Depleted shale reservoirs are potentially attractive targets to store carbon dioxide (CO2) as free and adsorbed phases. The time-dependent efficiency factors that depict dynamic storage performance in shale are estimated as a function of key reservoir parameters, shapes of stimulated reservoir volume, and injection scenarios. Efficiency of CO2 storage and flow regimes are determined dynamically as CO2 is injected in a depleted shale formation for a time period of 60 years. The most effective reservoir utilization for carbon storage was achieved during the transition from predominant flow in the stimulated reservoir volume to flow into unfractured zones. That transition occurs within first 15-30 years of injection depending on cases considered. The importance of adsorption as a mechanism of storage is determined based on sensitivity, which indicates that the amount of CO2 stored by adsorption would be on average similar to 26% of the CO2 stored as a free phase.