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| Titel |
Geochemical interactions between CO2 and minerals within the Utsira caprock: A long-term experimental study |
| VerfasserIn |
Keith Bateman, Christopher Rochelle, Gemma Purser, Simon Kemp, Doris Wagner, Antony Benham |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078893
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| Zusammenfassung |
| During the underground storage of carbon dioxide (CO2) in deep saline formations, the
containment of CO2 will be crucially dependent on the integrity of seals above the CO2.
These seals could be natural (e.g. a clay-rich caprock) or man-made (e.g. the engineered
seals around a borehole). It is important therefore, to assess how the CO2 might
impact these seals, as this could ultimately control the longevity of CO2 storage.
We have undertaken a long-term experimental study focused on the geochemical
reactions between CO2, synthetic porewaters and caprock material from the Sleipner
field.
The experiments utilised samples of disaggregated Utsira caprock, together with synthetic
formation waters based upon measured compositions. The experimental conditions were
representative of the in-situ environment (30Ë C, 8 MPa). Experiments were pressurised with
either nitrogen (N2) or CO2. The former provided a ‘non reacting’ reference point from
which to compare the reactive CO2 experiments.
Short-term experiments using disaggregated Utsira caprock were ran for up to 14 months.
Those without CO2 showed little or no reaction, indicating that the synthetic Utsira
porewater used in the experiments was a reasonable approximation for the actual in-situ
porewater composition. However, the experiments using high-pressure CO2 were
dominated by carbonate mineral dissolution. Dissolved Ca concentrations, showed a
rapid increase within the first few weeks to about 1400Â mg l-1. This reflects the
acidification of the synthetic porewater, with CO2, and subsequent carbonate mineral
dissolution. The fluid chemical data indicate that over two thirds of the calcite present in
the mudstone caprock dissolved in the experiments, with mineralogical analyses
possibly indicating even larger decreases (from 3.2-4.0% to 0.7% or less). Most of the
calcite is present as shell debris in the Utsira caprock mudstone, and the observed
reduction in calcite content is consistent with the dissolution of these. We found no
evidence for the formation of secondary precipitates such as Ca/Mg/Fe carbonates or
dawsonite.
Long duration experiments ran for up to 7 years, and these confirm the shorter-term
observations. The experiments pressurised with N2 showed little or no reaction. Reactions in
experiments involving high-pressure CO2 were dominated by carbonate mineral dissolution.
Fluid chemical data from the long duration tests confirm previous findings that over two
thirds of the calcite in the mudstone caprock dissolved, and dissolved Ca concentrations
remained at about 1400Â mg l-1. No definitive evidence of other changes in mineralogy
(including clay mineralogy, could be identified in either the CO2-pressurised or
N2-pressurised experiments.
In terms of the overall impact of storing CO2 at Sleipner, the results from these
experiments show no indication of major deleterious, geochemical, reaction processes
occurring with the caprock. The only process identified was some dissolution of carbonate
phases when CO2-rich fluids contact the caprock. |
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