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
modelling 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 ten thousand of microseismic events. The
transient numerical simulations with the HEX-S code match the main characteristics of
both, the microseismic and the hydraulic behaviour. Different model calculations
demonstrate the capabilities of our new approach. It is noteworthy that the modelling
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 behaviour 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.
Long term operation data are however rare at the current stage of EGS development.
The access of long term effects can however be estimated through modeling approaches.
Therefore, the same finte element kernel is used for a combined
hydro-thermo-mechanical simulation of long operation periods. Different examples will
be shown from 2D and 3D models. The models highlight especially the thermo-elastic
impact that can cause tensile fracturing perpendicular to the fracture surface. At
long term, the reservoir will thus behave fully dynamically and the risk of short
circuiting will be reduced.

Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy