We have developed a complete model designed to stimulate both the behaviour of the reservoir during an eventual eruption, and the development of the two-phase flow at each instant along the duct which links the reservoir to the ground. The model of the reservoir can contain fluids in different states, including (initially) gas, liquid or even hypercritical fluid. The simulation shows that the nature of the thermodynamic evolution of the feed fluid during the eruption depends upon the conditions of storage (e.g. temperature and pressure) in relation to the thermodynamic critical point. In the particular case of the reservoir which justified the study, a variation of only a few degrees would be sufficient to totally change the nature of the eruption. We have developed a "drift-flux model" for the duct, which takes account of the 2 phase head loss, and the effects of gravity. The feed fluid can be either saturated or very far from equilibrium (e.g. subcooled liquid, dry vapour or hypercritical fluid). This model alloxs us to determine the flow rate of the eruption and to consider the variation of the temperature, the pressure, the quality, and the velocity along the duct.