Ultrasonic wave propagation in dry and brine‐saturated sandstones as a function of effective stress: laboratory measurements and modelling¹

Abstract

Abst ractCompressional and shear-wave velocities have been measured and a novel approach using digital processing employed to study wave attenuation for brine-and gassaturated sandstones, over a range of effective stresses from 5 to 60 MPa. Also measured were the complex conductivity in the brine-saturated state and permeability in the gas-saturated state over the same range of stresses as for the velocity measurements. Broadband ultrasonic pulses of P-and orthogonally polarized S-waves in the frequency range 0.3-0.8 MHz are transmitted through the specimen to be characterized for comparison with a reference (aluminium) having low attenuation. The attenuation is calculated in terms of the quality factor Q from the Fourier spectral ratios, using the frequency spectral ratios technique. The corrections necessary for the effects of diffraction due to the finite size of the ultrasonic transducers have been carried out for the case of measurements under lower confining stress. To interpret the laboratory measured velocity and attenuation data under the physical conditions of this study and to estimate the effects of pore structure, numerical modelling of velocities and attenuation as functions of the confining stress have been performed, based on the MIT model. Theoretical models based on several hypothesized attenuation mechanisms are considered in relation to laboratory data of the effects of confining pressure, fluid saturation and pore structure on attenuation. Numerical calculations using these models with the experimental data indicate that friction on thin cracks and grain boundaries is the dominant attenuation mechanism for dry and brine-saturated sandstones at low effective stresses for the frequencies tested. However, for brine-saturated sandStones at moderately high effective stresses, fluid flow could play a more important role in ultrasonic S-wave attenuation, depending on the pore structure of the sample.

Petrophysical properties of bioclastic platform carbonates: implications for porosity controls during burial

“…At similar porosities, higher compressional velocities are observed in saturated than in dry conditions (Fig. 8), in agreement with previous studies (Winkler and Nur,1979;Yale,1985;Tao et al,1995;Adam et al, 2006).…”

“…10). Scatter in the V p values might be due to variations in types of cement , and pore geometry (Tao et al, 1995;Dü rrast and Siegesmund, 1999;Sayers, 2008;Verwer et al, 2008). V p -V s ratio is constant during tests and tends to be higher in the saturated plugs.…”

Acoustic wave propagation and permeability in sandstones with systems of aligned cracks

“…The transducer holders are similar to those described by Tao & King (1990), with band widths in the range 450-800kHz for P-wave and 350-750kHz for S-wave pulses. A block diagram of the ultrasonic measurement system is shown in Fig.…”

“…The system, described in a preliminary technical note by King et al (1995) load-actuating pressure characteristics) and ultrasonic transducer holders are mounted to provide orthogonal stresses on the cubic rock specimen in the horizontal plane. The horizontal principal stresses may be servo-controlled using facilities associated with a Schenk compression testing machine.…”

Uso da sísmica de reflexão de alta resolução e da sonografia na exploração mineral submarina

“…Por isso suas propriedades elásticas medidas são um valor resultante das propriedades elásticas de seus componentes, relativo a sua presença e volume. Tao et al (1995) mostrou que a propagação de ondas P em sedimentos marinhos está ligada ao módulo de compressão do fluido intersticial. Este fato é claramente observado ao compararmos a velocidade de propagação em um sedimento saturado com água e outro, com o mesmo conteúdo de sólidos mas contendo gás.…”