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1.
为研究裂缝、裂隙介质中波致流引起的衰减,将裂缝看作背景孔隙岩石中非常薄且孔隙度非常高的层状介质,并等价成White周期层状模型.分别考虑不同类型的裂隙和孔隙之间的挤喷流影响,结合改进的Biot方程,推导得到裂缝裂隙介质的刚度与频率的关系.当缝隙中饱含流体时,介质的衰减和速度频散受裂缝、孔隙之间和裂隙、孔隙之间流体流动的显著影响.在低频极限下,裂缝裂隙介质的性质由各向异性Gassmann理论和挤喷流模型获得;而在非常高的频率时,由于缝隙中的压力来不及达到平衡,波致流的影响可忽略.分析表明,裂隙密度主要影响波的衰减,而裂隙纵横比主要控制优势衰减频率和速度显著变化的频率范围;由于不同裂隙的衰减机制不同,衰减和速度频散大小有所差异,但基本趋势相同. 相似文献
2.
基于Biot理论,考虑液相的黏弹性变形和固液相接触面上的相对扭转,提出了含黏滞流体VTI孔隙介质模型.从理论上推导出,在该模型中除存在快P波、慢P波、SV波、SH波以外,还将存在两种新横波－慢SV波和慢SH波.数值模拟分析了6种弹性波的相速度、衰减、液固相振幅比随孔隙度、频率的变化规律以及快P波、快SV波的衰减随流体性质、渗透率、入射角的变化规律.结果表明慢SV波和慢SH波主要在液相中传播,高频高孔隙度时,速度较高;大角度入射时,快P波衰减表现出明显的各向异性,而快SV波的衰减则基本不变;储层纵向和横向渗透率存在差异时,快SV波衰减大的方向渗透率高. 相似文献
3.
Jixin Deng Shangxu Wang Gengyang Tang Jianguo Zhao Xiangyang Li 《Studia Geophysica et Geodaetica》2013,57(3):482-506
In sedimentary rocks attenuation/dispersion is dominated by fluid-rock interactions. Wave-induced fluid flow in the pores causes energy loss through several mechanisms, and as a result attenuation is strongly frequency dependent. However, the fluid motion process governing the frequency dependent attenuation and velocity remains unclear. We propose a new approach to obtain the analytical expressions of pore pressure, relative fluxes distribution and frame displacement within the double-layer porous media based on quasi-static poroelastic theory. The dispersion equation for a P-wave propagating in a porous medium permeated by aligned fractures is given by considering fractures as thin and highly compliant layers. The influence of mesoscopic fluid flow on phase velocity dispersion and attenuation is discussed under the condition of varying fracture weakness. In this model conversion of the compression wave energy into Biot slow wave diffusion at the facture surface can result in apparent attenuation and dispersion within the usual seismic frequency band. The magnitude of velocity dispersion and attenuation of P-wave increases with increasing fracture weakness, and the relaxation peak and maximum attenuation shift towards lower frequency. Because of its periodic structure, the fractured porous media can be considered as a phononic crystal with several pass and stop bands in the high frequency band. Therefore, the velocity and attenuation of the P-wave show an oscillatory behavior with increasing frequency when resonance occurs. The evolutions of the pore pressure and the relative fluxes as a function of frequency are presented, giving more physical insight into the behavior of P-wave velocity dispersion and the attenuation of fractured porous medium due to the wave-induced mesoscopic flow. We show that the specific behavior of attenuation as function of frequency is mainly controlled by the energy dissipated per wave cycle in the background layer. 相似文献
4.
Average elastic properties of a fluid‐saturated fractured rock are discussed in association with the extremely slow and dispersive Krauklis wave propagation within individual fractures. The presence of the Krauklis wave increases P‐wave velocity dispersion and attenuation with decreasing frequency. Different laws (exponential, power, fractal, and gamma laws) of distribution of the fracture length within the rock show more velocity dispersion and attenuation of the P‐wave for greater fracture density, particularly at low seismic frequencies. The results exhibit a remarkable difference in the P‐wave reflection coefficient for frequency and angular dependency from the fractured layer in comparison with the homogeneous layer. The biggest variation in behaviour of the reflection coefficient versus incident angle is observed at low seismic frequencies. The proposed approach and results of calculations allow an interpretation of abnormal velocity dispersion, high attenuation, and special behaviour of reflection coefficients versus frequency and angle of incidence as the indicators of fractures. 相似文献
5.
