Storage of carbon dioxide (CO2) in hydrocarbon reservoirs and saline aquifers is considered as one of the promising mitigation strategies to reduce the negative impact of this greenhouse gas. The static and dynamic behaviour of CO2 in these storage sites which are located at various depths and geographical locations, affects the efficiency of this strategy. Understanding the impact of the conditions of these storage sites on mechanisms involved in CO2 flow, displacement and trapping is also critical for the purpose of site selection and the design of CO2 storage projects. In this paper we report the results of a series of CO2 injection (CO2I) flow visualisation (micromodel) experiments conducted using high-pressure transparent porous media representing various aquifer and depleted oil reservoirs storage conditions. The impact of pertinent parameters on the interaction between the stored CO2 and the reservoir fluids were investigated. Both sub-critical and supercritical CO2 were used to investigate the effect of pressure (depth) of the storage site on CO2 trapping mechanisms. A faster CO2 breakthrough (BT) was observed in the micromodel test simulating CO2I into depleted oil reservoirs, compared to that into aquifers, reducing the sequestration capacity of the depleted oil reservoirs. Compared to the injection of supercritical CO2, the BT of gaseous CO2 happened faster, adversely affecting the CO2 displacement performance. The results of these direct flow visualization experiments significantly improve our understanding of the complex mechanisms and interactions involved in CO2I and storage in geological formations. This knowledge is essential for identifying storage conditions that would lead to maximising CO2 storage capacity, for better understanding the ultimate fate of the stored CO2 and the storage safety. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.