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| Titel |
Characterisation of a geothermal reservoir using microseismic data |
| VerfasserIn |
Anton Reshetnikov, Makky Jaya, Stefan Buske, Joern Kummerow, Serge Shapiro |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054731
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| Zusammenfassung |
| In this paper we considered the geological structure at the German deep drill site (KTB) as a
prototype of a geothermal system. We used microseismic data in combination with surface
seismic imaging results for characterization of the geothermal reservoir.
The long-term fluid injection experiment in the KTB pilot hole at 4 km depth started at
May 27, 2004. The seismicity was monitored by a seismic network, which consisted of a
borehole seismometer in KTB main hole at  200 m horizontal distance from the injection
source and variable number of near-surface stations (on average 10–15 stations). Most of the
stations were installed within radius of 3 km from the KTB. All instruments were
3-component seismometers and data were recorded continuously at sample rates 200–1000
Hz. The cumulative number of events detected by near-surface stations was 146, and more
than 3000 for the borehole sensor. All events were small with magnitudes in the range of -3.0
to +0.3.
Only the events which were also recorded by near-surface stations could be precisely
located. We applied a two-step location procedure: absolute location was first determined by
a grid-search algorithm in a local 3D velocity model. These locations were then refined
by a double-difference relocation. Then using the only recordings from borehole
instrument and similarity of the events we additionally located approximately 3000 small
events.
In order to get a large scale model of the fault zones we used the image obtained from a
3-D prestack depth migration of the ISO89-3D data set. To enhance specific geological
features which were we not able to obtain using only the original migrated seismic data we
also conducted additional seismic attributes analysis. The seismic attributes were represented
in the form of absolute energy values calculated using the Hilbert transform of migrated
seismic data.
We applied Seismicity Based Reservoir Characterization (SBRC) approach to the three
stages of the fluid injection experiment and estimated rock diffusivity using three clouds of
microseismicity at 9 km depth which corresponded to SE1 fault zone, at 4 km depth (SE2
reflector) and 5.4 km depth (background rocks). In order to construct 3D permeability cube
we made an assumption about the positive correlation between the seismic reflectivity and the
hydraulic diffusivity. Our suggested permeability model consists of three bodies: isotropic
body of background permeability, low permeable SE2 fault zone and high permeable SE1
zone. Using seismic attributes as a reference map of the reflectivity, we separated
the whole volume into three parts by specifying two thresholds for the attribute
values.
In order to get a high resolution image of SE2 fault zone we applied our microseismic
imaging approach to the waveforms recorded at the borehole sensor. Using
Fresnel-Volume-Migration technique we constructed 3D images of the data between P- and
S- first arrivals which we interpreted as PP reflections. There is complicated network
of reflectors revealed in the vicinity of microseismicity cloud which belongs to
SE2 reflector. Obtained result is mainly consistent with seismic attributes by the
location and dip. Furthermore, it provides a more detailed image of the fine structure
of the fault zone due to the higher frequencies used in our study (60 – 350 Hz). |
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