The effect of orthorhombic anisotropy and its implication for oil recovery and reservoir exploitation 总被引：1，自引：0，他引：1
Jamal M. Assad 《Geophysical Prospecting》2005,53(1):121-129
Results of an experimental study of shear‐ and compressional‐wave propagation in an orthorhombically anisotropic medium are presented. The experiments were performed on a physical model consisting of two sets of fractures. The first set consisted of orientated rubber inclusions simulating weak material‐filled cracks. The second set consisted of a system of closely spaced parallel fractures simulated by thin plates of epoxy resin, superimposed orthogonally on the first set. Three cases of fracture orientations within the model were identified and studied. Case 1 is analogous to a jointed fracture reservoir with one vertical set of fluid‐filled cracks or fractures and one non‐filled horizontal set. This case is referred to as JFV. Case 2 is analogous to a double fracture reservoir with one horizontal set of fluid‐filled fractures or cracks and one non‐filled vertical set. This is referred to as DFH. Case 3 is analogous to a double fracture reservoir with two vertical sets of fractures or cracks, with only one fluid‐filled. Case 3 is referred to as DFV. A pulse transmission method was performed on all three modelled cases along the three principal axes. A directional variation in the compressional‐ and shear‐wave velocities, as well as distinct shear‐wave splitting, was observed. The elastic constants for each case were determined and differences between them were noted and compared with the controlled results of both layered (transverse isotropy, TI) and vertically fractured (azimuthally anisotropic models, VF) media. The differences in elastic moduli and velocities indicate the potential of recognizing the different fracture orientations and suggest an approach to designing a method of drilling to further enhance oil recovery and reservoir exploitation. 相似文献
6.
地下多孔介质中的孔隙类型复杂多样,既有硬孔又有扁平的软孔.针对复杂孔隙介质,假设多孔介质中同时含有球型硬孔和两种不同产状的裂隙(硬币型、尖灭型裂隙),当孔隙介质承载载荷时,考虑两种不同类型的裂隙对于孔隙流体压力的影响,建立起Biot理论框架下饱和流体情况含混合裂隙、孔隙介质的弹性波动方程,并进一步求取了饱和流体情况下仅由裂隙引起流体流动时的含混合裂隙、孔隙介质的体积模量和剪切模量,随后,在此基础上讨论了含混合裂隙、孔隙介质在封闭条件下地震波衰减和频散的高低频极限表达式.最后计算了给定模型的地震波衰减和频散,发现地震波衰减曲线呈现"多峰"现象,速度曲线为"多频段"频散.针对该模型分析讨论了渗透率参数、裂隙纵横比参数以及流体黏滞性参数对于地震波衰减和频散的影响,表明三个参数均为频率控制参数. 相似文献
7.
Measurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity. Analysis of the data is based upon equivalent-medium theories that describe the elastic response of a rock containing cracks or fractures in the long-wavelength limit. Conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures. The latter, however, control the fluid flow in many subsurface reservoirs. Therefore, the fracture size is essential information for reservoir engineers. In this study we apply a new equivalent-medium theory that models frequency-dependent anisotropy and is sensitive to the length scale of fractures. The model considers velocity dispersion and attenuation due to a squirt-flow mechanism at two different scales: the grain scale (microcracks and equant matrix porosity) and formation-scale fractures. The theory is first tested and calibrated against published laboratory data. Then we present the analysis and modelling of frequency-dependent shear-wave splitting in multicomponent VSP data from a tight gas reservoir. We invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves. The derived fracture length matches independent observations from borehole data. 相似文献
8.
