During the last five years, the European Hot Dry Rock Project at Soultz-sous-Forets (France) has known major advances. Three deep wells were drilled into the crystalline basement at 5000 m depth. Stimulation of the wells by high rate fluid injections gave many information thanks to the microseismic events recorded on site. In order to better understand the development of the reservoir, a numerical model based on a Discrete Fracture Network approach was written to interpret and predict the pressure distribution in the reservoir and the hydraulic behaviour of such a system. The hydraulic part of this finite volume code is strongly coupled with the mechanical behaviour of the fractures and with the temperature of the fluid, what implies important density changes of the fluid.

The last years experiments showed that the density difference between autochthonous fluid (density 1.06) and injected fluid (heavy brine or fresh water) might play a significant role during the stimulation phases of the wells. It was then decided to enhance the numerical code, under the assumption of immiscibility of the two fluids. A new variable was introduced in the code, saturation; flow equations are now solved with an IMPES scheme.

These recent improvements allow us to evaluate the impact of fluid density on the pressure distribution in the reservoir during stimulation or circulation tests. Therefore we provide better estimates of both extension and shape of the stimulated areas and we are able to define new stimulation strategies. The new version is also proven fruitful in predicting the evolution of flow distributions in fractures intersecting the bore holes as well as in giving breaktrough estimates for tracers (chlorure contend) migration in between injection and production bore holes.

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