A Broadband Laboratory Study of the Seismic Properties of Cracked and Fluid-saturated Synthetic Glass Materials

Abstract

The presence and migration of fluids in Earth’s upper crust is a topic of broad interest in geophysics, with applications ranging from imaging earthquake fault zones, through hydrocarbon and geothermal reservoir exploration, to the monitoring of sequestered supercritical carbon dioxide. The mechanical response of a fluid-saturated rock to an applied oscillatory stress depends on the scale of stress-induced pore-fluid flow within solid matrix, and hence is time (or frequency) dependent. Uncertainty, therefore, arises in applying ultrasonic measurements on elastic moduli/velocities of rocks at MHz frequencies, as the most commonly used technique in laboratory, to field data mainly acquired at frequencies of tens of Hz to a few kHz. A precise interpretation of the field data requires characterization of such frequency dependence or dispersion of seismic-wave velocities related to fluid flow, over the entire range of frequencies from mHz to MHz. Broadband mechanical measurements were performed on a suite of synthetic media made either by sintering soda-lime-silica glass beads or from glass rods of similar composition with artificially controlled microstructures involving both equant pores and cracks. The goal was an improved understanding of the origin of wave-induced fluid flow and the influence of microstructure on fluid-flow related dispersion. Various fluid-flow regimes are accessed either by using pore fluids with contrasting viscosity or exciting fluid-saturated rocks at different oscillating frequencies. Synthetic samples, therefore, were measured under dry, argon- and water-saturated conditions in sequence, with a combined use of three techniques, namely, forced oscillation at mHz-Hz frequencies, resonant bar at kHz frequency, and ultrasonic wave transmission at MHz frequency, to cover a wide frequency range. Complementary measurements on permeability were also conducted on these synthetic glass samples with either argon or water. Pressure dependent crack closure has been inferred for the cracked samples from the measured pressure dependence of the elastic and hydraulic properties. The microstructure of each cracked sample has been inferred from the measured pressure-dependent modulus deficit relative to the uncracked medium through a micromechanical model. A water-saturated glass-rod specimen tested at mHz frequencies has a systematically higher shear modulus than its dry counterpart – evidence of the saturated isolated regime at seismic frequencies. Accordingly, the application of the Gassmann equation for the saturated isobaric regime, usually considered suitable for seismic frequencies, is inappropriate in this case. With argon and water saturation, a dispersion of shear modulus as high as ~ 10% has been observed over the frequency range from mHz to MHz on the cracked samples, and various fluid flow regimes have been assigned based on the change in modulus due to fluid saturation and estimated characteristic frequencies. The observed dispersion indicates that conventional ultrasonic lab measurements of wavespeeds on cracked and fluid-saturated rocks cannot be directly applied in the interpretation of field data. Water with much greater viscosity than argon lowers the frequency for the squirt-flow transition on a cracked glass-rod specimen. The fluid-saturated samples with various equant porosities respond differently to the applied stress at the same frequency, indicating the influence of microstructure on the fluid-flow related dispersion.