Effects of partial water saturation on attenuation in Massilon sandstone and Vycor porous glass

Abstract

The advent of new high resolution seismic reflection and borehole sonic techniques has stimulated renewed interest in what information stress wave propagation may carry about rock properties and pore fluids in situ.We have measured extensional and shear wave velocities. I• e and I•,, and their specific attenuation, Q•-i and Q•-i, in Massilon sandstone and Vycor porous glass as a function of continuously varying partial water saturation and relative humidity. Measurements were made at frequencies from 300 Hz to 14 kHz using a resonant bar technique and from 25-400 Hz using a torsional pendulum technique. Energy loss is very sensitive to partial water saturation. In Massilon sandstone, Q•-i is maximum and greater than Q;-1 only at full saturation. Q•-i rises to a strong peak at 85% water saturation. Energy loss drops significantly as the Massilon becomes "very dry." Q•-i and Q•-i in partially saturated Massilon and Vycor are strongly frequency dependent throughout the acoustic range, exhibiting peaks between 1-10 kHz. Q-1 in dry Massilon and Vycor is independent of frequency, at least in the acoustic range. Two pore fluid mechanisms absorb energy. Viscous dissipation due to fluid flow in pores dominates in fully and partially water saturated materials. A surface capillary film mechanism dominates at low moisture contents. Nonlinear frame mechanisms such as frictional grain sliding are not signficant at normal acoustic strains, even in "dry" rocks. Compressional wave velocity and specific attenuation, l•p and Q•-i, and bulk compressional specific attenuation, Q;-1, were calculated at given frequencies. While the dependence of velocities on water saturation agrees well with a very simple explanation, there is no satisfactory theory yet available for attenuation. Vp/V, and Q;1/Qs-1 provide sufficient information to distinguish between fully and partially water saturated Massilon sandstones, yet are insufficient to resolve the degree of partial water saturation. PACS numbers: 43.35.Cg, 91.60. --x, 43.40.Ph INTRODUCTION Specific attenuation, Qq, is a dimensiontess measure of the energy loss per cycle as a stress wave propagates through a dissipative material. It is preferably defined as 1/Q =/l•/4rrW, (1) where ß is the energy toss per cycle and W is the, average energy stored per cycle. gccurate estimates of Q-t in sands and sandstones are necessary in order to reliably predict the path and decay of tow-frequency sound which travels through the ocean floor. 2'• In geophysical exploration, an understanding of how acoustic energy toss depends on pore structure and water saturation may advance the interpretation of borehole sonic togs and high resolution seismic reflections. 7 Consider that much of the original technical motivation for the analysis of"bright spots," observed in seismic reflections, arose from a limited understanding ultrasonic velocities as a function of partial water saturation. TM Despite the pioneering results of Biot, t3'l* Winkler, •s'19 and others, 2ø'23 I think it safe to say that in geophysics as ...

Hydro-bio-geomechanical properties of hydrate-bearing sediments from Nankai Trough

“…Black curves are measured while the specimen is within the hydrate stability conditions, red curves are measured after u cell is reduced below the hydrate stability pressure at 6 C. Amplitude loss for pressures below the hydrate stability pressure (red curves) is in agreement with empirical results for the addition of gas to an otherwise water-saturated porous material (Murphy, 1982;Santamarina et al, 2001). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.…”

“…To test whether gas bubbles produced during hydrate dissociation could be responsible for the observed amplitude loss, the water saturation S w is related to the Pwave attenuation using an empirical relationship in which the attenuation 1/Q P is related to the waveform amplitude A by Murphy (1982); Santamarina et al (2001): Figure 4. Evolution of specimen void ratio during the complete loading history for ESC core 8P (A) and core 10P (B).…”

Effect of Partial Water Saturation on Attenuation Characteristics of Low Porosity Rocks

“…This region covers the saturation from 0 to approximately 10 or 20%. In contrast, in high-porosity rocks (e.g., Murphy 1982;Cadoret et al 1998), the region that provides no or slight increase in attenuation with increasing water saturation covers a wider range of water saturation, i.e., from 0 to approximately 60 or 70%. This phenomenon is due to the fact that the high porosity rock requires more water to reveal Biot effect, which causes water-solid coupling and decoupling.…”

“…Compressional wave velocity changes little until the pore spaces are fully saturated with water because the air in partially saturated pore fluids diminishes the stiffness of the pore fluids and hardly contributes to strengthening the rock frame. On the other hand, when there is an increase in water saturation, the compressional wave attenuation-that is related to energy dissipation-tends to increase more sensitively than velocity (Gardner et al 1964;Toks} oz et al 1979;Mavko and Nur 1979;Murphy 1982;Winkler and Nur 1982;Cadoret et al 1998).…”

“…Several previous researchers have investigated the fluid effect on the wave velocity and attenuation, mainly focusing on high porosity rocks (mostly greater than 20% of porosity), such as limestone (Cadoret et al 1998) and sandstone (Murphy 1982;Winkler and Nur 1982). This study, therefore, examines how water saturation affects the attenuation characteristics of low porosity rocks.…”