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21st European Symposium on Improved Oil Recovery, IOR 2021 -- 19 April 2021 through 22 April 2021 -- -- 169492, Increasing global trend of unconventional production impelled the oil and gas companies to adapt the EOR mothods to unconventionals to increase the recovery. In this study, a deep review of cyclic gas (Natural Gas, CO2, N2) injection processes and the methodology to apply in unconventional shale reservoirs is described by investigating all the geological, petrophysical, production and reservoir engineering aspects. The significance of each uncertainty and control variable throughout the process is outlined. A full-physics commercial simulator is used to identify the significance of control variables, and also the level of uncertainty which are directly affecting the production and recovery functions. The challenges encountered during implementation of cyclic gas injection processes are outlined in order to provide a comprehensiveand practical implementation perspective rather than only theoretical or a simulation work. Besides, the theory, advantages, drawbacks, benefits are given in details. Results of real cases are being compared and matched with the simulation results. The study indicates the incremental advantages of appliying huff`n puff process in unconventional shale reservoirs by comparing the primary production performance with the performances of cyclic injection cases that uses various gases. Key factors that directly affect the reservoir, completion and operational attitudes are circumstantially given to technically handle a successful project in a feasible way. In the model, the objective function is built by considering the economic parameters to help NPV maximization in a realistic perspective. Scurves and tornado charts is used to visualize the results and illustrate the significance of parameters and ranges of the developed probabilistic economic model. The effect of huff`n puff method and how a successful cyclic injection gives better recovery and feasibility results is clearly shown. Petroleum literature includes several studies related to cyclic gas injection. However, these works are either only focus on the simulation/theoretical parts or only includes a case study that focuses reservoir/production analysis. This comprehensive study closes the gap aims to be a reference by including a deep look into details of the theory and combines it with real field cases and solutions by clearly describing the candidate selection, modeling, physical parameters/key factors, economics, and success stories. © IOR 2021 - 21st European Symposium on Improved Oil Recovery.