Common Evolution of Mechanical and Transport Properties in Thermally Cracked Westerly Granite at Elevated Hydrostatic Pressure

Abstract

Increasing the damage and crack porosity in crustal rocks can result in significant changes to various key physical properties, including mechanical strength, elastic and mechanical anisotropy, and the enhancement of transport properties. Using a Non-Interactive Crack Effective Medium (NIC) theory as a fundamental tool, we show that elastic wave dispersion can be inverted to evaluate crack density as a function of temperature and is compared with optically determined crack density. Further, we show how the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness (K IC ) of rocks as measured via a suite of tensile failure experiments (chevron cracked notch Brazilian disk). Finally, we include fluid flow in our analysis via the Guéguen and Dienes crack porosity-permeability model. Using the crack density and aspect ratio recovered from the elastic-wave velocity inversion, we successfully compare permeability evolution with pressure with the laboratory measurements of permeability.

Anomalous Vp/Vs Ratios at Seismic Frequencies Might Evidence Highly Damaged Rocks in Subduction Zones

“…Permeability was either (i) measured (black symbols) as a function of P eff (hence increasing p f ) on the microcracked Fo3 sample, the "intact" Fo7, and the microcracked WGs at lowest P eff = 1 MPa or (ii) attained (red symbols) from published works (Delle Piane et al, 2015;Loaiza et al, 2012;Nasseri et al, 2009). In the relevant frequency range, we measured extreme values of V p /V s in crustal rocks of very different mineralogical composition, hence very different matrix Poisson's ratio.…”

“…The thermal treatment method was shown to induce a large and isotropic degree of microfracturing due to the mismatch in thermal expansion of the grains in initially isotropic rocks (Fredrich & Wong, 1986;Nasseri et al, 2007Nasseri et al, , 2009). The thermal treatment method was shown to induce a large and isotropic degree of microfracturing due to the mismatch in thermal expansion of the grains in initially isotropic rocks (Fredrich & Wong, 1986;Nasseri et al, 2007Nasseri et al, , 2009).…”

Thermal‐elastic stresses and the criticality of the continental crust

“…In order to account for this discrepancy, we regarded the thermal expansivities of our damaged mineral phases as free fitting parameters and reduced them empirically until our numerical experiment matched the radial deformation history of the physical experiment (see below). Intracrystalline damage also lowers the elastic moduli of the affected minerals and thereby internal stresses [Heard and Page, 1982;Nasseri et al, 2009]. However, we lumped this effect solely into the downscaling of thermal expansivities for two reasons: first, Bruner [1984] demonstrated that, in quartzo-feldspathic rocks, thermal-expansion mismatch makes the main contribution to internal stresses for geologically reasonable geotherms; second, the degrees of freedom in the numerical calibration are reduced.…”

“…Hitherto, thermal-elastic internal stresses were mainly investigated in the context of cooling brittle rocks, for three reasons. First, they can generate micro-cracks (Figure 1) in the upper $4 km of the continental crust [e.g., Heard and Page, 1982], which affect rock properties such as elasticity, strength, fracture toughness, thermal conductivity, and permeability [Cooper and Simmons, 1977;Meredith and Atkinson, 1985;Nasseri et al, 2009]. This is relevant for resource extraction (e.g., mining, quarrying, and drilling), nuclear waste disposal, and reservoir stimulation [Heard and Page, 1982;Nasseri et al, 2007].…”

Laboratory measurement of elastic anisotropy on spherical rock samples by longitudinal and transverse sounding under confining pressure

“…Very often, polyhedron-type samples such as octadecahedrons, tetratrioctahedrals [12][13][14] and/or cubes with beveled edges [15,16] are used. Cylindrical samples are mostly used to determine elastic anisotropy [17][18][19][20] and other quantities as well.…”