Incorporating Nanoscale Effects into a Continuum-Scale Reactive Transport Model for CO<sub>2</sub>‑Deteriorated Cement

Wellbore cement deterioration is critical for wellbore integrity and the safety of CO<sub>2</sub> storage in geologic formations. Our previous experimental work highlighted the importance of the portlandite (CH)-depleted zone and the surface dissolution zone in the CO<sub>2</sub>-attacked cement. In this study, we simulated numerically the evolution of the CH-depleted zone and the dissolution of the cement surfaces utilizing a reduced-dimension (1D) reactive transport model. The approach shows that three nanoscale effects are important and had to be incorporated in a continuum-scale model to capture experimental observations: First, it was necessary to account for the fact that secondary CaCO<sub>3</sub> precipitation does not fill the pore space completely, with the result that acidic brine continues to diffuse through the carbonated zone to form a CH-depleted zone. Second, secondary precipitation in brine begins via nucleation kinetics, and this could not be described with previous models using growth kinetics alone. Third, our results suggest that the CaCO<sub>3</sub> precipitates in the confined pore space are more soluble than those formed in brine. This study provides a new platform for a reduced dimension model for CO<sub>2</sub> attack on cement that captures the important nanoscale mechanisms influencing macroscale phenomena in subsurface environments.