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| Titel |
Tracer-based prediction of thermal reservoir lifetime: scope, limitations, and the role of thermosensitive tracers |
| VerfasserIn |
I. Ghergut, H. Behrens, S. Karmakar, T. Licha, M. Nottebohm, M. Sauter |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061980
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| Zusammenfassung |
| Thermal-lifetime prediction is a traditional endeavour 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 evolutions
(‘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 (say, fracture aperture and
porosity), 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 only ‘few more’ fractures are
considered, this clear-cut correlation is broken. A given geothermal reservoir can
simultaneously feature a single-fracture behaviour, in terms of heat transport, and
a multiple-fracture behaviour, 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.
We demonstrate this issue at the example of a five-fracture system, representing a
deep-geothermal reservoir, with well-doublet placement inducing fluid flow ‘obliquely’ to the
fractures. Thermal breakthrough in this system is found to strongly depend on fracture
apertures, whereas conservative-solute tracer signals from inter-well tests in the same system
do not show a clear-cut correlation with fracture apertures. Only by using thermosensitive
substances as tracers, a reliable correlation between (early) tracer signals and (later) thermal
breakthrough can be re-established.
Thus, thermosensitive tracers are indispensable for predicting thermal
breakthrough, in such geothermal reservoirs whose ‘hydrogeological personality’ is
given by a finite set of fractures, with flow occurring both across and along the
fractures. In terms of the ‘gebo benchmark-model’ typology investigated by
Hördt et al. (2011)  [http://eposters.agu.org/abstracts/models-of-geothermal-reservoirs-as-a-basis-for-interdisciplinary-cooperation/] ,
such systems combine flow and transport patterns of the ‘petrothermal’ type and of the
so-called ‘deep-aquifer’ type:
across the fractures, heat is travelling faster than conservative-solute tracers;
along the fractures, conservative-solute tracers experience much less retardation
by transversal exchange (matrix diffusion), than heat;
fluid (and tracer) flow is not limited to the fractures; matrix flow yields essential
contribution to prolonging the fluid (and tracer) residence time.
Thermal lifetime results from the opposite effects of fracture aperture as an:
advection-related parameter: fluid travel time increases with increasing fracture
aperture
advection-unrelated parameter: fracture – matrix exchange rate increases with
decreasing fracture aperture, which accelerates transport across the fracture, but
retards transport along the fracture.
In conservative-solute tracer signals, all these fracture aperture effects on tracer transport are
masked by the very long residence time associated with the matrix flow component.
Thermosensitive tracers are able to ‘magnify’ the visibility of fracture aperture effects against
matrix flow effects.
Acknowledgment: This study benefits from thermosensitive-tracer research conducted
within the projects Smart Tracers and LOGRO, funded by the German Ministry for
Environment, Nature Conservation and Nuclear Safety (BMU, 0327579 and 0325111B) and
by Energie Baden-Württemberg (EnBW). |
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