Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 478

− The acoustic borehole televiewer (BHTV) scans the borehole by means of an
ultrasonic beam. While the ultrasonic beam is well reflected by the smooth well
wall, it is more or less absorbed by fissures. Colour representations of the am-
plitude of the reflected signal generate optical illustrations of the wall surface
and allow detection and characterisation of even fine fractures, including their
orientation. The travel time of the signal is used to determine the radius of the
borehole and allows the construction of a three-dimensional model of the bore-
hole geometry that is of great interest in unstable well sections, where break-
outs of the borehole wall are to be observed.
− The formation micro scanner uses the signals from electrical electrodes arrays
placed on scratch patches along the borehole wall. Colour representations of
electrical resistance between each pair of electrodes generate an oriented image
of the borehole wall. Fluid filled fractures have a much higher electric conduc-
tivity than the rock and are easily detected. The resolution of the formation mi-
cro scanner is even higher than that of the acoustic borehole televiewer.
Seismic Fracture Mapping. For the completion of Hot-Dry-Rock (HDR) systems
it is very important to determine the shape, the extent and the orientation of the
fracture system created in the first borehole in order to design the trace of the
second borehole. Various methods have been tested for this purpose. The best
results have been achieved by using the seismicity induced during the fracturing
process. Ten thousands of small seismic events are induced during massive water-
frac-experiments in crystalline rock /10-23/. These signals can be recorded with
geophones at the surface or in shallow boreholes around the Hot-Dry-Rock site.
The detection times of the signals on the various stations allow localising the hy-
pocentres of these signals. A three-dimensional representation of these hypo-
centres gives a detailed image of the induced or activated fracture system.
Also seismic tomography seems promising, which investigates the rock section
between two boreholes by seismic waves generated by a probe lowered in one of
the borehole. This method permits to localise fracture systems acting as seismic
absorbers or reflectors.
In the vicinity of the borehole, fissure orientation can be determined by means
of a Variable Acoustic Logging Tool. This method is based on the reflection of
tube waves on fracture surfaces intersecting the borehole.
10.3.1.2 Aboveground system
The power plant technology applied for geothermal power generation is divided
into three groups that are discussed as follows:
− open systems which directly use the geothermal fluid as a working fluid within
the power plant,
− closed systems which transmit the heat of the geothermal fluid to another work-
ing fluid which is then used within the power plant and
− combined systems, a mixture of both open and closed systems.