The focus of this research is to characterize the risk of carbon sequestration under downhole conditions with regards to wellbore cement integrity. The primary research objective was to understand the geochemical interactions of simulated sequestration brines and wellbore cement. There are four key focuses of this study with regards to cement interaction: 1) Simulated brines saturated with Supercritical CO2 and CO2-O2; 2) Adjacent formation host rock under sequestration conditions; 3) Geochemical modeling using CHIM-XPT based on the experimental studies of points 1) and 2) above; 4) Geomechanical strength tests of CO2 exposed cements. First, the interaction of cement with supercritical CO2, CO2-saturated brines, and CO2-O2 mixtures at 50 °C, 85 °C, and 28.9 MPa was evaluated. Cement exposed to a lower salinity brine (1 M) had a larger alteration depth (1.25 mm) compared to cement exposed to a higher salinity brine (2.2 M) (0.836 mm). CO2-O2 exposed cements displayed alteration depths of 1.6 mm at 85 °C and 0.7 mm at 50 °C for 1 M. The cement exposed to the 2.17 M solution had an alteration extending 1.685 mm. The second focus studied the influence of adjacent formations and observed mineral trapping in the Grande Ronde Basalt-cement. Studies of Mt. Simon Sandstone-cement interaction provided evidence for high CO2 storage capacity. The third focus is geochemical modeling using CHIM-XPT. Pure CO2 saturated brines yielded a pH of 3.0 at 1 M and 3.85 at 2.17 M. The CO2-O2 gas run calculated a pH of 3.96 at 1 M and 3.85 at 2.17 M. The Pasco solution after exposure had a pH of 6.97 and an estimated dissolved 161 g of CO2 (g). The pH of the Illinois Basin brine and Mt. Simon sandstone [and cement paste] was 5.8 with an estimated 139 g of CO2 consumed. Lastly, geomechanical tests on CO2 exposed cements indicated that addition of oxygen or changes in temperature play a key role in compressive and tensile strength, decreasing strength by 20-35%. This research highlighted the importance of representing the in-situ wellbore environment when conducting laboratory experiments. This dissertation includes previously published and unpublished co-authored material.