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| Titel |
Fluid flow properties of Volterra Gypsum during experimental deformation at low strain rates monitored through simultaneous permeability measurement |
| VerfasserIn |
S. Llana-Fúnez, D. R. Faulkner, J. Wheeler |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250019776
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| Zusammenfassung |
| Understanding the evolution of fluid pathways in low permeability rocks during natural
deformation is key in understanding rock deformation processes and rheology. Furthermore,
it also has a direct application to studies concerning the migration of fluids in natural
rock reservoirs (e.g. oil, CO2), particularly under stress (tectonic or not). We use
gypsum as an analogue rock material for low porosity materials but also to understand
gypsum itself, as it can control the migration of fluids at shallow depths in the upper
crust.
The starting material in the experiments is Volterra Gypsum, a very pure gypsum rock
(>95 % gypsum), with low porosity (~0.5 %) and low permeability (~10-18 to
10-20 m-2, depending on effective pressure). We measure permeability and pore
volume change by using the pore pressure oscillation technique combined with
pore volumometry. The technique involves applying a pore pressure sinusoidal
wave in one end of the sample while monitoring the attenuation and phase shift
of the wave at the other end, from which the permeability and storage capacity
of the sample is calculated. The advantage over other techniques is that it allows
continuous monitoring of permeability changes as other experimentally controlled
parameters, for example effective pressure, differential stress and temperature, are
varied.
We present experimental results that show consistent variations of permeability in
Volterra gypsum with effective pressure and how the permeability evolves over time with
differential stress at low strain rate during constant loading (at strain rate of approx. 10-6
s-1), stress relaxation, and during cooling from 50 ºC to room temperature. Variations of
permeability beyond one order of magnitude when samples are loaded hydrostatically and
during non-hydrostatic loading reproduce previous experimental work. However, we also find
smaller but consistent decreases of permeability, on the order of several times, when
specimens relax over time from hydrostatic and non-hydrostatic load and also during cooling
from low temperatures. It has been reported previously that gypsum deforms by
diffusion creep at laboratory rates in powdered specimens. We suggest that the changes
we report in the fluid flow properties of intact gypsum specimens when subject to
small non-hydrostatic stresses may be related to the operation of diffusion creep. |
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