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| Titel |
Joint inversion for thermal and petrophysical properties from wireline and temperature observations |
| VerfasserIn |
Andreas Hartmann, Volker Rath |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035087
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| Zusammenfassung |
| High resolution temperature logs are extremely useful in geothermal
studies but the analysis of their detailed structure requires good
control on the thermal properties of the subsurface. This is often
achieved by a two step procedure where first thermal conductivity is
derived from well logging data and then used to infer the heat flux
density of the temperature log. Here, an algorithm is presented that
incorporates both steps into a single inversion procedure.
In addition to the petrophysical inverse problem and steady-state heat
conduction, transient temperature signals, e.g., originating from past
surface temperature change, are included. Computational requirements
differ from conventional algorithms for the petrophysical problem
because the differential equation for transient heat conduction needs
to be solved numerically, resulting in a more complex forward problem.
The inverse problem is solved by a Quasi-Newton iterative scheme,
allowing for a Bayesian approach as well as a Tikhonov type
regularization. The Jacobian matrix may be calculated either by
traditional finite-differences, or by using automatic
differentiation. The technique of matrix compression is considered to
achieve a most efficient implementation. Matrix compression can
drastically improve the computational speed for calculating the
Jacobian for certain classes of problems. Those involving transient
heat conduction cannot be compressed efficiently whereas purely
petrophysical problems have a much higher compression ratio. Results
of the algorithm are verified by using well-known programs for
paleoclimate and petrophysical inversion.
A borehole in Southern Germany serves as a case study for the
algorithm. The algorithm is able to match both wireline and
temperature data with good accuracy and in a consistent manner. The
ground surface temperature history of the last glacial cannot be
reconstructed without ambiguity because the log terminates too shallow
for a full reconstruction. An important result is the correlation
between the transient temperature perturbation and the shale
petrophysical properties which are usually not well-known but may play
an important role when transient problems in sedimentary rocks are
considered. |
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