Carbonate rock is a potential host for carbon dioxide storage. It is likely to react with carbonated water, following CO2 dissolution and dissociation in formation fluids. This might lead to carbonate dissolution or precipitation, depending on pH and formation fluid composition. In case of dissolution, the formation gets mechanically weaker, which could pose a risk for the mechanical stability of the formation, requiring a deeper understanding. In this paper, we used a direct CT scanning – DEM (Discrete Element Method) combined method to investigate how limestone rock mechanical properties change during CO2 injection. The results show that the minor dissolution happened after scCO2 injection, but such damage was much weaker when compared with the sample after live brine flooding. Related morphology change directly affects the rock mechanical response where the maximum compressive stress dropped from 17.2 MPa (for dead brine saturated), to 14.8 MPa (for scCO2 injection), and to 7.9 MPa (under live brine injection). We thus conclude that CO2 injection into pure carbonate reservoirs can impact the mechanical strength of matrix rock, and that the applied DEM models work well since they predicted a reliable stress-strain curve.