Abstarct: With increasing well developments in gas reservoirs the need for a proper completion in these zones of interest is felt intensely. Wellbore cementing is an essential part of every completion plan. The primary target of a cement job is zonal isolation and avoiding formation fluids to invade the well. Failure of cements in this important task results in disasters, significant financial damages and many more consequences. Good Cement slurry design is the key element of having a gas resistant cement column. In this thesis a gas invasion optimization plan is introduced. baxsed on this plan effective parameters are recognized and cement slurry is optimized according to these factors. Performance of several additives are also examined and assessed. At the end of the slurry optimization plan the results are verified using FMA test. In this test cementing job condition is simulated. In this thesis performances of the conventional slurries facing gas invasion were reviewed and compared with modified slurry containing special gas migration additive, using fluid migration analyzer device. Results of this study reveals importance of proper additive usage in slurry formulations. Rate of gas flow through slurry in neat cement is very high; by using different types of additives we observe obvious changes in performance of the cement system.