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Defining, exploring, imaging and assessing reservoirs for potential heat exchange - Potsdam, Germany, Workshop1
Defining, exploring, imaging and assessing reservoirs for potential heat exchange - Potsdam, Germany, Workshop1
6-8 November 2006 GeoForschungsZentrum Potsdam (GFZ)
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Borehole imagery contribution to EGS reservoir exploration
For Enhanced Geothermal Systems (EGS) controlled by pre-existing fracture networks, 
it is necessary to explore the reservoir by borehole imagery techniques during the 
phase of drilling reconnaissance. As pre-existing fractures represent the permeable 
pathways, such techniques permit to get at the end of the drilling a consistent 
fracture characterisation in terms of location, geometry, thickness, aperture and 
alteration. The main borehole imagery tools, which are based on acoustic or 
electrical methods, provides an oriented mapping of the borehole wall. In this 
paper, the contribution of borehole imagery to EGS reservoir is based on the 
exhaustive database of the deep Soultz-sous-Forêts wells which were systematically 
investigated by means of images logs.
On these high resolution images, different structures, detected due to their own 
geophysical contrast, could be interpreted such as pre-existing fractures, fracture 
zones, alteration zones or locally lithological variations. Structural analysis of 
the borehole image logs permits to reconstruct the natural fracture network in 3D at 
different scale (small-scale fractures, fracture zones) Induced fractures could also 
be interpreted and measured on the borehole image logs. Their relationship with the 
present-day stress field could be derived and computed. Comparison between the main 
fracture sets, or the main fracture zones, and the present-day stress field brings 
lot of information for understanding fluid circulation in the EGS fractured 
However, despite of the high resolution of the tools, all fractures present in the 
rock mass are not systematically visible on the image logs and about 20% of 
fractures are only detected properly in crystalline rocks. Moreover, lot of 
fractures are not clearly visible on the images or are not ideal 3D planes. Thus, 
the measurements of their orientation could be difficult to obtain and extrapolate 
in 3D and could bring some biases which have to taken into account. Similarly, there 
is another limit by using borehole image logs to characterize the fracture network 
which is related to the absence of vision beyond the drill wall of the fracture 
In conclusion, the borehole imageries constitute reliable tools to detect fractures, 
which are natural or induced, fracture zones and altered zones. These methods permit 
to obtain high quality fracture datasets of a geothermal EGS reservoir and 
contribute to a better understanding of the fluid circulation through a deep seated 
fractured rocks.
Id: 17
Place: GeoForschungsZentrum Potsdam (GFZ)
14473 Potsdam, Telegrafenberg
Starting date:
07-Nov-2006   10:25
Duration: 03'
Contribution type: Poster
Primary Authors: Dr. DEZAYES, Chrystel (BRGM)
Co-Authors: Dr. GENTER, Albert (BRGM)
Mr. VALLEY, Benoit (ETH Zürich)
Dr. SAUSSE, Judith (University of Nancy)
Presenters: Dr. DEZAYES, Chrystel
Material: slides Slides
poster Poster

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