Experimental study of Young’s modulus dispersion and attenuation in fully saturated sandstones

Abstract

Although seismic wave dispersion and attenuation have been found to occur in sedimentary rocks, it remains challenging to experimentally observe these effects. A new experimental setup has been developed to measure the Young's modulus and Poisson's ratio of rocks over a wide range in pressure (P c ∈ ½0; 30 MPa) and frequency (f ∈ ½5.10 −3 ; 10 2 Hz). Calibration with standard samples determined the following: (1) no dependence of the apparatus to pressure and frequency and (2) a good fit between published data and the measured and inferred elastic properties. The measured Young's modulus dispersion and attenuation of Plexiglas were also consistent with the published data. The Young's modulus and the attenuation of Fontainebleau sandstone samples saturated by water and glycerin were then measured. Although small variations were observed for one sample, the second one exhibited strong pressure-and frequencydependent variations of Young's modulus and attenuation. A frequency-dependent fluid flow was simultaneously measured. The characteristic frequency for these variations was highly fluid dependent. Accounting for the in situ fluids' viscosity using an apparent frequency parameter, we determined the Young's modulus and attenuation of a fluid-saturated Fontainebleau sandstone over an apparent frequency band of f à ∈ ½10 −3 ; 10 5 Hz. The measurements under water and glycerin saturation compared favorably, and two frequency-dependent phenomena were observed that were interpreted as the drained/undrained and undrained/unrelaxed transitions. The undrained/unrelaxed transition occurred in a large frequency range, which was attributed to a distribution in aspect ratio of the rock's microcracks.

Dispersions and attenuations in a fully saturated sandstone: Experimental evidence for fluid flows at different scales

“…The frequency dependence is obtained by using Zener rheologic model. Figure modified from Pimienta et al (2015b;2016a).…”

“…Using again the hydraulic properties of the sample, now in terms of a pseudoconsolidation parameter (Pimienta et al, 2015b), evidence for the disappearance of global fluid flow is observed at about f * = 100 Hz. Consistent with the isotropic solicitation (Figure 3c), the sample is thus undrained at this value of f *.…”

“…The frequency range attained by the experimental setups is (1) f ∈ [10 -3 ; 10 0 ] Hz for the isotropic solicitation ( Figure 2a); and (2) f ∈ [10 -3 ; 10 2 ] Hz for the axial solicitation ( Figure 2b). More details can be found on the calibrations, and measuring procedure, in Pimienta et al (2015aPimienta et al ( , 2015bPimienta et al ( , 2016a.…”

“…The experimental evidence for no macroscopic fluid flow is highlighted in green. Figure modified fromPimienta et al (2015b).…”

Dynamic bulk and shear moduli due to grain-scale local fluid flow in fluid-saturated cracked poroelastic rocks: Theoretical model

“…Recently, laboratory measurements by Pimienta et al (2015a, b) show that pore microstructural effect (specifically, the squirt-flow mechanism) are likely to play a role in producing seismic attenuation in fluid-saturated rock samples (e.g., see data of Fo9 in Pimienta et al, 2015a; see also Pimienta et al, 2015b).…”