fate of $$\hbox {CO}_{2}$$ and to investigate short-to-long-term consequences of $$\hbox {CO}_{2}$$ storage into deep saline reservoirs. In this paper, the injection of $$\hbox {CO}_{2}$$ in a deep structure located offshore in the Tyrrhenian Sea (central Italy) was simulated. The results of a methodological approach for evaluating the impact that $$\hbox {CO}_{2}$$ has in a saline aquifer hosted in Mesozoic limestone formations were discussed. Seismic reflection data were used to develop a reliable 3D geological model, while 3D simulations of reactive transport were performed via the TOUGHREACT code. The simulation model covered an area of $$>$$ 100 km $$^{2}$$ and a vertical cross-section of $$>$$ 3 km, including the trapping structure. Two simulations, at different scales, were carried out to depict the local complex geological system and to assess: (i) the geochemical evolution at the reservoir–caprock interface over a short time interval, (ii) the permeability variations close to the $$\hbox {CO}_{2}$$ plume front, and (iii) the $$\hbox {CO}_{2}$$ path from the injection well throughout the geological structure. One of the most important results achieved in this study was the formation of a geochemical barrier as $$\hbox {CO}_{2}$$ -rich acidic waters flowed into the limestone reservoir. As a consequence, a complex precipitation/dissolution zone formed, which likely plays a significant role in the sequestration of $$\hbox {CO}_{2}$$ due to either the reduction of the available storage volume and/or the enhancement of the required injection pressure.