Influence of fluid saturation in soft pores on elastic modulus dispersion and attenuation of rocks under partial saturation condition at seismic frequencies
A micro-and mesoscopic dual-scale fluid flow model with different soft pore saturations was developed in this paper,so as to reveal the influence of micro-and mesoscopic dual-scale fluid flow on the modulus dispersion and attenuation of partially saturated rocks at seismic frequencies.Based on the Betti-Rayleigh reciprocity theorem,frequency-dependent wet frame moduli with different soft pore saturations were derived,and then the wet frame moduli were incorporated into the White spherical patchy saturation model to obtain the model.The numerical calculation using the model showed that:(1)When the saturation of the rock was constant,the modulus dispersion and attenuation of the partially saturated rock increased as the soft pore saturation increased.When the soft pores were completely saturated,the modulus dispersion and attenuation reached their maximum values.On the other hand,there were two attenuation peaks:one at lower frequencies was related to the mesoscopic flow,and the other at higher frequencies was related to the microscopic flow.(2)When the difference between the characteristic frequencies of the mesoscopic and microscopic fluid flows was large,the different saturations of the soft pores had a small effect on the mesoscopic fluid flow but a larger effect on the microscopic fluid flow.When the soft pores were fully saturated,the effect on the microscopic fluid flow was the largest.As the characteristic frequencies of the mesoscopic and microscopic fluid flows approached each other,the effect of soft pore saturation on the mesoscopic fluid flow increased,and the effect was enhanced with an increase in soft pore saturation.(3)Compared with the micro-and mesoscopic dual-scale model of Li et al.(2018),the model of Li et al.(2018)can effectively predict the variation in the moduli of partially saturated rocks at high frequencies,but may fail at seismic and sonic frequencies.After modification of the model,it could effectively predict the variations in the moduli of partially saturated rocks at different frequencies.Finally,the newly developed model reasonably explained the attenuation data of partially saturated sandstone,and the effectiveness of the model was verified,suggesting that when the rock is partially fluid-saturated,the partial saturation of soft pores is more consistent with reality.
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