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| Titel |
Sampling and analytical methods of stable isotopes and dissolved inorganic carbon from CO2 injection sites |
| VerfasserIn |
Robert van Geldern, Anssi Myrttinen, Veith Becker, Johannes A. C. Barth |
| 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 |
250041452
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| Zusammenfassung |
| The isotopic composition (δ13C) of dissolved inorganic carbon (DIC), in combination with
DIC concentration measurements, can be used to quantify geochemical trapping of CO2 in
water. This is of great importance in monitoring the fate of CO2 in the subsurface in CO2
injection projects. When CO2 mixes with water, a shift in the δ13C values, as well as an
increase in DIC concentrations is observed in the CO2-H2O system. However, when using
standard on-site titration methods, it is often challenging to determining accurate in-situ DIC
concentrations. This may be due to CO2 degassing and CO2-exchange between the sample
and the atmosphere during titration, causing a change in the pH value or due to other
unfavourable conditions such as turbid water samples or limited availability of fluid
samples.
A way to resolve this problem is by simultaneously determining the DIC concentration
and carbon isotopic composition using a standard continuous flow Isotope Ratio Mass
Spectrometry (CF-IRMS) setup with a Gasbench II coupled to Delta plusXP mass
spectrometer. During sampling, in order to avoid atmospheric contact, water samples taken
from the borehole-fluid-sampler should be directly transferred into a suitable container, such
as a gasbag. Also, to avoid isotope fractionation due to biological activity in the sample, it is
recommended to stabilize the gasbags prior to sampling with HgCl2 for the subsequent stable
isotope analysis.
The DIC concentration of the samples can be determined from the area of the
sample peaks in a chromatogram from a CF-IRMS analysis, since it is directly
proportional to the CO2 generated by the reaction of the water with H3PO4. A
set of standards with known DIC concentrations should be prepared by mixing
NaHCO3 with DIC free water. Since the DIC concentrations of samples taken from
CO2 injection sites are expected to be exceptionally high due to the additional high
amounts of added CO2, the DIC concentration range of the standards should be set
high enough to cover the sample concentrations. In order to assure methodological
reproducibility, this “calibration set” should be included in every sequence analysed with the
Gasbench CF-IRMS system. The standards, therefore, should also be treated in
the same way as the samples. For accurate determination, it is essential to know
the exact amount of water in the vial and the density of the sample. This requires
weighing of each vial before and after injection of the water sample. For stable
isotope analysis, the required signal height can be adjusted by the sample amount.
Therefore this method is suitable for analysing samples with highly differing DIC
concentrations. Reproducibility and accuracy of the quantitative analysis of the dissolved
inorganic carbon need to be verified by independent control standards, treated as
samples.
This study was conducted as a part of the R&D programme CLEAN, which is funded by
the German Federal Ministry of Education in the framework of the programme
GEOTECHNOLOGIEN. We would like to thank GDF SUEZ for permitting us to conduct
sampling campaigns at their site. |
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