地震波在含孔隙、裂隙斑块饱和介质传播过程中会诱发多个尺度孔隙流体流动而产生衰减和速度频散.在含有宏观尺度“Biot流”和介观尺度“局域流”衰减诱导机制的周期性层状孔隙介质模型基础上,引入了微观尺度硬币型和尖灭型裂隙“喷射流”的影响,构建了周期性层状含孔隙、裂隙介质模型.利用双解耦弹性波动方程的方法数值计算了该模型地震频带的纵波衰减和速度频散并与周期性层状孔隙介质模型做了对比研究.分析了该模型在不同裂隙参数（裂隙密度、裂隙纵横比）及裂隙体积含量下的纵波衰减和频散特征,裂隙密度越高对于纵波衰减和频散的影响越大,裂隙纵横比越小,由裂隙引起的纵波衰减部分向高频段移动,裂隙体积含量越少,纵波衰减先降低后小幅增加再降低,频散速度增加,并逐渐接近于周期性层状孔隙介质模型的纵波衰减和频散速度曲线.最后研究了周期性层状含孔隙、裂隙介质模型有效平面波模量的高低频极限以及流固相对位移在该模型中的分布特征. 相似文献
9.
Wave‐induced fluid flow plays an important role in affecting the seismic dispersion and attenuation of fractured porous rocks. While numerous theoretical models have been proposed for the seismic dispersion and attenuation in fractured porous rocks, most of them neglect the wave‐induced fluid flow resulting from the background anisotropy (e.g. the interlayer fluid flow between different layers) that can be normal in real reservoirs. Here, according to the theories of poroelasticity, we present an approach to study the frequency‐dependent seismic properties of more realistic and complicated rocks, i.e. horizontally and periodically layered porous rock with horizontal and randomly orienting fractures, respectively, distributed in one of the two periodical layers. The approach accounts for the dual effects of the wave‐induced fluid flow between the fractures and the background pores and between different layers (the interlayer fluid flow). Because C33 (i.e., the modulus of the normally incident P‐wave) is directly related to the P‐wave velocity widely measured in the seismic exploration, and its comprehensive dispersion and attenuation are found to be most significant, we study mainly the effects of fracture properties and the stiffness contrast between the different layers on the seismic dispersion and attenuation of C33. The results show that the increasing stiffness contrast enhances the interlayer fluid flow of the layered porous rocks with both horizontal and randomly orienting fractures and weakens the wave‐induced fluid flow between the fractures and the background pores, especially for the layered porous rock with horizontal fractures. The modelling results also demonstrate that for the considered rock construction, the increasing fracture density reduces the interlayer fluid flow while improves the dispersion and attenuation in the fracture‐relevant frequency band. Increasing fracture aspect ratio is found to reduce the dispersion and attenuation in the fracture‐relevant frequency band only, especially for the layered porous rock with horizontal fractures. 相似文献
10.
Brittle deformation of oceanic lithosphere due to thermal stress is explored with a numerical model, with an emphasis on the spacing of fracture zones. Brittle deformation is represented by localized plastic strain within a material having an elasto-visco-plastic rheology with strain softening. We show that crustal thickness, creep strength, and the rule governing plastic flow control the formation of cracks. The spacing of primary crack decreases with crustal thickness as long as it is smaller than a threshold value. Creep strength shifts the threshold such that crust with strong creep strength develops primary cracks regardless of crustal thicknesses, while only a thin crust can have primary cracks if its creep strength is low. For a thin crust, the spacing of primary cracks is inversely proportional to the creep strength, suggesting that creep strength might independently contribute to the degree of brittle deformation. Through finite versus zero dilatation in plastic strain, associated and non-associated flow rule results in nearly vertical and V-shaped cracks, respectively. Changes in the tectonic environment of a ridge system can be reflected in variation in crustal thickness, and thus related to brittle deformation. The fracture zone-free Reykjanes ridge is known to have a uniformly thick crust. The Australian-Antarctic Discordance has multiple fracture zones and thin crust. These syntheses are consistent with enhanced brittle deformation of oceanic lithosphere when the crust is thin and vice versa. 相似文献
11.
