CO2 storage in depleted oil and gas reservoirs is considered to be one of the most practical options for reducing CO2 emissions in the atmosphere and has been practiced in different locations worldwide. It is commonly believed that the sealing capacity of the caprock, which had successfully sealed the original hydrocarbon in the reservoirs for a geological time, is sufficient to prevent the injected CO2 from escaping into the upper formations. However, the sealing capacity of the caprock is significantly reduced due to the much lower interfacial tension of the CO2/water system than that of the hydrocarbon/water system when the hydrocarbons in a reservoir are replaced with the injected CO2 As a result, the injected CO2 may migrate through the caprock into the upper formations by volume flow. This paper examines the occurrence of the volume flow and measures the gas effective permeability for selected Weyburn Midale caprock samples after the caprock was broken through by the injection of CO2 Calculations based on the measured gas effective permeabilities show that CO2 leakage by volume flow is disastrous once it occurs. To avoid the volume flow, the caprock sealing pressure should be determined before the outset of a CO2 storage project, and it should not be exceeded during the CO2 injection process. The calculated storage capacity for a given void reservoir volume shows that once the reservoir pressure reaches a certain high level, a continuous increase in pressure is not effective in enhancing the storage capacity. However, much more capacity can be achieved by removing a portion of the remaining water in the reservoir. (c) 2005 Elsevier Ltd. All rights reserved.