| Title | Heat-and Fluid-Flow at the Soultz Hot Dry Rock System in the Rhine Graben |
|---|---|
| Authors | Daniel Pribnow & Christoph Clauser |
| Year | 1999 |
| Conference | European Geothermal Conference |
| Keywords | Hot Dry Rock, Rhine Graben, heatflow, fluid flow |
| Abstract | Hot Dry Rock (HDR) is a concept for using Earth's heat as an energy source. An artificial underground heat exchanger at a depth of 3.0 to 3.5 km was created in the granitic basement of the Rhine Graben, close to the town of Soultz in France, by hydraulically connecting two deep boreholes over a horizontal distance of 500 m with the hydrofiac technique. During a 4-month circulation test, over 240,000 m3 of water were injected and produced at flow rates of 20 to 25 l/s and with outflow temperatures above 140 "C. The net output of thermal power exceeded 10 MW. The experiments indicate that the circulation system in the underground is hydraulically open. Numerical models of coupled heat and fluid flow help to understand the observations. At the regional scale, previous models suggested deep fluid circulation from East to West through a sandstone aquifer across the Rhine Graben causing a heat flow anomaly at Soultz. This is in contradiction to recent geochemical analyses of pore fluids. Models of the regional flow-system that agree with the pore-fluid chemistry now predict deep flow through the granitic basement. These models show that in the area of the HDR heat exchanger fluids generally move upwards. The results of the regional simulations are incorporated as boundary conditions in a local 3- D model of the underground heat exchanger at Soultz. This model considers two flow regimes: (1) flow in the artificially fractured (stimulated) volume of the granite and (2) flow channelled along natural faults. The stimulated volume is a heat exchanger in the classical HDR concept: it is the hydraulic connection between injection and production hole and it provides the surface for heat exchange between the fluid and the fractured rock. Additionally, the hydraulic fracturing of the granite created a connection with the regional fault system of the graben. Although not part of the HDR concept, this extension of the involved hydraulic system improves the long-term heat extraction process because the regional fault system acts as a large heat and fluid reservoir. The numerical models are used to predict the performance of a pilot plant that is planned to utilise an underground heat exchanger at 200 "C over a period of more than 30 years. |