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| Titel |
An experimental study of iron release from red sandstones |
| VerfasserIn |
Gemma Purser, Christopher Rochelle, Jeremy Rushton, Jonathan Pearce |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089807
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| Publikation (Nr.) |
 EGU/EGU2014-4020.pdf |
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| Zusammenfassung |
| An experimental study has been conducted to better understand the features of a natural CO2
–rich system at Saltwash Graben, Utah, USA. This site is associated with numerous CO2 rich
springs linked to faults and fractures. In this area, a key feature of the red Entrada sandstone
formation is the presence of significant rock bleaching (iron reduction and mobilisation) that
occurs subparallel to bedding, typically at the base of large sandstone units and adjacent to
some subvertical fractures. The difference in total iron content between the bleached and
unbleached sandstones is very small, with the bleached sandstone containing slightly less
total iron. In contrast to widely-reported regional bleaching, attributed to hydrocarbon
accumulations towards structural crests, it has been suggested that the bleaching may be
associated with the presence of modern day CO2 in the area and we sought to test
this.
Laboratory experiments were conducted to assess reaction processes that may have
caused the observed iron reduction and mobilisation. Fixed volume batch reactors, containing
either small cores of red or bleached sandstone were exposed to representative local ground
waters (a dilute or a saline fluid), which were pressurised with either CO2 or N2
(the latter as a control) to 50 bar and placed inside an oven at 40°C to simulate
subsurface conditions . The experiments ran for up to nine months with fluids being
sampled periodically, though solids were only analysed once experiments were
completed. Very little reaction was found to occur in the presence of CO2. It seems
possible therefore that the modern CO2 rich fluids were not the cause of the sandstone
bleaching.
The study therefore assessed how the presence of reducing agents such as methane (CH4)
and hydrogen sulphide (H2S) may result in the bleaching of the bulk sandstone. H2S was
introduced into the experiments as a breakdown product of thioacetamide (0.1% v/v fluid
containing thioacetamide was added to the experiments). CH4 was added at a 5%
concentration in the CO2 gas (or N2) used for pressurisation. The presence of H2S caused a
significant release of iron (II) from the sandstone into solution, even in the absence of CO2,
though the reaction did not progress far enough to cause bleaching of the sandstone cores. It
seems possible therefore that sandstone bleaching may have been a consequence of ancient
flow of highly reducing water.
It is therefore proposed that reducing sulphur-rich fluids migrated along fractures, and
mobilised iron from the rock, resulting in the bleaching observed. Further evidence from
petrographical observations of fracture mineralisation from the centre of bleached
zones have also supported the experimental observations and hence the proposed
model.
It has been demonstrated that laboratory experiments can help to improve understanding
of fluid-rock interaction processes within this natural CO2 –rich system. The results also have
wider implications for longer-term CO2 trapping mechanisms within CO2 storage
schemes.
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