Environmental Engineering Reference
In-Depth Information
480
onstrated that the hydraulic conductivity of self-propped fractures in crystalline
rock is considerably higher than the hydraulic conductivity of propped fractures
created with the conventional technique in sedimentary rock. The success in these
research projects give good hope that commercial HDR-systems may be installed
in the near future with technical success. The most advanced project in this re-
spect is the European HDR-project Soultz where a HDR-system of commercial
size is in preparation at a depth of 5,000 m and rock temperatures of 200 °C.
Reservoir evaluation.
Various parameters have to be determined to describe the
characteristics of the geothermal reservoir. The most important are depth, thick-
ness, temperature, formation fluid pressure, porosity, and permeability. In igneous
rock types fracture distribution, orientation and characteristics are of similar im-
portance. Many of these parameters can be determined directly or indirectly by
geophysical borehole measurements. The given possibilities are discussed below.
For Hot-Dry-Rock (HDR) systems it also necessary to determine the spatial ex-
tension the fracture system created in the first well in order to be able to link the
second well into the periphery of this system. For this purpose, the method of
seismic fracture mapping has been developed which is described below.
Borehole measurements.
Geophysical borehole measurement (borehole logging)
is a mature technique in the oil and gas industry, and many of the methods devel-
oped there can be applied for geothermal wells. The severe environmental condi-
tions, especially the higher temperatures and more aggressive fluids, however,
require some adjustments of the logging equipment for geothermal wells that is
not always possible, and limits the number of available tools. High temperature
versions of geophysical logging probes use either high temperature electronic
parts, or the probes are shielded by Dewar housings, sometimes in combination
with heat sinks in the inner of the probe. For temperatures up to 150 °C almost the
whole suite of logging equipment from the oil and gas industry is available. About
180 °C is the limit for most of the high temperature electronic parts and the num-
ber of available tools for this temperature is already limited. Temperatures above
180 °C require specialised high temperature tools containing either selected high
temperature electronic parts or using heat shields. Some tools, such as temperature
or pressure probes, can operate without electronic parts and can therefore more
easily be adjusted to higher temperatures.
Of special importance for geothermal wells are production wells. These meas-
ure temperature, pressure, and flow velocity during production or injection and
are used for determining static and dynamic fluid pressure and for localising and
quantifying the major inlets or outlets in the borehole.
For geothermal wells in igneous rocks, especially for Hot-Dry-Rock (HDR)
systems image tools are very helpful. Today, two types of image probes are avail-
able.
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