裂缝性多孔介质纵波频变特性研究

Frequency-dependent characteristics of P-wave in fractured porous media

各向异性介质地震波衰减的研究一般只针对横向各向同性介质,即包含一组垂向裂缝。实际储层中往往存在多组正交或斜交裂缝,因此研究含多组裂缝储层的地震波频散和衰减非常必要。Chapman建立了包含两组裂缝的中观尺度喷射流模型,但是没有讨论两组裂缝对特征频率、频散和衰减幅度的影响。为此,建立了计算含多组裂缝储层纵波频变特性的方法。首先对Chapman模型数值模拟,讨论裂缝参数对特征频率和频散、衰减幅度的作用;然后分析Chapman模型和标准线性固体模型之间的关系;最后建立利用广义标准线性固体模型计算包含多组裂缝储层的频变纵波模量的方法,即利用Chapman模型和各向异性Gassmann方程得到低频模量,利用Chapman模型得到高频模量。所提方法利用低频、高频极限模量和模量损失表征地震波的频散和衰减特性,其中模量损失可表征每组裂缝的贡献,并得出以下认识:(1)裂缝方位和地震波入射方向只影响频散幅度和衰减幅度,而不影响频散频带和衰减频带;(2)不同裂缝发育情况的纵波速度V(或纵波逆品质因子Q_(P)^(-1))曲线间存在代数关系,因此可以利用单组裂缝的V(或Q_(P)^(-1))曲线表示两组甚至多组裂缝的V(或Q_(P)^(-1))曲线;(3)当地震波传播方向与裂缝平行时,V随频率的变化很小,P波衰减最小。所提方法对于研究各向异性介质的地震波场特征及地震响应机理具有一定实际意义。

Most studies on seismic wave attenuation in anisotropic media only focus on transversely isotropic media containing a set of fractures.Since there are often several sets of orthogonal or oblique fractures developed in actual reservoirs,it is necessary to study the seismic wave dispersion and attenuation of reservoirs with multiple sets of fractures.Chapman builds a mesoscale squirt flow model including two sets of fractures but does not discuss the influence of those fractures on the characteristic frequency,dispersion,and attenuation magnitude.Therefore,a method for calculating the P-wave frequency-dependent characteristics of reservoirs with multiple sets of fractures is proposed.Firstly,the Chapman model is numerically simulated,and the effects of fracture parameters on characteristic frequency,dispersion,and attenuation amplitude are discussed.Then,the relationship between the Chapman model and the standard linear solid model is analyzed.Finally,a method is developed to calculate the frequency-dependent P-wave modulus of reservoirs with multiple sets of fractures by the generalized standard linear solid model.This means that the low-frequency modulus is obtained by the Chapman model and anisotropic Gassmann equation,and the highfrequency modulus is obtained by the Chapman model.The proposed method employs the low-frequency and high-frequency limit modulus and modulus loss to characterize the frequency dispersion and attenuation properties of seismic waves.The modulus loss can represent the contribution of each set of fractures,and the following conclusions are obtained.(1)Fracture orientations and seismic wave incident direction only affect the velocity(frequency dispersion)and attenuation magnitude but do not affect the frequency dispersion and attenuation frequency band.(2)The P-wave velocity VP(or P-wave inverse quality factor Q_(P)^(-1))curves of different fracture development conditions have some algebraic relationship.Therefore,the VP(or Q_(P)^(-1))curves of a single set of fractures can be utilized to represent the VP(or Q_(P)^(-1))curves of two or more sets of fractures.(3)When the propagation direction of seismic waves is parallel to the fractures,the variation of VPwith frequency is slight,and the attenuation of P-wave is the smallest.The proposed method is of practical significance for studying the seismic wave field characteristics and seismic response mechanism of anisotropic media.