Dispersion measurement and analysis of sandstone

Abstract:

Velocity dispersion is a common phenomenon for fluid-charged porous rocks, meaning the propagation velocities of acoustic wave in a rock vary with frequency. Previous research have shown that pore structures and fluid content of a porous rock can be revealed by studying velocity dispersion.

In this research, we study velocity dispersion in a low-frequency range (Hz-KHz) relative to ultrasonic frequency band (MHz) using 30 sandstone samples with various fluid viscosities and confining pressures. For a certain rock sample, different velocities are obtained by frequency scanning in a range of 1-3000Hz using a low-frequency acoustic system modified from Batzle (2006).

The experiment results show that: (1) the obtained velocity dispersion curves do not well match the theoretical models, such as Biot model, BISQ model, classical squirt-flow model, dual porosity model, etc., where the former is sometimes linear, while the latter usually has a ladder shape; (2) with increasing confining pressure, the degree of velocity dispersion decreases; (3) the characteristic frequency of the curve moves with increasing fluid viscosity.

We also find that the pore structure (especially the aspect ratio) has a great influence on the characteristic frequency of the model. Therefore, we introduce the pores with different aspect ratios to the classical squirt-flow model to simulate the dispersion and attenuation of the samples.

The new model and the experiment results are consistent in trend generally. Moreover, the results of some samples are in good agreement with numerical value derived from the new theoretical model.

Through the low-frequency experiments, we conclude that: (1) fluid viscosity has a great influence on velocity dispersion, specifically, the characteristic frequency of the dispersion curve moves toward low-frequency direction with an increasing fluid viscosity; (2) squirt-flow effect between different pores may be the main reason of velocity dispersion in seismic frequency band.

References

[1] Batzle, et al. 2006, Geophysics Journal of the Society of Exploration Geophysicists, 2006, 71(1):N1.[2] David & Zimmerman. 2012, Journal of Geophysical Research Atmospheres, 117(B7), 7210.[3] Gurevich, et al. 2010. Geophysics, 75(6), N109-N120.[4] Yin H, et al. Journal of Geophysical Research Solid Earth, 2017.