Predictive modeling of reservoir response to hydraulic stimulations at the European EGS site Soultz-sous-Forêts

With the beginning of the new century, the European EGS Project got into its decisive state by reaching the final reservoir depth of 5 km. The three boreholes, GPK2, GPK3 and GPK4 have been successfully targeted at their predefined reservoir positions. Improvement of the reservoir conditions by stimulation with a minimized seismic risk represents now a primary challenge to enable economic operation and future extension. In this context, the new HEX-S code has been developed to simulate the transient hydro-mechanical response of the rock matrix to massive hydraulic injections. The present paper describes the successful forecast of the pressure response and shearing locations for the GPK4 stimulation in September 2004. As basis for this predictive modeling, the reservoir model was derived from data analysis of the stimulation of GPK3 in May 2003. Stimulation flow rates up to 45 l/s at GPK4 and of >60 l/s at GPK3 have been applied, triggering several 10,000 of microseismic events. The transient numerical simulations with the HEX-S code match the main characteristics of both, the microseismic and the hydraulic behavior. Different model calculations demonstrate the capabilities of our new approach. It is noteworthy that the modeling became possible only due to the excellent data quality at the Soultz project. The results demonstrate that simulations based on solid physical ground can reveal the complex reservoir behavior during hydraulic stimulation. The use of HEX-S also provides perspectives for future developments such as design calculations that enable optimizing cost-intensive hydraulic stimulations before hand.