Geological investigations are extensively used in all phases
of exploration of
EGS fields. At large scale, geological key elements for the
geothermal resource
evaluation include temperature data distribution, field
geology, geological
mapping, seismic profile interpretation, structural analysis
of large-scale faults
or volcano-tectonic structures.
During the drilling exploration phase, a lot of geological
data are acquired in
order to characterize the deep lithology, the fracture
system organisation, and
some information about the present-day stress field. The
knowledge of the
reservoir composition (lithology, mineralogy) is derived
from drilling data such as
core samples, cuttings, and various geophysical loggings.
The detailed chemical
and mineralogical composition of the hosted rocks and
hydrothermal alteration
is investigated from petrographical studies (thin sections,
X-ray, dating).
The knowledge of the fracture system during the phase of
drilling
reconnaissance is mainly based on core data and borehole
imagery logs.
Additional information about the fracture system could
derive from classical
geophysical logs. Moreover, the fluid flow characterisation
of the permeable
fractures is deduced from temperature and flow logs.
Compilation and interpretation of geological and geophysical
data obtained in
the different phases of the EGS exploration conduct to
conceptual models which
could be modelled in 3D for a better understanding of fluid
flow circulation and
heat exchange.
Based on the Soultz experience, the main needs identified
are related to the
lack of information about the fracture extension in 3D and
the difficulty for
characterizing crystalline altered rocks at great depth. In
order to fill these
gaps, fracture mapping on well-exposed analogues,
geophysical methods such
as vertical seismic profiles (VSP) or new high resolution
geophysical logging
techniques run in the boreholes could improve significantly
the image of EGS
reservoirs.
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