Q factor of elastic wave propagation in poroelastic medium

Abstract

Sonic logging has been widely used for many years to understand physical properties of hydrocarbon reservoirs. When gaseous phase exists in the formation fluid, the compressional waves traveling through the formation could be strongly attenuated due to low bulk modulus of gas in the fluid, while the shear waves are not. For acquiring physical properties of fluid in the formation, Biot physics or poroelastic analysis could be the best method. Among the available technologies, quality factors based on the Biot's equation could be used. Although the Biot's theory considers the viscous attenuation induced at the interface between rocks and pore fluids, the intrinsic attenuation caused by the internal friction in the rock matrix is ignored. The intrinsic attenuation indicates the energy loss due to the heat generated by the friction between soil grains. In the present study, we investigate how large are the effects of the intrinsic attenuation of compressional waves through the evaluation of the reservoir properties based on the quality factor. We employ a 2D finite-difference scheme to simulate seismic wave propagation in a poroelastic medium. The intrinsic attenuation is included in our model by using the filter of frequency-independent quality factor (constant-Q). We then compare the results with and without the intrinsic attenuation in our numerical simulations. Our results clearly show that on compressional and shear waves, the amplitude and phase of the waveforms are strongly affected by the intrinsic attenuation, and the calculated quality factors are dramatically different from each other. We conclude that the evaluations of hydrocarbon reservoir require the consideration of the intrinsic attenuation as well as the viscous attenuation predicted by the Biot's theory.