Numerical study of the impact of fluid density on the pressure distribution and stimulated volume in the Soultz HDR reservoir

During the last 5 years the European hot dry rock project (HDR) at Soultz-sous-Forêts (France) has made significant progress. Three approx. 5000-m deep wells have been drilled into the crystalline basement and hydraulically stimulated. These operations have provided a great deal of information on the subsurface, especially from the interpretation of the monitored induced microseismic events. To better understand the development and hydraulic behaviour of the fractured reservoir, a numerical model based on a discrete fracture network approach has been written to interpret and predict the pressure distribution in the HDR reservoir. The hydraulic part of this finite volume code is strongly coupled with the mechanical behaviour of the fractures.Recent modelling studies have demonstrated that the density difference between the in situ (native) reservoir fluid (density 1060 kg/m3) and the injected fluid (dense brine or fresh water) might play a significant role in the hydraulic stimulation of the reservoir. The decision was therefore taken to enhance the numerical code, under the assumption of immiscibility of the two fluids.The improved code permits us to evaluate the impact of fluid density differences on the pressure distribution in the reservoir during stimulation or fluid circulation tests. It provides better estimates on both the extension and shape of the stimulated areas, and can be used to define new stimulation strategies. The code has also been successful in predicting changes in fluid flow distributions in the reservoir, especially in fractures intersecting the wells, as well as in estimating trace breakthrough times.