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| Titel |
Compaction creep of granular calcite under upper crustal conditions: Effects of aqueous pore fluids and supercritical CO2 |
| VerfasserIn |
Christopher J. Spiers, Emilia Liteanu, Xiangmin Zhang |
| 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 |
250037552
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| Zusammenfassung |
| The compaction creep behaviour of granular calcite is directly relevant to the healing and slip
behaviour of faults in carbonates terraines, to porosity and permeability evolution in
carbonates, and to the response of depleted carbonate reservoirs to CO2 storage. There is
therefore much interest in quantifying the rates and mechanisms of compaction creep in
carbonates. We have conducted one-dimensional compaction creep experiments on granular
calcite (grain size 1-250 μm) at temperatures in the range 25-150 Ë C and effective axial
stresses up to 50 MPa. Tests have been performed dry, using saturated CaCO3 solution as
pore fluid (wet tests), and using pore fluids consisting of saturated CaCO3 solution in
equilibrium with supercritical CO2 at pressures up to 10 MPa (wet/CO2 tests). Pore fluid
salinity has also been varied. Dry experiments show negligible creep, whereas significant
creep is obtained in wet and wet/CO2 tests. Wet samples without CO2show two
main regimes of behaviour. At fine grain size ( 100 Ë C), the mechanical behaviour and
microstructures obtained resemble those expected for diffusion and/or precipitation
controlled intergranular pressure solution. Samples deformed at higher stresses and lower
temperatures, show a direct dependence of creep rate on stress and grain size, along with
SEM evidence for grain scale cracking, indicating a switch to creep controlled by
subcritical microcrack growth. Introduction of CO2 into wet samples leads to an
acceleration of creep by typically 1-2 orders of magnitude, with mechanical behaviour and
microstructure suggesting that pressure solution dominates in fine grained material
(-¤100 μm), giving way to subcritical crack growth and grain failure at coarser
grain sizes. In the pressure solution regime, increasing CO2 pressure causes an
increase in creep rate that roughly corresponds to predictions assuming diffusion
controlled pressure solution. Increasing temperature tends to decrease creep rates under
all conditions investigated. Pore fluid salinity increases or decreases creep rate,
depending on temperature and CO2 pressure. The effects of the variables investigated are
therefore complex and it remains difficult to arrive at simple compaction creep “laws”.
On the other hand, using data from samples showing behaviour consistent with
diffusion controlled pressure solution, the grain boundary diffusion product Dδ for
pressure solution is estimated to be around 10-19 m3/s under in situ conditions. |
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