An experimental investigation of factors influencing compressional‐ and shear‐wave velocities and attenuations in tight gas sandstones

Abstract

Results are presented for compressional and shear velocities and attenuations in fully brine-saturated tight gas cores with porosities from 3 to 11.9 percent and clay contents from 1 to 38 percent. The influence of porosity, clay content, frequency, and stress on velocities and attenuations were examined using the amplitude spectra of P-and S-waves in the frequency domain. Attenuations of samples were obtained using the spectral ratio method. For a few selected samples the attenuations were also measured using the length correlation method and these results were compared with the spectral ratio results.In tight gas sandstones, the attenuations obtained were 2 to 5 times greater than the attenuation obtained for Berea sandstone. In general, the presence of clay softens the rock grain contacts causing smaller values of compressional (Vp) and shear (Vs) velocities as the clay content increases. However, the VplVs ratio was found to increase with clay content. Compressionaland shear-wave amplitude spectra exhibited a shift in peak frequency toward lower frequencies for samples with higher clay content when compared to clean samples. Velocities and attenuations were found to be frequency dependent, but the positive slope of both compressional and shear attenuations indicate that scattering starts to dominate at the lower frequency end of the ultrasonic measurements. Both V p and Vs increased while both compressional and shear attenuations decreased when stress was increased.

Prediction of petrophysical properties from seismic quality factor measurements

“…Their quality factor-clay relationship has been confirmed by Tutuncu et al (1994) for tight gas sandstones measured at ultrasonic frequencies in the laboratory. Tang & Strrack (1995) have demonstrated a significant difference between the quality factors of clean and shaley sandstones from sonic log data.…”

Estimates of Formation Sound Speed From Ultrasonic Reflections

“…This may be the result of a grain contact "softening" effect caused by the presence of the clays. Tutuncu et al 9 also observed this "softening" effect in experiments.…”

“…Tutuncu et al 9 describe these experiments. An acoustic gel had been applied to the ends of the cores to provide acoustic coupling with the load-frame transducers.…”

Microplasticity effect in low-velocity zone induced by seismic wave

“…Usual data show decrease of wave velocity, the increase of attenuation with increasing amplitude (Mavko, 1979;Ostrovsky and Johnson, 2001;Tutuncu et al, 1994;Winkler et al, 1979). Other data show unusual increase of wave velocity and the decrease in attenuation (Johnston and Toksoz, 1980;Mashinskii, 2004;Mashinskii, 2007a;Zaitsev et al, 1999).…”