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| Titel |
Experimental Rock Deformation under micro-CT: ERDμ |
| VerfasserIn |
Nicola Tisato, Qi Zhao, Anton Biryukov, Giovanni Grasselli |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104166
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| Publikation (Nr.) |
 EGU/EGU2015-3590.pdf |
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| Zusammenfassung |
| Typically, the static elastic moduli of a rock differ from the corresponding dynamic
rock-moduli. Such frequency-dependent characteristic, called modulus dispersion, implies
also velocity dispersion (i.e. Vp- and Vs-dispersion). Velocity dispersion can be seen, in fact,
as the result of a viscoelastic response of the geo-material to the externally imposed stress
(e.g. seismic wave). Viscoelasticity can be conveniently expressed as attenuation (1/Q),
which describes the loss of elastic energy for each stress cycle and comprises the
measurement of the complex elastic modulus. As 1/Q at frequencies < 100 Hz is strongly
influenced by the presence of saturating fluids, 1/Q represents an important seismic
attribute as it can aid the subsurface imaging accuracy. For instance, the study of 1/Q
is fundamental for the oil and gas industry as valuable natural resources, such as
oil-sands or gas-shales, exhibit significant attenuation (e.g. 1/Q ~ 0.3 at 1 Hz), or for
volcanic and earthquake related studies as fluids are often involved in those natural
processes.
In the last five years, employing the Broad Band Attenuation Vessel (BBAV), the attenuation
of partially saturated rocks has been investigated along with the fluid pressure transient
caused by a sudden increase of stress. In particular, those studies shed light on the
relationships between 1/Q and i) saturation, ii) confining pressure, and iii) strain. The
combination of laboratory and numerical results helped demonstrating that wave
induced fluid flow (WIFF) on the mesoscopic scale is responsible for the large and
frequency-dependent attenuation observed in the laboratory measurements of a partially
saturated sandstone.
However, these studies lay bare limitations: the behavior of attenuation as a function of i) the
distribution of fluids in the pore space and ii) the role of dissolution-precipitation of new
mineral phases are still unclear. For instance, when CO2 is injected in the Earth’s crust to
pursuit carbon sequestration it would be extremely useful understanding the impact of the
gas-water-rock reactions on the rock elastic properties. Potentially, the imaging
of the internal structure and fluid distribution in the sample, combined with the
measurement of 1/Q, could serve to this goal helping subsurface monitoring and
surveying. This is the primary purpose of our research: uncovering the relationships
between i) saturation and dissolution-precipitation, and ii) the elastic properties of a
rock.
The present contribution reports the design of a new high-pressure X-Ray transparent vessel
which can fit and perform measurements inside the X-Ray computed tomography
apparatus (μCT) installed at the University of Toronto. Hence, the scientist can
measure changes in 1/Q in the sample and, simultaneously, link them to saturation
variations, or precipitation-dissolution of minerals. We discuss how the use of the
μCT will allow shedding light on the physics of 1/Q, and present the preliminary
results obtained with the new vessel in the μCT. This technological development,
together with the results already obtained, will enrich the knowledge of seismic wave
attenuation mechanisms for partially saturated rocks to aid geophysical methods. |
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