Theoretical modeling on chemical composition and mechanical properties of well cement under carbonation reactions

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. •The stoichiometric model has been applied to track the phase transformations of cement under the carbonation reaction.•The rate of carbonation reactions for each phase is calculated in the model with the application of CO2 diffusion.•The two-phase general self-consistent model has been used to calculate the mechanical properties change of the cement.•The changes of of Class G cement has been predicted in the underground CO2 storage system.