FRACTAL ANALYSIS OF FRACTURES APPLIED TO SOULTZ-SOUS-FORETS HOT DRY ROCK GEOTHERMAL PROGRAM

A fractal analysis has been performed on a fracture set observed on gr anite cores in order to determine preferential paths for fluids inject ed during Hot Dry Rock (HDR) geothermic experiments. This analysis was based on the sorting of fractures on criteria such as their direction according to the present stress field, their dip, their hydrothermal filling thickness (vein width) which is linked to the intensity of alt eration and thus to the physico-chemical properties of the surrounding rock. The results show a global increase of the fractal dimension (D) with depth which may be related to the lithostatic pressure gradient. Analysis of the total fracture set has showed zones in which fracture s are clustered (low D values) while others are characterized by regul arly spaced fractures (higher D values). Fractures parallel to the max imum horizontal stress, narrow veins, and fractures with a dip greater than 40-degrees represent 35% to 79% of the total fracture set. Their fractal dimensions are similar to those of the total fracture set. La rge veins linked with high porosity zones show a cluster pattern all a long the drilling. In the context of HDR geothermics, one should seek highly fractured zones especially if the fractures they contain are cl ustered and if they have developed wide alteration zones linked to a h igh porosity. These conditions may optimize the HDR heat exchanger pos sibilities.