Ensuring cement mechanical integrity of geothermal wells is of paramount importance for the success of a project. Indeed, a failed cement sheath can lead to loss of production from inner-zonal fluid flow or well shut-in due to environmental or safety issues. In many cases, it is difficult to repair a failed cement sheath, and this may require an expensive work-over program, and subsequent re-completion of the well. In extreme cases, a failed cement sheath can result in loss of reservoirs due to early well abandonment. Over time, cement mechanical integrity (CMI) software applications have been proposed for the industry to design wells that do not leak during their entire life. However, the efficiency of such tools to solve CMI problems depends not only on if their equations are verified, but also on if the models are validated versus the downhole conditions encountered by the wells to be simulated. In previous papers, the authors have shown the importance of simulating the cement hydration processes in order to evaluate the state of stress in the cement after the cement has set. They also have highlighted how cement plastic behavior can lead to opening micro-annuli at the boundaries of a cement sheath. Recently they have started research work in the area of cement integrity of ultra-high temperature geothermal wells (up to 550°C) and on how the effects of high well temperatures, either naturally occurring or induced, would affect the cement’s mechanical integrity. This work focused on modeling the increase in pore pressures, the opening of micro-annuli at the cement sheath’s boundaries, and the changes of phases which take place in the cement when it is heated to very high values. This paper will present all three topics to highlight that, when all these features are taken into account, it is feasible to design cement sheaths that do not fail, even at very high temperatures.