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@PHDTHESIS{Siebert:716212,
author = {Siebert, Philipp},
othercontributors = {Ziegler, Martin and Clauser, Christoph},
title = {{L}aborversuche zur hydraulischen {R}isserzeugung in
dreiaxial belasteten {G}ranitquadern - {G}rundlagen,
{V}ersuchsentwicklung, -durchführung und {A}nalyse},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
reportid = {RWTH-2018-221026},
pages = {1 Online-Ressource (XXII, 171 Seiten) : Illustrationen},
year = {2017},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2018; Dissertation, RWTH Aachen University, 2017},
abstract = {In hydraulic fracturing, cracks are generated, opened and
propagated by high fluid pressure. Hydraulic fracturing as a
technical process is used, among other applications, to
install artificial geothermal systems - so-called engineered
geothermal systems (EGS). In EGS, a fluid is pumped through
the hydraulically generated cracks in the deep subsoil in
order to promote geothermal energy. Since EGS are basically
independent of water-bearing layers and temperature
anomalies, a significant contribution of geothermal energy
to Germanys energy supply is expected. Various international
pilot projects have confirmed the feasibility of engineered
geothermal systems. However, these projects were still
inefficient in terms of profitability. For a successful
realization of an EGS, it is necessary to improve, inter
alia, the understanding and the numerical simulation of
hydraulic fracturing in plutonic rock to make it more
predictable. Against this background, the process of
hydraulic fracturing in crystalline bed-rocks is
investigated experimentally and numerically in a new
research group at RWTH Aachen University under the
leadership of the Institute of Applied Geophysics and
Geothermal Energy. The projects approach is to use
experimental results to examine and improve various
numerical methods in order to develop a numerical tool for
planning of hydraulic fracturing for future geothermal
applications. Since field-scale experiments are very costly
due to the need of deep drilling, a variation of some
boundary condition is only possible by changing the location
of the experiment and because of the poor reproducibility
due to the natural heterogeneity of the subsoil,
laboratory-scale experiments were carried out in this
project. For this purpose a new experimental apparatus has
been developed. In the presented test series, cuboidal rock
samples (300 x 300 x 450 mm3) made from Tittlinger Feinkorn
granite are loaded triaxially with flat-jacks to simulate
the influence of the initial stress state on the fracturing
process. Then, dyed glycerin is pressed into a delimited
borehole section of the centrically drilled samples with a
high-pressure precision-pump. When the fluid pressure
reaches a critical value, a crack initiates at the loaded
borehole section, and is propagated by the injection of
further fluid. In order to decrease the speed of fracture
growth and to keep the cracks within the specimens, a
special injection method is used: in a so-called
"pre-fracturing cycle", fluid is injected until the peak
pressure is reached. Then the pressure is discharged
abruptly. In the second injection cycle, the previously
produced "flaw" is opened and propagated with a very low,
constant injection rate of Q = 0.05 cm3 / min. Acoustic
emissions of the fracturing process are recorded and
subsequently localized to monitor the fracture propagation.
In addition, the pressure in the injection string and the
control volumes of the control-device connected to the
flat-jacks are recorded. After the test, the sample is split
in the crack plane and the "colored fracture surface" is
scanned with a 3D scanner. The present work does not include
a description of the numerous preliminary experiments which
were necessary to develop the final experimental procedure.
Instead, the six last test series are presented, each
consisting of three individual tests with the same settings.
In five series, cracks were generated from a circumferential
notch on the wall of the borehole and propagated transvers
to the borehole axis. The duration of injection, the
injection process and the normal stress on the crack plane
(z) were varied. In the sixth series, fractures parallel to
the borehole-axis were produced. The tests show that the
hydraulic fracturing in the developed test stand is
reproducible with regard to the injection pressure level as
well as the size and shape of the created fractures. In
addition, the observations show that the hydraulic fracture
propagation is dominated by different influences, varying
with time: Directly after reaching the peak pressure, the
volume flow, which is increased by the decompression of the
fluid, causes the crack to propagate very rapid initially.
With the reduction of the excess in elastic energy, the
influence of fluid losses increases and the fracture
propagation slows down significantly. In some experiments,
even a crack stop is observed since the fluid losses exceed
the injection rate in the meantime. The numerical difference
between the control volume change of the loading apparatus
(Vz), which correlates with the elongation of the sample,
and the injected volume (Vp), shows that a high proportion
of the injected glycerin migrates into the partially
saturated rock without creation of new crack volume. By
comparing the test results with a simple analytical model
this conclusion could be confirmed. The preliminary
assumption that the experimental rock is to be regarded as
impermeable does not appear to be justified for the
simulation of the presented experiments with its very low
injection rate as well as the injection fluid and the type
of rock used. In planning of the presented experiments, the
question of scalability was deliberately ignored. For
subsequent investigations it is recommended to derive the
experimental setting from the field scale and to scale it
into the laboratory scale. Corresponding scaling approaches
were developed by "hydraulic fracturing research". Their
transferability to the geothermal background has to be
checked. Thus, in the future it could be excluded that in
the future purely laboratory-specific phenomena will
determine the experimental and numerical developments for
hydraulic fracturing. To conclude, it can be stated that the
newly developed experiment is a strong foundation for future
investigations. Improvements in the described strain
measurements by analyzing the volume changes in the
flat-jacks and optimizations on the technique for the
acoustic emission analysis promise additional insights in
the interpretation of the experiments.},
cin = {314310},
ddc = {624},
cid = {$I:(DE-82)314310_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2018-221026},
url = {https://publications.rwth-aachen.de/record/716212},
}
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