CO2 storage in a depleted gas reservoir in Italy: Modeling the diffusion of CO2-rich fluids within the wellbore cement of an abandoned well

The geochemical processes occurring in cement-based materials in contact with clay and arenaceous rock formations under high-PCO2 conditions have been investigated using reactive transport simulations. The site of interest is a depleted gas reservoir in Italy where CO2 geological sequestration is under evaluation. On the basis of field data, simulations have been carried out under liquid saturated, isothermal (T = 45°C) and isobaric (P = 140 bar) conditions. Two main reactive stages were predicted inside the cement sheath: a first "carbonation" stage, characterized by the dissolution of primary portlandite and CSH, the precipitation of calcite and some reduction in porosity, and a second "acidification" stage, characterized by a severe drop in pH, the dissolution of the newly-formed calcite and some increase in porosity.