为了检测定向裂隙介质中横波分裂的方位属性特征,分析地震属性随裂隙密度和方位变化,采用人工吸收边界和反周期扩展边界,用伪谱法获得不同裂隙密度和不同方位地质模型三分量地面记录;应用时频分析和剪切波偏振分析研究由于裂隙方位和密度引起的横波分裂.结果显示,裂隙密度和方位决定着横波分裂的时差和偏振.快慢横波的延迟时间随裂隙密度增大而增加,不同方位相同裂隙密度的横波分裂时差有微小的变化.在45°方位检测时间延迟时间最大.通过时频分析,可以看到不同方位的瞬时主频有显著的变化,在横波分裂处瞬时主频有明显变化.因此,瞬时主频和快横波的偏振以及延迟时间可以作为裂隙方位和密度的指示. 相似文献
12.
The presence of fractures in fluid‐saturated porous rocks is usually associated with strong seismic P‐wave attenuation and velocity dispersion. This energy dissipation can be caused by oscillatory wave‐induced fluid pressure diffusion between the fractures and the host rock, an intrinsic attenuation mechanism generally referred to as wave‐induced fluid flow. Geological observations suggest that fracture surfaces are highly irregular at the millimetre and sub‐millimetre scale, which finds its expression in geometrical and mechanical complexities of the contact area between the fracture faces. It is well known that contact areas strongly affect the overall mechanical fracture properties. However, existing models for seismic attenuation and velocity dispersion in fractured rocks neglect this complexity. In this work, we explore the effects of fracture contact areas on seismic P‐wave attenuation and velocity dispersion using oscillatory relaxation simulations based on quasi‐static poroelastic equations. We verify that the geometrical and mechanical details of fracture contact areas have a strong impact on seismic signatures. In addition, our numerical approach allows us to quantify the vertical solid displacement jump across fractures, the key quantity in the linear slip theory. We find that the displacement jump is strongly affected by the geometrical details of the fracture contact area and, due to the oscillatory fluid pressure diffusion process, is complex‐valued and frequency‐dependent. By using laboratory measurements of stress‐induced changes in the fracture contact area, we relate seismic attenuation and dispersion to the effective stress. The corresponding results do indeed indicate that seismic attenuation and phase velocity may constitute useful attributes to constrain the effective stress. Alternatively, knowledge of the effective stress may help to identify the regions in which wave induced fluid flow is expected to be the dominant attenuation mechanism. 相似文献
13.
In fractured reservoirs, seismic wave velocity and amplitude depend on frequency and incidence angle. Frequency dependence is believed to be principally caused by the wave‐induced flow of pore fluid at the mesoscopic scale. In recent years, two particular phenomena, i.e., patchy saturation and flow between fractures and pores, have been identified as significant mechanisms of wave‐induced flow. However, these two phenomena are studied separately. Recently, a unified model has been proposed for a porous rock with a set of aligned fractures, with pores and fractures filled with two different fluids. Existing models treat waves propagating perpendicular to the fractures. In this paper, we extend the model to all propagation angles by assuming that the flow direction is perpendicular to the layering plane and is independent of the loading direction. We first consider the limiting cases through poroelastic Backus averaging, and then we obtain the five complex and frequency‐dependent stiffness values of the equivalent transversely isotropic medium as a function of the frequency. The numerical results show that, when the bulk modulus of the fracture‐filling fluid is relatively large, the dispersion and attenuation of P‐waves are mainly caused by fractures, and the values decrease as angles increase, almost vanishing when the incidence angle is 90° (propagation parallel to the fracture plane). While the bulk modulus of fluid in fractures is much smaller than that of matrix pores, the attenuation due to the “partial saturation” mechanism makes the fluid flow from pores into fractures, which is almost independent of the incidence angle. 相似文献
14.
本研究对涪陵地区井下产气的龙马溪组页岩岩芯,采用三轴压机压裂制造裂缝,用工业CT扫描压裂前后岩芯,应用图像识别技术统计出裂缝体密度,又用超声脉冲透射法测定样品破裂前后裂缝方向上的纵波、单偏振方向与裂缝不同夹角的横波,来研究裂缝对页岩岩芯声学性质的影响.实验结果表明:压裂后样品的纵波速度略微降低,只有含较多内部裂隙的150#样品纵波速度减小幅度明显.压裂前后样品的纵波波形差别不大,纵波主频随裂缝体密度呈下降趋势,即压裂后纵波频谱主频向低频端移动.压裂前横波速度随自身与裂隙方位角变化而变化,与0°和180°相比,在45°和135°时略微减小,在90°时速度降低幅度最为明显并且发生相位反转.典型样品的横波主频随偏振方向与裂缝夹角的增大而逐渐向低频移动;压裂后,横波频谱杂乱,出现多处局部峰值,速度和主频较压裂前更低,平均横波波速随裂缝体密度呈明显减小趋势.平均纵横波速比随裂缝密度呈近线性增加,表明其与裂缝体密度有较强相关性. 相似文献
15.
