Hydro-fracturing stimulation is a classical method to achieve better flow rate in 
hard geothermal rock reservoirs.  The stimulation is performed by maintaining a 
high fluid pressure (several MPa) into pilot boreholes.  The pore pressure 
increases gradually from the injection source to the surrounding rocks, implying 
local changes in the stress field that induces shears and/or opening of sealed 
micro fractures or faults, and, when the differential pressure is greater than the 
rock cohesion, leads to normal faulting.  Each of these events induces small 
magnitude micro-seismic events which can be recorded from the surface by tri-axial 
geophones located deep in observation boreholes.  This induced micro-seismicity 
provides indirect information on the reservoir permeability.  A new methodology is 
proposed to characterize the geothermal reservoir potentialities by estimating its 
3D anisotropic heterogeneous diffusivity and its time evolution during the 
stimulation.  This approach has been applied to the Soultz-s-Forêts, France HDR 
geothermal reservoir for depths ranging from 3500 to 5000m.  It provides a detailed 
map of the permeability and of its variations before and after stimulation with a 
resolution of tenths of meters over an extension of several hundred meters around 
the injection source.  Then, by interpreting and processing the boreholes UBI 
images, the main active fractures zones were identified on the three available 
wells.  Orientation and opening of each fracture were evaluated using the GMI 
software, and integrated into a comprehensive 3D model using the gOcad geomodeler.  
The 3D fracture network seems to be coherent with a strike slip model related to 
the formation of the Soultz horst.  The relationship between the induced miro-
seismicity and the identified fracture zones are then discussed in terms of 
hydraulic potentialities.

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