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| Titel |
Electromagnetic characterization and monitoring of CO2 sequestration sites: feasibility studies and first field results from Ketzin, Germany |
| VerfasserIn |
Rita Streich, Michael Becken, Oliver Ritter |
| 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 |
250052449
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| Zusammenfassung |
| Bulk electrical resistivity of sedimentary units depends strongly on the pore fluid content
and pore connectivity. Since carbon dioxide exhibits significantly higher electrical
resistivity than saline pore fluids, electrical and electromagnetic methods are key
geophysical techniques for investigating CO2 sequestration sites. Electrical resistivity
tomography has been successfully applied for monitoring the immediate vicinity of CO2
injection wells; however, techniques having a wider spatial footprint are required for
imaging larger regions beyond the wells, particularly for prospective industrial-scale
injection.
Low-frequency electromagnetic methods are sensitive to thin resistive layers and may
thus provide a suitable tool for larger-scale injection monitoring. The resolution power can be
expected to depend strongly on local geology and the layout of the sensor network. Here, we
present results of synthetic controlled-source electromagnetic (CSEM) resolution studies, and
initial results of a large-scale field experiment carried out at the Ketzin CO2 sequestration
site.
For synthetic studies, we have developed 1D quasi-analytic and 3D finite-difference
modelling tools. To enable accurate CSEM simulations in the frequency range and for the
geometries of interest, our modelling tools include exact representations of long grounded
wire sources, and numerical stabilization at low frequencies by explicitly prohibiting
non-physical current sources. Because inherent limitations of the numerical approaches
employed or improper use of modelling tools may lead to inaccurate results and possibly
false conclusions, we first assess the modelling accuracy. Comparison of 1D to
analytical results for homogeneous models indicates errors mostly less than 0.01%, and
comparison of 1D and 3D results for layered models indicate 3D modelling errors
generally no more than 1-3%, with exceptions in regions of very low EM field
amplitudes.
We then study the sensitivity of surface-to-surface and borehole-to-surface source-receiver
configurations for layered and 3D resistivity models roughly mimicking the geology of the
Ketzin site. We find that a resistive layer having the resistivity and thickness of the
CO2-bearing sandstone should be detectable, but the present CO2 reservoir is probably too
small to be resolvable by surface measurements alone. Configurations involving borehole
instruments positioned more closely to the CO2 reservoir exhibit much higher sensitivity and
may thus be particularly useful for monitoring applications. We also observe a
trade-off between absolute field amplitudes and sensitivity in terms of relative EM field
changes due to the anomalous layer. Hence, low-noise instrumentation capable
of accurately measuring low-amplitude fields is essential for successful CSEM
measurements. Most information on subsurface structure is contained in the electric
field components, whereas the magnetic field is largely insensitive to the small
resistors.
For a first large-scale CSEM field experiment at Ketzin, we deployed a newly developed
CSEM transmitter equipped with three grounded source electrodes at eight different locations
to inject currents in the frequency range of 1/64 to 64Â Hz. The horizontal electric and
three-component magnetic fields were recorded by 39 surface receivers; additionally,
natural-source EM fields and ambient noise were recorded during transmitter-off periods.
Initial data analysis indicates that the lower-frequency source signals can be traced over
distances of ~10Â km. We present data examples and first transfer functions indicative of
subsurface resistivity distribution. |
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