The relationships between the velocities, attenuations and petrophysical properties of reservoir sedimentary rocks¹

TLDR

The study measured compressional and shear wave velocities and attenuations in 29 water‑saturated sandstone and shale samples at 60 MPa using a pulse‑echo method, determining velocities from diffraction‑corrected traveltimes and attenuations from amplitude ratios, and reporting results as quality factors Qp and Qs. Results show Qs strongly correlates with Vs, Qp weakly with Vp but strongly with Qs, and Qp depends on the volume fraction of intra‑pore minerals, leading to the conclusion that attenuation arises from local fluid flow caused by differential dilation between the rock frame and intra‑pore mineral assemblage.

Abstract

Abstract We have measured the velocities and attenuations of compressional and shear waves in 29 water‐saturated samples of sandstones and shales at a confining pressure of 60 MPa and at frequencies of about 0.85 MHz. The measurements were made using a pulse echo method in which the samples (diameter 5 cm, length 1.5 cm to 2.5 cm) were placed between perspex buffer rods inside a high‐pressure cell. The velocity of each seismic wave was determined from the traveltime difference of equivalent phase points (corrected for diffraction effects) of the signals reflected from the top and from the base of each sample. Attenuation was determined in a similar way by comparison of the diffraction corrected amplitudes of the signals. The attenuation data are presented as ‘quality factors’: Q p and Q s for compressional and shear waves respectively. The results show that Q s is strongly correlated with V s , that Q p is weakly correlated with V p , and that Q p is strongly correlated with Q s . Q p is strongly dependent on the volume percentage of the assemblage of intra‐pore minerals, whether they are clays or carbonates. It is concluded that the attenuation mechanism is due to the local fluid flow arising from the differential dilation of the solid rock frame and the intra‐pore mineral assemblage, which is a result of their very different elastic moduli.