Fluid effects on velocity and attenuation in sandstones

Abstract

Ultrasonic velocity and attenuation measurements in sandstones with a variety of saturating fluids are compared with the predictions of Biot’s theory of porous media acoustics. Velocity data show systematic deviations from Biot theory as a function of pore fluid viscosity. Ultrasonic attenuation is much larger than the Biot prediction and for several sandstones is nearly constant for a three decade variation in viscosity. This behavior contrasts with synthetic porous media such as sintered glass beads, where Biot theory provides accurate predictions of velocity and attenuation. A sandstone was modified by curing a residual saturation of epoxy in the pore space, filling small pores and microcracks. This altered rock has a significantly reduced attenuation, demonstrating the dominance of small pores in controlling ultrasonic attenuation. Discrepancies between Biot theory and ultrasonic measurements are due to complex pore shapes in sandstones that are not present in simpler synthetic porous media such as sintered glass beads. Two non-Biot viscous attenuation mechanisms are needed: local flow in microcracks along grain contacts, and attenuation from pore-wall surface roughness. The combination of these mechanisms with Biot theory can explain ultrasonic measurements on sandstones. A model calculation of attenuation for a pore wall with surface roughness shows a significant increase in attenuation compared to the smooth wall calculation of Biot theory. This attenuation enhancement explains the absence of a Biot slow wave in fluid saturated rocks, and the observed correlation between ultrasonic attenuation and clay content in sandstones. When extended to the lower frequencies of geophysical field measurements, attenuation from all viscous dissipation mechanisms is negligible. This implies that ultrasonic attenuation measurements of sandstones provide no useful information about seismic attenuation in fluid-filled rocks in the earth, and mechanisms other than viscous dissipation must be invoked to describe the attenuation in geophysical field measurements.