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| Titel |
Analysis of propagation mechanisms of stimulation-induced fractures in rocks |
| VerfasserIn |
Michael Krause, Joerg Renner |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250128317
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| Publikation (Nr.) |
 EGU/EGU2016-8299.pdf |
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| Zusammenfassung |
| Effectivity of geothermal energy production depends crucially on the heat
exchange between the penetrated hot rock and the circulating water.
Hydraulic stimulation of rocks at depth intends to create a network of
fractures that constitutes a large area for exchange. Two endmembers of
stimulation products are typically considered, tensile hydro-fractures that
propagate in direction of the largest principal stress and pre-existing
faults that are sheared when fluid pressure reduces the effective normal
stress acting on them. The understanding of the propagation mechanisms of
fractures under in-situ conditions is still incomplete despite intensive
research over the last decades. Wing-cracking has been suggested as a
mechanism of fracture extension from pre-existent faults with finite length
that are induced to shear. The initiation and extension of the wings is
believed to be in tensile mode. Open questions concern the variability of
the nominal material property controlling tensile fracture initiation and
extension, the mode I facture toughness $K_{\text{IC}}$, with in-situ conditions,
e.g., its mean-stress dependence. We investigated the fracture-propagation
mechanism in different rocks (sandstones and granites) under varying
conditions mimicking those representative for geothermal systems. To
determine $K_{\text{IC}}$-values we performed 3-point bending experiments. We varied
the confining pressure, the piston velocity, and the position of the chevron
notch relative to the loading configuration. Additional triaxial experiments
at a range of confining pressures were performed to study wing crack
propagation from artificial flaws whose geometrical characteristics, i.e.,
length, width, and orientation relative to the axial load are varied. We
monitored acoustic emissions to constrain the spacio-temporal evolution of
the fracturing. We found a significant effect of the length of the
artificial flaw and the confining pressure on wing-crack initiation but did
not observe a systematic dependence of wing-crack initiation on the
orientation of the initial flaw in the range of tested angles. In fact,
wings do not develop for artificial flaws shorter than 3 mm. The force
required to initiate wing cracking increases with increasing confining
pressure as does the apparent fracture toughness. So called ``anti-wing
cracks'' were observed too, probably an artifact of the geometrical
constraints imposed on the sample in a conventional triaxial compression
test. |
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