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| Titel |
Evaluating uncertainties in geochemical modelling for CO2 storage: multivariate sensitivity analysis of database entries |
| VerfasserIn |
Marco De Lucia, Susanne Stadler, Matthias Rateitzak, Mich. Kühn |
| 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 |
250056300
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| Zusammenfassung |
| Geochemical and reactive transport modelling are powerful tools used for the assessment of
the stability and long-term safety of CO2 storage sites. Due to the complexity of the natural
systems and the consequent experimental difficulties, and despite the large effort of the
scientific community in filling the gaps, the data which forms the available thermodynamical
and geochemical databases needed for the simulations are incomplete and patchy, and
include measurement errors, inconsistencies, fitting and extrapolation errors (i.e.
unknown or partial dependency on temperature and pressure). Such sources of
uncertainty are usually disregarded by the modellers (end-users) and not easy to
quantify.
The goal of the study is to assess the reliability of chemical simulations for simple
and more complex settings, in a range of conditions relevant for CO2 storage in
depleted gas reservoir, taking as reference the Altmark gas field (Germany). The
considered temperatures range from 25 to 130 °C and pressures up to 200 bar,
accompanied by a high salinity of the involved brines. More in detail, the data we are
focusing on are on one hand Pitzer coefficients, required to model with sufficient
accuracy the activities of the dissolved species due to the high ionic strength of the
formation fluids; on the other hand, the equilibrium constants of aqueous and mineral
reactions. We performed a systematic multivariate sensitivity analysis based on
randomly generated small perturbations of the parameters tabulated in available
databases, in order to investigate the propagation of the errors through the chemical
speciation system. The geochemical simulator PHREEQC was used in the test cases. We
show that with perturbations of less than 5% in the initial equilibrium constants and
Pitzer parameters, which is already optimistic for the considered range of ionic
strengths and temperatures, the response on the simulations in unfavorable cases
can reach relative differences in the order of 15%. This compares in magnitude to
uncertainties arising from other unknown features of the underground like spatial
heterogeneity or uncertainties in reaction kinetics. More complex settings with an increased
number of reacting mineral phases may, however, not necessarily show increased
discrepancies. |
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