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| Titel |
Large-scale DC geoelectric measurements at the CO2 pilot storage Ketzin: Time-lapse inversion and evaluation of imaged volume |
| VerfasserIn |
Peter Bergmann, Cornelia Schmidt-Hattenberger, Dana Kiessling, Tim Labitzke, Carsten Rücker |
| 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 |
250049887
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| Zusammenfassung |
| The geophysical monitoring programme at the Ketzin pilot site comprises DC geoelectric
measurements in three different setups (surface-to-surface, surface-to-downhole and
crosshole). The first two setups are performed as Dipole-Dipole measurements on crossed
profiles as well as two rings centred around the injection well. The latter two setups make use
of a permanent well installation with 45 electrodes deployed in three wells. These
measurements are intended to monitor subsurface resistivity changes associated with the
migration of the injected CO2. Laboratory measurements on core samples and Archie fluid
substitution indicate a resistivity increase of a factor of about 2-3. Based on this increase a
model is created, which integrates the geologic setup and well logged resistivities. The model
is used for sensitivity studies and cross-evaluation with field data. Pre-inversion field data of
the surface-downhole measurements indicate a relevant resistivity increase in the injection
depth interval being consistent with the observations of the crosshole geoelectric
monitoring.
Inversion is performed with BERT (Boundless Electrical Resistivity Tomography) using
unstructured triangulated meshes and structural constraints. Structural constraints are
collected from the lithology and interpretation of seismic measurements. These
constraints allow to consider for the sparseness of the surface measurements and to
regularize the equivalent model space. The inversion scheme takes the data quality in
an error-weight scheme into account, which is shown to be essential to deal with
the ambient noise and the temporally progressing degradation of the subsurface
installation. Inversion results show a clear increase in resistivity in the injection
formation, but raise question for the extent of the imaged volume. This question is
addressed by pre-inversion analysis of electrode configuration specific sensitivities and
post-inversion analysis of the depth/volume of investigation. Sensitivities are observed
to decrease rapidly with increasing distance to the wells, making imaging of thin
layered CO2 migration at a distance of several electrode distances from the well
infeasible. The contribution is finished with a discussion about modification of the
geoelectric imaging by variation of surface-downhole acquisition based on the Ketzin
experience. |
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