Physical and transport properties of isotropic and anisotropic cracked rocks under hydrostatic pressure

A key consequence of the presence of microcracks within rock is their significant
influence upon elastic anisotropy and transport properties. Here two rock types (a basalt
and a granite) with contrasting microstructures, dominated by microcracks, have been
investigated using an advanced experimental arrangement capable of measuring porosity,
P wave velocity, S wave velocity, and permeability contemporaneously at effective
pressures up to 100 MPa. Using the Kachanov (1994) noninteractive effective medium
theory, the measured elastic wave velocities are inverted using a least squares fit,
permitting the recovery of the evolution of crack density and aspect ratio with increasing
isostatic pressure. Overall, the agreement between measured and predicted velocities is
good, with average error less than 0.05 km/s. At larger scales and above the percolation
threshold, macroscopic fluid flow also depends on the crack density and aspect ratio.
Using the permeability model of Gue´guen 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. We also calculate the evolution of the crack porosity with increasing
isostatic pressure, on the basis of the calculated crack density, and compare this directly
with the experimentally measured porosity. These combined experimental and modeling
results illustrate the importance of understanding the details of how rock microstructures
change in response to an external stimulus when predicting the simultaneous evolution
of rock physical properties.

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