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| Titel |
Matching tracer selection to georeservoir typology - A note on geothermal reservoir classification |
| VerfasserIn |
Julia Ghergut, Horst Behrens, Tobias Licha, Martin Sauter |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250075172
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| Zusammenfassung |
| Thermal-lifetime prediction is a traditional endeavor of inter-well tracer tests conducted in
geothermal reservoirs. Early tracer test signals (detectable within the first few years of
operation) are expected to correlate with late-time production temperature drop (so-called
’thermal breakthrough’, supposed to not occur before some decades of operation) of a
geothermal reservoir.
Whenever a geothermal reservoir can be described as a single-fracture system, its
thermal lifetime will, ideally, be determined by two parameters, whose inversion from
conservative-tracer test signals is straightforward and non-ambiguous (provided that the
tracer tests, and their interpretation, are performed in accordance to the rules of the
art).
However, as soon as just few more fractures are considered, this clear-cut correlation is
broken. A given geothermal reservoir can simultaneously exhibit a single-fracture behavior,
in terms of heat transport, and a multiple-fracture behavior, in terms of solute tracer transport
(or vice-versa), whose effective values of fracture apertures, spacings, and porosities are
essentially uncorrelated between heat and solute tracers. Solute transport parameters derived
from conservative-tracer tests will no longer characterize the heat transport processes
(and thus temperature evolutions) taking place in the same reservoir. Parameters
determining its thermal lifetime will remain invisible to conservative tracers in inter-well
tests.
Non-conservative tracers, in particular sorptive and thermo-sensitive compounds, can be
used to overcome this gap between heat and tracer transport. However, significant differences
exist, w. r. to tracer functionality, between different geothermal systems:
(I) hot natural aquifers (with predominantly ’porous media’ character),
(II) aquifer-based EGS,
(III) petrothermally-based EGS,
(IV) naturally-fractured systems.
Conservative tracers are indispensable to characterizing any of (I) – (IV), but their
residence time distribution (with mean residence time MRT) correlates differently with
thermal lifetime: the more pronounced the petrothermal character (with effective
aperture w), the more the quadratic term (proportional to Dth*MRT2/w2) will prevail
within thermal lifetime. It therefore turns out that thermo-sensitive tracers are less
useful (roughly speaking) in petrothermal, than in aquifer-based reservoirs; whereas
sorptive tracers prove more useful in petrothermally-dominated, than in aquifer-based
reservoirs.
Acknowledgement: This study was conducted within task unit G6 of the project gebo
(’Geothermal Energy and High-Performance Drilling’), funded by the Lower-Saxonian
Ministry of Science and Culture and by Baker Hughes (Celle), Germany. |
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