TO DETERMINE THE BEHAVIOUR OF NATURAL AND STIMULATED FRACTURES IN CRYSTALLINE ROCKS IN THE CONTEXT OF GEOTHERMAL HDR DEVELOPMENT.
The European Geothermal Project involved teams from France and Germany who collaborated to test a site in the Upper Rhine Valley for its suitability for terrestrial heat mining (hot dry rock (HDR) energy production). Some British scientists participated in specific tasks.
The site was chosen near Soulz-sous-Forets in Alsace at the location of the old oil field of Pechelbronn which was the first oil field field exploited in Europe since the 18th century.
It is situated on 1 of the summits of a very large thermic anomaly (200 km long and 20 km wide) where the mean geothermal gradient between the surface and 1500 m is known to be higher than 6.5 C/100 m. The programme began in July 1987 with a 2000 m deep borehole.
Below at 1375 m thick sediment cover, the granitic basement was penetrated to a depth of 2000 m. The temperature at the bottom of the hole was 140 C. The geothermal gradient within the sediments was unusually high (10 C per 100 m) and diminished to a normal after a series of fractures inside the Bundsandstein producing some water at 116 C with a total salinity 98 g/l. At the depth of 1820 m, hydraulically active natural fissure was reached. The artesian outflow from this zone was 0.15 l/s, with well head pressure of 1.6 bars. The thermal water produced from the well had a high chloride contents and clearly had an identical origin with the fluid collected from the Buntsandstein just above the granite. During the water injection tests, a second active natural fissure was detected normally closed out but which seemed to aquire a noticeable permeability at a well head pressure of about 40 bars.
THE WORK FORMS PART OF THE FRANCO-GERMAN COLLABORATIVE HDR PROJECT (SEE ALSO CONTRACT NOS.G0052D, G0055D, G0081D, G0082D AND G0072F). THE MAIN EXPERIMENTAL WORK WILL OCCUR AT SOULTZ, BUT OPPORTUNITIES MAY ALSO BE TAKEN TO WORK AT OTHER RELEVANT SITES IN FRANCE OR GERMANY.
THE WORK WILL INVOLVE HYDRAULIC TESTING OF OPEN-HOLE AND PACKED-OFF SECTIONS OF BOREHOLE TO DETERMINE:
- THE TRANSMISSIVITY AND WIDTH OF NATURAL AND STIMULATED FRACTURES;
- FRACTURE CHARACTERISTICS AS A FUNCTION OF FLUID PRESSURE AND IN-SITU STRESS;
- THE EXTENSION OF FRACTURES DURING STIMULATION;
- WATER LOSS FROM FRACTURES UNDER FLOWING CONDITIONS.