Cement CO2-alteration propagation at the well-caprock-reservoir interface and influence of diffusion

A geochemical model of wellbore cement-caprock-reservoir interfaces is developed. The model predicts the mechanisms of chemical alteration of cement due to interaction with the acid brine obtained after the injection of supercritical CO2 into the reservoir rock. Numerical outputs indicate that, at the well-reservoir interface, intrusion into the cement phase of acid brine accompanied by aqueous CO2 quasi-instantaneously transforms Portlandite and CSH into Calcite and hydrated Si-gel, leading to a slight but moderate porosity decrease, from 28% to ~20%. Conversely, at the well-caprock interface, Portlandite first leaches and the Ca2+ ions released from it migrate towards the caprock, where they eventually combine with slowly diffusing aqueous CO2 to form Calcite, up to the porosity collapse of the interface. This can significantly affect the sealing behaviour of the geological storage. We can finally deduce a time-space characterisation of the system enhancing the main chemical mechanisms in view of a future export to a geomechanical model.