Study On Velocity Dispersion And Attenuation Based On Laboratory Measurements And Rock Physics Theories

Seismic velocity dispersion and attenuation provides a siginificant theoretical basis for fluid identification and reservoir prediction.It is also an important but difficult frontier problem in seismic petrophysics.However,the factors that affect the velocity dispersion and attenuation are complex.At present,the rock physics experimental and theoretical studies on this topic are still relatively weak-based and unsubstantial.More in-depth research and discussion are urgly required to clarify the influence of pore type,fluid properties,pressure and other factors on the velocity dispersion and attenuation,and further to reveal its internal physical mechanism and figure out the characteristics of the AVO response from dispersive and attenuative media,so that the experimental and theoretical basis for reservoir prediction and fluid identification can be established.To this end,we combine the multi-band direct laboratory measurement,the rock physics modeling and the frequency-dependent AVO technique together.Using the rock physics experimental data,an inversion method for calculating pore aspect ratio distribution using the thought experiment is developed.On this basis,we establish an extended Gurevich-Biot model based on the classical Gurevich’s squirt model,so as to reveal the scientific law of velocity dispersion and attenuation for real rock samples.Finally,driven by the rock physics experiment and theory,the inversion of velocity dispersion gradient and curves is implemented.Firstly,based on the multi-band direct laboratory measurements,the influence of the pore structure and the pore fliud on velocidy dispersion and attenuation is analyzed.The experimental results show that the characteristic frequency of velocity dispersion and attenuation is closely related to the pore structure of the rock,that is,the fractured rock has a lower characteristic frequency while the rock with more siffer pores has a higher frequency.Moreover,the higher the pore fluid viscosity is,the greater the dispersion and attenuation are,which verifies the effectiveness of the dispersion gradient as a fluid indicator.Then,the applicability of several classical rock physics models is discussed and assessed.It is found that the classical models usually consider only one certain mechnism and contain quantities hard to obtain in laboratory,and thus only limits to ideal cases.For general real rocks,the velocity dispersion and attenuation predicted by these theoretical models have difficulties to match with the measured data.In view of this,we develop a new extended Gurevich-Biot squirt model based on the classical Gurevich’s model by introducing fliud flow effects between different pores.This extended model extracts the aspect ratio and porosity of different pores from the pressure dependency of ultrasonic velocities in dry rocks using thought experiment.Then,the squirt-flow effects between the micro-cracks and between the cracks and the stiff pores are modeled according to the pressure relaxation in the pore space.The global fluid flow in the stiff pores is described by the Biot theory.The modeled results indicate that the global Biot flow often occurs at ultrasonic or even higher frequencies,whileas the squirt flow,which is closely related to the pore structure,plays a role at the whole frequency range.In addition,the velocity dispersion and attenuation predicted by the extended Gurevich-Biot model show a more consistent trend with the measured data as compared to the classical model.On the basis of the petrophysical studies,we further discuss the influence of velocity dispersion and attenuation on seismic AVO response,and then achieve frequency-dependent AVO inversion and velocity dispersion inversion.Firstly,we derive the reflection coefficients of a layered structure based on the wave equations and analyze the characteristics of AVO response from the dispersive and attenuative media.Secondly,taking the dispersion gradient as a fluid indicator,we construct a unified equation for frequency-dependent AVO inversion based on the spectral decomposition technique by combining several classical Zoeppritz approximations.The frequency-dependent AVO inversion is then applied to the real seismic field data to predict the location of reservoir.Finally,under the constraint of the experimental data,the global optimization ideas of Metropolis and Heat bath simulated annealing methods are combined to realize the fine inversion of reservoir velocity dispersion and the reservoir thickness prediction.