The goal of the workshop was to discuss all parameters that
should be known before
drilling for exploitation of potential geothermal
reservoirs. The workshop was
strongly focused on debates about the definition of targets,
characterization of
reservoirs and optimisation of investigation methodology for
EGS. To achieve this
focus, four thematic sessions were defined (Signatures of
temperature field for
defining and exploring potential geothermal reservoirs,
Signatures of fluid transport
in Earth's crust, State of the Art in the exploration of
potential geothermal
reservoirs, Processes in geothermal reservoirs).
Definition of targets, characterization of reservoirs and
optimisation of
investigation methodology for EGS require, among other
topics, a significant
improvement of the imaging and modelling of fault and
fracture systems, of the
knowledge of the paleostress field as well as of the heat
flow and temperature
distribution at depth. Following the workshop, it is stated
that heat, stress and
pathways as well as the structural inventory of the
subsurface are the key elements
that could be considered as priority for research needs.
Four main research targets were defined, and the main
conclusions can be summarized
as follows.
Structural Geology: imaging potential geothermal reservoirs
Geothermal reservoirs are sections at drillable depths
containing enough heat for
geothermal utilisation. Geophysical methods are suitable to
determine the
architecture, geometry, and quality of target intervals.
However improvement of
existing methods and in particular reasonable combination of
different, most
sensitive techniques (passive and active seismic, MT, and
others) are needed to meet
the requirements of modern geophysical exploration. The
interpretation of geophysical
features must be supported and validated by petrophysical
laboratory and borehole
measurements, as well as modelling. Experiences made in
hydrocarbon exploration must
be modified for EGS. EGS requires usually more knowledge
about fracture and fault
systems with respect to their role as potential water
conduits. The reservoir imaging
strategy should include large scale approaches supplemented
by high-resolution
experiments. Further benefit should come from adapted
processing techniques.
Heat: finding heat at depth
For large-wavelength anomalies of the surface heat flow, the
accuracy of their
extension at depth often is very limited due to improper
knowledge of the causes of
the heat-flow anomaly. For example, the existence of
convective and advective cells,
well characterized at Soultz and in the Rhinegraben area,
preclude a temperature
extrapolation to greater depth and can lead to wrong
evaluation of thermal gradients.
Maps of the surface heat flow and of the depth and heat flow
at the crust-mantle
boundary provide farfield conditions for the definition of
possible targets for EGS.
However, such maps require a basic knowledge of the main
lithologies and their
thermal properties. To properly define temperatures, an EGS
database of the thermal
conductivity and the radiogenic heat production are now
feasible for better
constrained modelling. Several physical parameters (density,
wave velocity…) are
coupled with temperature and can be imaged by different
geophysical methods. Thus,
the definition of possible targets for EGS could be improved
by the use of a 3D
modelling platform, in which all solutions from geological
and geophysical modelling,
direct and inverse, could be combined and analysed.
Stress: understanding and stimulating fluid circulation
There is abundant evidence for the influence of the stress
field on hydro fracturing.
The knowledge of spatial stress distribution (map and depth)
on a local as well as on
a regional scale is thus fundamental for any future
experiment. Mechanisms of rupture
and propagation of an existing fault system and related
displacement remain debated,
especially in connection with the circulation of fluids and
success rate of improving
sustainable permeability. The circulation and accumulation
of fluids in the crust are
fundamentally controlled by the geometry of the fault and
fracture systems.
The ability of these systems for the channeling of fluids is
directly dependant on
the stress field (orientation and intensity). Favourable and
unfavourable conditions
exist depending of the
tectonic context and geological environment.
Pathways: defining integrated conceptual models
What starting conditions are needed to develop/stimulate an
EGS? What are the
conditions classifying a thermally well-suited area for the
development of an EGS?
There is a need to refer to conceptual models of the main
geothermal sites, from
extended active geothermal sites to EGS for which heat
distribution and permeability
networks are available for modelling pathways for fluid
circulation and heat
exchange. The basis of such models should be the geometry of
the regional geology.
They could be built by integrating the most significant
datasets and their
interpretation on reference key areas, such as Larderello,
Bouillante, Soultz, Groß
Schönebeck. Such models must be updated as soon as new data
or new experiences and
results are available. A significant improvement of the
knowledge is expected from
natural analogues on which hypotheses could be tested about,
for example, circulation
of fluids in relation with seismicity and heterogeneity of
the lithologies, thermal
imprint of fluid circulation. The links with other
investigation programmes such as
nuclear waste storage, capture and storage of CO2 and oil
and gas field development
will be developed to take advantage of existing
installations and experiences.
Workflows encompassing fault interpretation from 3D seismics
and geostatistic tools,
3D retro-deformation and fracture interpretation from well
data should be further
developed to give a base for possible pathway interpretation
through time. To decide
about open or closed fractures, palaeostress maps are
needed. Technological platforms
could be promoted to develop new methods and tools, test
hypotheses in situ or the
accuracy of conceptual models.
Investigation for EGS is of strategic importance for
reduction of costs and increase
of efficiency in the development of geothermal projects, and
Workshop 1 of the ENGINE
project has certainly made an important contribution.
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