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

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.