At the Soultz-sous-Forêts EGS (Enhanced Geothermal Systems) site and in general in other crystalline reservoirs, the fluid flow paths between the injection and the production boreholes are mainly controlled by a set of permeable and non-permeable structures occurring from the hectometric to micrometric scales. The objective of our work is thus to define the architecture of these paths within the heat exchanger, which was currently poorly constrained and difficult due to the great depth of the hot granite.

Thus we will expose an up to date global synthesis of our ongoing works, already detailed in some other past ENGINE workshops:

Direct investigation:
The direct investigation of the reservoir consists in some in situ geophysical data acquisitions and the following interpretations. Both logging and seismic methods are used for the multi-scale delineation of the structures intersected by the well, or the faults affecting the sedimentary cover of the reservoir basement. A special attention is paid to VSP (Vertical Seismic Profiling) data after an adapted isotropic processing which reveals the location of some permeable faults affecting the reservoir, even several hundreds of meter away from the acquisition well. Other structures are identified as well, like diffracting faults intersections or strongly attenuating zones.  In addition, at well location these VSP results are in full agreement with the local results of structural logging data (UBI, ARI, FMI...).

Field analogue analysis:
As the Soultz-sous-Forêts granitic basement is covered by thick sediments (1500 m thick), an indirect characterization is led on an analogue outcropping batholith located north of Barcelona (Catalunya – Spain). This granitic massif indeed presents strong analogies with the geothermal reservoir in Soultz-sous-Forêts: these two hercynian granites exhibiting very close petrologic compositions have recorded similar tectonic histories (Pyrenean or Alpine compression, Oligocene rifting). These two granites currently present alike thermal anomaly, seismic and geothermal (hot springs) activities, and evidences of hydrothermal alteration. Based on field measurements and observations, the preliminary results of a 3D structural recognition and detailed fracturation analysis will be presented at different scales (from hectometer to centimeter). A special attention is paid to fault zones, studied by consideration of their inner architecture (fault core, damaged zone, protolith) and their petrophysical properties resulting in different behavior regarding fluid flows (porosity, dissolution, precipitation, permeability, thermal conductivity). In addition, these intragranitic faults will be investigated by seismic and radar methods in order to make a link between the geological structures and their geophysical responses, in order to help the interpretation of the Soultz-sous-Forêts VSPs.

Conclusion:
Thus, from these two approaches, a comparison is proposed so as to complete the Soultz-sous-Forêts EGS multi-scale characterization. As the structural data acquired by logging are collected only locally in the wells, the macro-structural pattern of the reservoir deduced by seismic methods can be completed by the analogue field 3D fracturation data. In addition, petrophysical properties of fault zones measured on Soultz-sous-Forêts cores and on the analogue samples are compared. An analogue allows to have a conceptual idea of a hidden reservoir with a better fidelity. 
In the future, thanks to these approaches, the drilling strategy of an EGS should absolutely take in account a preliminary targeting of permeable structures before and during the drilling phase, as not rigorously done in the past at Soultz-sous-Forêts EGS due to the lack of knowledge of the deepest granite rocks. Thus, by this way, a better global productivity could be reach by a lower number of producing deviated boreholes (improvement of the ratio (fluid flows)/(drilled length)). In addition, an accurate reservoir knowledge could help with benefit the management of the heat resource in the exploitation phase.

Vilnius, Lithuania