| Title | Steps Towards a Comprehensive Thermo-Hydraulic Analysis of the HDR Test Site Soultz-Sous-Forets |
|---|---|
| Authors | Thomas Kohl, D. Bachler and L. Rybach |
| Year | 2000 |
| Conference | World Geothermal Congress |
| Keywords | heat flow, non-Darcy, HDR, numerical modelling, fractured rock, Soultz-sous-ForÍts |
| Abstract | During the exploration of the HDR test site Soultz-sous- ForÍts (France) numerous unexpected thermal and hydraulic features have been encountered. The HDR reservoir at the Soultz-sous-ForÍts site is targeted in a granitic horst zone bounded by subvertical N-S trending normal faults. The high surface heat flow at HDR test site, Soultz-sous-ForÍts, being a main reason for its selection dropped from nearly 150 mW m-2 at surface to <25 mW m-2 in 2-3 km depth and recovers at greater depth to ~70 mW m-2. In the depth range between 2-3.8 km (i.e. the range with low heat flow) numerous hydraulic experiments have been performed which highlighted the importance of far field drainage systems. The analysis of these features, jointly investigated by numerical models, elucidate that the local temperature field can be explained by a convective flow pattern which develops in the domain with the highest degree of fracturation. The convection cell can be localised between the top of the Buntsandstein at ~1000 m and a depth of ~3700 m in the Granitic basement. Another clue to the dominant role of the ambient fracture system is provided by investigations of numerous multiple level flow rate experiments performed in the GPK1 and GPK2 boreholes. These short time (t<20 days) tests have highlighted the importance of turbulent-like hydraulic behaviour and could be accurately fitted by transient models of simple geometry. Herein, particular attention to the influence of geometry and of far field fault systems was paid. Another effect of the dominant role of natural faults and fractures in the realm of the boreholes are thermo-elastic stresses developing during the 4 month circulation test from 1997 which cause dilation of the fracture voids. These results allow to derive a conceptual model of the upper reservoir (3000-3800 m) at Soultz with a dominance of the natural fracturation and fault system. It is especially demonstrated that numerical simulation of hydraulic and thermal data can yield a recognition of flow processes and of flow geometries and provides the necessary basis for more elaborated 3-D models which then will allow to predict future HDR performance. |