One of the major economic risks in the exploitation of geothermal reservoirs is the
cost of exploration and drilling combined with the uncertainty of success.
Exploration methods employed have largely been developed for the hydrocarbon
industry, where drilling costs do not constitute such a high proportion of the budget
compared to that of geothermal reservoirs. In addition, geothermal reservoirs differ
from those of oil and gas in several ways, such that exploration methods cannot be
transferred one to one.
The main need for a reduction of the mining risk is therefore a geophysical method
that is cost-effective and tailored to the specific needs of geothermal exploration
and the geological situation of a reservoir.
The EU project IGET is funded to investigate and develop such a method by integrating
several geophysical methods that are commonly used independently. In the field,
seismic 2-D lines with 3-component geophones record seismic waves in 3 orientations,
such that anisotropic structures can be deduced. The same profile is used for
magnetotelluric recordings, complementing the structural information with that of
water content. The combined interpretation and the joint inversion of these
measurements has the potential to provide more information than the sum of the two
used independently.
The anisotropy of the structures is investigated by AVO, with the potential to detect
fluid-filled cracks.
Another major question for high-enthalpy geothermal reservoirs is the physical signal
of the liquid-steam transition, as the position of that transition is of great
economic interest. This question is first addressed by laboratory experiments at
in-situ conditions, using core samples from wells in the areas investigated by IGET.
These areas comprise one reservoir in a metamorphic geological environment,
(Larderello/Travale, Italy), where rock porosity is low and the reservoir
permeability is provided by cracks and fault zones, one in a volcanic environment
(Hengill, Iceland), and one in a deep sedimentary basin (Groß Schönebeck, Germany) as
well as one in shallower sedimentary rocks (Skierniewice, Poland), where initial rock
porosity is relatively high.
The information provided by field measurements, well-logging and laboratory
experiments will be used to develop numerical models of the reservoirs in 3D in order
to produce a static image of the reservoirs and calculate the fluid-dynamic behaviour
of the fracture systems.

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