A Broadband Laboratory Study of the Seismic Properties of Cracked and Fluid‐Saturated Synthetic Glass Media

Abstract

For better understanding of frequency dependence (dispersion) of seismic wave velocities caused by stress-induced fluid flow, broadband laboratory measurements were performed on a suite of synthetic glass media containing both equant pores and thermal cracks. Complementary forced oscillation, resonant bar, and ultrasonic techniques provided access to millihertz-hertz frequencies,~1 kHz frequency, and~1 MHz frequency, respectively. The wave speeds or effective elastic moduli and associated dissipation were measured on samples under dry, argon-or nitrogen-saturated, and water-saturated conditions in sequence. The elastic moduli, in situ permeability, and crack porosity inferred from in situ X-ray computed tomography all attest to strong pressure-induced crack closure for differential (confining-minus-pore) pressures <30 MPa, consistent with zero-pressure crack aspect ratios <4 × 10 À4 . The low permeabilities of these materials allow access to undrained conditions, even at subhertz frequencies. The ultrasonically measured elastic moduli reveal consistently higher shear and bulk moduli upon fluid saturation-diagnostic of the saturated-isolated regime. For a glass rod specimen, containing cracks but no pores, saturated-isolated conditions apparently persist to subhertz frequencies-requiring in situ aspect ratios (minimum/maximum dimension) <10 À5 . In marked contrast, the shear modulus measured at subhertz frequencies on a cracked glass bead specimen of 5% porosity, is insensitive to fluid saturation, consistent with the Biot-Gassmann model for the saturated-isobaric regime. The measured dispersion of the shear modulus approaches 10% over the millihertz-megahertz frequency range for the cracked and fluid-saturated media-implying that laboratory ultrasonic data should be used with care in the interpretation of field data. Gurevich et al. (2010) incorporated the influence of "squirt" flow, as originally proposed by Mavko and Nur (1975), for a unified model of Biot and squirt-flow dispersion. In contrast, in effective medium theory, individual inclusions with assigned geometries are embedded within a solid to estimate the overall elastic properties of the composite from the elastic properties of individual constituents (Eshelby, 1957;Kachanov, 1993;Kuster & Toksöz, 1974;Wu, 1966;Zimmerman, 1991). Consideration of frequency-dependent fluid communication between the isolated fluid phases of conventional effective medium models provides insight into the dispersion of fluid-saturated medium and the influence of microstructure on dispersion (Adelinet et al.

Permeability and seismic-frequency elasticity of cracked glass

“…To assess the effect of cracking, the specimen dimensions were measured before and after the thermal treatment. About 0.2% increase in sample volume, i.e., crack porosity, was found after thermal cracking (Li et al, 2017a). In this follow-up study, an improved sample fabrication approach is presented.…”

“…Sintered aggregates of beads of soda lime silica glass provide a suitable synthetic analogue of a well sorted, matured siliciclastic rocks which has a very huge economic importance as hydrocarbon reservoir rocks (Figure 1a-d), while initially fully dense glass rods of soda-lime-silica (Figure 1e, f) are used to investigate the effects of crack porosity alone (Li et al, 2017a). The sintered analogue material has residual equant porosities between 2-5%, while the glass rod is fully dense with zero porosity.…”

“…Additional data for the cracked glass-rod material FDSL1 of Li et al (2017b), from further permeability measurements and forced oscillation tests, along with the results of exploratory flexural mode tests, and progress in the sintering of improved glass-bead materials is presented with preliminary results concerning the pressure dependence of permeability and torsional/flexural compliance.…”

“…Acquisition of data complementary to those of torsional oscillation from flexural tests, that were precluded in the study by Li et al (2017a) by poor alignment of the experimental assembly and finally V.…”

“…Assessment of the feasibility of measurements at differential pressures below the 10 MPa minimum of the Li et al (2017a) study to better document the pressure-induced closure of cracks of very low aspect ratio IV.…”