The development of carbon capture and storage (CCS) brings challenges for safety issues regarding carbon dioxide (CO2) transmission pipelines. Once a pipeline is punctured or full-bore ruptured, the leaked CO2 is hazardous to personnel and the environment. Small-scale devices were established with the aim of studying the release and dispersion behaviour of gas and liquid CO2 from a punctured underground pipeline. A sandbox was built to simulate the underground conditions. The parameters of the sand used in the experiments were tested. CO2 concentrations on the ground and temperatures around the release orifice in the sand were analysed. The results indicate that in the CO2 gas release experiments, the CO2 concentration on the sand surface decreases with increasing horizontal distance in the form of a power function. CO2 concentrations in upward release are slightly larger than those in horizontal release at the same location but are obviously bigger than values in downward release. The temperature-drop region is much smaller than that in air. A frozen ice ball can be generated near the release orifice during the gas phase of the CO2-release process. In the liquid phase of CO2-release experiments, a large amount of dry ice is generated near the release orifice. Dry ice can only be generated in the area close to the release orifice, especially in the near-field area. (C) 2016 Elsevier Ltd. All rights reserved.