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| Titel |
Permeability Enhancement in Enhanced Geothermal System as a result of Hydraulic Fracturing and Jacking |
| VerfasserIn |
Mohammadreza Jalali, Maria Klepikova, Hansruedi Fisch, Florian Amann, Simon Loew |
| 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 |
250128770
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| Publikation (Nr.) |
 EGU/EGU2016-8796.pdf |
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| Zusammenfassung |
| A decameter-scale in-situ hydraulic stimulation and circulation (ISC) experiment has been
initiated by the newly-founded Swiss Competence Centre for Energy Research – Supply
of Electricity (SCCER-SoE) at Nagra’s Grimsel Test Site (GTS) as a part of the
work-package WP1 of the Deep Underground Laboratory (DUG-Lab) initiative. The
experiment area is situated in the southern part of the GTS in a low fracture density
volume of the Grimsel granodiorite. The hydraulic properties of the granitic rock
mass are supposed to be similar to those expected in the crystalline basement of the
alpine foreland where deep enhanced geothermal systems might be developed in
future. The main objectives of the multi-disciplinary experiment are to provide a
high resolution pre- and post-stimulation characterization of fracture permeability
and connectivity, to investigate patterns of preferential flow paths, to describe the
pressure propagation during the stimulation phases and to evaluate the efficiency of the
fracture-matrix heat exchanger. A comprehensive test & monitoring layout including a fair
number of boreholes instrumented with a variety of sensors (e.g. pressure, strain,
displacement, temperature, and seismic sensors) is designed to collect detailed
data during multiple hydraulic stimulation runs. The diffusion of fluid pressure is
expected to be governed mainly by the properties and geometry of the existent fracture
network. The hydraulic transmissivity of fractures are in the range of 10−7 to 10−9
m2/s whereas the matrix rock has a very low hydraulic conductivity (K ∼ 10−12
m/s).
As part of the stress measurement campaign during the pre-stimulation phase of the ISC
experiment, a series of hydraulic fracturing (HF) and hydraulic tests in pre-existing fractures
(HTPF) were conducted. The tests were accompanied by micro-seismic monitoring within
several observation boreholes to investigate the initiation and propagation of the
induced fractures. Together with results from over-coring tests, these data were used
to conclude on the local stress orientation and stress magnitudes. The hydraulic
response of the rock mass under hydro-mechanical perturbations was investigated by
conducting various hydraulic packer tests (e.g. pulse, constant rate and constant
head) in multiple hydraulically isolated borehole sections before and after the stress
measurements.
Hydraulic testing of borehole sections which were previously fracked (during HF tests)
didn’t show a distinct increase in permeability. For the tested borehole sections without
natural fractures, this can be explained by the fact that hydraulic fracturing was
dominated by fracture normal opening (mode I). In this case, the implemented
pressure range (less than 2 MPa) during the hydraulic packer tests was not sufficient to
re-open the tensile fractures and permeability would remain unchanged. Conversely,
in borehole sections with pre-existing ductile and/or brittle fractures and where
HTPF-tests were conducted, the permeability increased by two orders of magnitude, from
10−11 m2/s to∼10−9 m2/s (results of hydraulic tests pre and post HTPF). These
findings could be explained by permanent enhancement of permeability as a result of
shear dilation of existing structures. Considering the efficiency of the hydraulic
stimulation process observed at low differential injection pressures, even more significant
permeability enhancement is expected during the upcoming stimulation experiments. |
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