With the high demand for oil and gas operators are becoming increasingly interested in unconventional sources of hydro-carbon. One of the major sources in the world is heavy oil, which has-been defined by the API as oil that has API gravity of less than 22.3 degrees API at 15.6 degrees C (60 degrees F) (extra heavy oil or bitumen, has 10 degrees API or less). Because heavy oil does not flow on its own I, the wellbore, a mechanism must be used to recover the oil by making it mobile. Among the different experimental and, operational methods, steam-assisted gravity; drainage (SAGD) has been the method of choice for the past few decades. This process uses steam to heat the immobile oil, and reduce its viscosity so it can be extracted, usually, by using pump jacks These:heavy oil wellbores need to be zonally isolated to:help ensure that the re serves are produced properly without environmental or,production optimization challenges. In Alberta and Saskatchewan,: many wells-are drilled and,cemented in designated heavy oil fields where SAGD is applied to stimulate production. Most of these wells are: successfully cemented, however, some wells experience zonal isolation failures that result in steam breakout of steam loss into non-targeted. zones. While the problem may be related to the primary cement job; it may also have been created by post-cementing operations and/or conditions. Sometimes the problem may be temporarily solved by conventional methods, however, the ultimate. remedy may have to be applied eventually. The Tong-term integrity of the cement sheath behind the casing should be planned with drilling completion, production/injection and abandonment stages considered. A properly designed and engineered cement slurry system can help save a wellbore/field from,additional unplanned.] expenses., The process is shown below Cement slurries are-designed under defined wellbore conditions and exposed to field conditions in the laboratory The mechanical behaviour of the cement sheath under downhole conditions is studied using; three dimensional; fit finite element analysis-software. The endurance of the cement sheath under applied conditions is predicted and optimized. Operation and placement of the selected slurry is studied to; help ensure optimum-execution. Stresses exerted on the cement sheath from wellbore operations during construction, injection or production that could damage the cement sheath are integral to the study. These conditions are analyzed to help engineers understand the post-cementing operational effects on the sealant system. This paper presents the design criteria for finding the fit-for purpose seal ant l systems that can successfully protect wellbores under harsh environmental conditions of SAGD,and cyclic steam stimulation (CSS) operations.