We observed cement/sandstone interfaces after reaction with supercritical CO2 using SEM-EDS, μ-XRD, and μ-Raman spectroscopy in order to evaluate chemical reactions of well-cement in CO2 storage site. To model actual well in geological formation, we prepared a well composite sample consisting of steel casing, Portland cement, and Berea sandstone. The batch experiment was performed for 56 days under a condition of 10 MPa and 50? C. After the batch experiment, a carbonation zone appeared at the cement/sandstone interface, however, the carbonation depth was limited within a few millimeters and the inner part of the cement was not altered. The Ca concentration in the carbonation zone increased 13% in comparison to that in the unaltered cement zone while the Mg, Si, and S concentrations decreased significantly. The predominant crystalline phases in the carbonation zone were calcite, aragonite, and vaterite. In addition, sparse precipitation of CaCO3 was observed in the pore spaces of the sandstone along the cement/sandstone interface.