Geologic storage of CO2 is one option for avoiding CO2 emissions from a large-scale point source such as a thermal power plant and a gas refinery. The alteration of well materials by CO2 under reservoir conditions requires characterization because the wells are the main possible leakage pathways for CO2 from a geological reservoir. This paper presents a numerical modeling of interaction experiments involving a composite well sample formed from steel casing surrounded by Portland cement, itself surrounded by sandstone and CO2-saturated brine at 10 MPa and 50 °C during a period of up to 8 weeks, as reported by Mito et al. (2015). A reactive-transport model was developed to simulate diffusion of the CO2-saturated brine in the well sample and the resulting successive dissolution/precipitation reactions in the sandstone, cement and steel. The observed changes in mineralogy (which primarily consist of dissolution of portlandite and Ca-rich CSH phases and precipitation of calcite, amorphous silica and zeolite) and the associated evolution in brine composition were reproduced by the model. A buffering role of sandstone on the cement degradation was evidenced, thus avoiding the re-dissolution of calcite usually observed in experiments with direct interaction between cement and CO2-saturated brine. Interestingly, the model results also noted a possible perturbation in the measured pH and Ca content due to CO2 outgassing during solution sampling. The Si behavior control linked with the uncertainty in zeolite stability is also discussed.