HR: 1340h
AN: T13B-1332    [Abstracts]
TI: Linking elastic, mechanical and transport properties in anisotropically cracked rocks
AU: * Schubnel, A
EM: aschubnel@geologie.ens.fr
AF: Laboratoire de Geologie - ENS/CNRS, 24 rue Lhomond, Paris, 75005, France
AU: Benson, P
AF: Lassonde Institute - University of Toronto, Canada, 170 College Street, Toronto, ON M5S3E3, Canada
AU: Benson, P
AF: Mineral, Ice and Rock Laboratory - University College London, Gower Street, London, WC1E 6BT, United Kingdom
AU: Nasseri, F
AF: Lassonde Institute - University of Toronto, Canada, 170 College Street, Toronto, ON M5S3E3, Canada
AU: Gueguen, Y
AF: Laboratoire de Geologie - ENS/CNRS, 24 rue Lhomond, Paris, 75005, France
AU: Meredith, P
AF: Mineral, Ice and Rock Laboratory - University College London, Gower Street, London, WC1E 6BT, United Kingdom
AU: Young, R
AF: Lassonde Institute - University of Toronto, Canada, 170 College Street, Toronto, ON M5S3E3, Canada
AB: Damage and crack porosity can result in a decrease of the mechanical strength of the rock, the development of elastic and mechanical anisotropy and the enhancement of transport properties. Using Non-Interactive Crack Effective Medium (NIC) theory as a fundamental tool, it is possible to calculate dry and wet elastic properties of cracked rocks in terms of a crack density tensor, average crack aspect ratio and mean crack fabric orientation using the solid grains and fluid elastic properties. Using the same tool, we show that the anisotropy, the shear wave splitting and the dispersion of elastic waves can be derived for anisotropic crack fabrics. Mechanically, the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness (K_IC) of rocks. We show that K_IC can show large amounts of anisotropy as well, the degree and orientation of which being largely constrained once again by the microcrack fabric. NIC can predict relatively well K_IC at high crack density, by simply using dimensionless crack densities inverted from velocities. A decrease of 50% for crack densities larger than 1, 80% for crack densities larger than 5 is predicted, in close agreement with our observed experimental variation of K_IC. At the microscale, this can be interpreted by the fact that the main fracture is strongly interacting with the pre-existing microcrack fabric. Finally, and above the percolation threshold, macroscopic fluid flow also depends on the porosity, crack density and aspect ratio. Using the permeability model of Guéguen and Dienes (1989) and the crack density and aspect ratio recovered from the elastic wave velocity inversion, we successfully predict the evolution of permeability with pressure for direct comparison with the laboratory measurements. These combined experimental and modelling results illustrate the importance of understanding the details of how rock microstructures change in response to an external stimulus in predicting the simultaneous evolution of rock physical properties.
DE: 3909 Elasticity and anelasticity
DE: 5102 Acoustic properties
DE: 5112 Microstructure
DE: 5114 Permeability and porosity
DE: 8031 Rheology: crust and lithosphere (8159)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting