Geochemical Barriers in CO2 Capture and Storage Feasibility Studies

Abstract

CO2 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 CO2. Theoretical calculations carried out by numerical geochemical modeling play an important role to understand the fate of CO2 and to investigate short-to-long-term consequences of CO2 storage into deep saline reservoirs. In this paper, the injection of CO2 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 CO2 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 km2 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 CO2 plume front, and (iii) the CO2 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 CO2-rich acidic waters flowed into the limestone reservoir.