This paper presents theoretical models of the phase transformation and the change of mechanical properties of oil well cement due to underground carbonation reactions. Application of oil well cement has become common practice in CO₂ underground storage formations. However, the CO₂ stored underground may leak through damaged well cement, and it can cause carbonation reactions which may significantly decrease the mechanical properties of well cement. Extensive research has been conducted by using experimental methods in laboratories to characterize the carbonation reactions of cement underground. However, no theoretical formulations have been developed considering the mechanical properties change due to the carbonation effect. In this paper, the CO₂ profile was predicted first by the error function solution of a 1D CO₂ diffusion equation and further the carbonation reaction rate was calculated. The phase transformation of well cement due to carbonation were then analyzed by using a stoichiometric model. After that, the changes of mechanical properties of well cement were modeled using a two-phase generalized self-consistent model at different scales. Specifically, the decrease of the elastic modulus of well cement during the carbonation process was used as an example application of the model. Finally, the modeling result was validated with published experimental results.