Gui-Hong Guo Jian-Ping Yan Zhi Zhang José Badal Jian-Wu Cheng Shuang-Hu Shi Ya-Wei Ma 《应用地球物理》2018,15(2):299-310
Elastic wave propagation and attenuation in porous rock layers with oriented sets of fractures, especially in carbonate reservoirs, are anisotropic owing to fracture sealing, fracture size, fracture density, filling fluid, and fracture strike orientation. To address this problem, we adopt the Chapman effective medium model and carry out numerical experiments to assess the variation in P-wave velocity and attenuation, and the shear-wave splitting anisotropy with the frequency and azimuth of the incident wave. The results suggest that velocity, attenuation, and anisotropy vary as function of azimuth and frequency. The azimuths of the minimum attenuation and maximum P-wave velocity are nearly coincident with the average strike of the two sets of open fractures. P-wave velocity is greater in sealed fractures than open fractures, whereas the attenuation of energy and anisotropy is stronger in open fractures than sealed fractures. For fractures of different sizes, the maximum velocity together with the minimum attenuation correspond to the average orientation of the fracture sets. Small fractures affect the wave propagation less. Azimuth-dependent anisotropy is low and varies more than the other attributes. Fracture density strongly affects the P-wave velocity, attenuation, and shear-wave anisotropy. The attenuation is more sensitive to the variation of fracture size than that of velocity and anisotropy. In the seismic frequency band, the effect of oil and gas saturation on attenuation is very different from that for brine saturation and varies weakly over azimuth. It is demonstrated that for two sets of fractures with the same density, the fast shear-wave polarization angle is almost linearly related with the orientation of one of the fracture sets. 相似文献
16.
Angular and Frequency-Dependent Wave Velocity and Attenuation in Fractured Porous Media 总被引：1，自引：0，他引：1
José M. Carcione Boris Gurevich Juan E. Santos Stefano Picotti 《Pure and Applied Geophysics》2013,170(11):1673-1683
Wave-induced fluid flow generates a dominant attenuation mechanism in porous media. It consists of energy loss due to P-wave conversion to Biot (diffusive) modes at mesoscopic-scale inhomogeneities. Fractured poroelastic media show significant attenuation and velocity dispersion due to this mechanism. The theory has first been developed for the symmetry axis of the equivalent transversely isotropic (TI) medium corresponding to a poroelastic medium containing planar fractures. In this work, we consider the theory for all propagation angles by obtaining the five complex and frequency-dependent stiffnesses of the equivalent TI medium as a function of frequency. We assume that the flow direction is perpendicular to the layering plane and is independent of the loading direction. As a consequence, the behaviour of the medium can be described by a single relaxation function. We first consider the limiting case of an open (highly permeable) fracture of negligible thickness. We then compute the associated wave velocities and quality factors as a function of the propagation direction (phase and ray angles) and frequency. The location of the relaxation peak depends on the distance between fractures (the mesoscopic distance), viscosity, permeability and fractures compliances. The flow induced by wave propagation affects the quasi-shear (qS) wave with levels of attenuation similar to those of the quasi-compressional (qP) wave. On the other hand, a general fracture can be modeled as a sequence of poroelastic layers, where one of the layers is very thin. Modeling fractures of different thickness filled with CO2 embedded in a background medium saturated with a stiffer fluid also shows considerable attenuation and velocity dispersion. If the fracture and background frames are the same, the equivalent medium is isotropic, but strong wave anisotropy occurs in the case of a frameless and highly permeable fracture material, for instance a suspension of solid particles in the fluid. 相似文献
17.
应用三维交错网格应力-速度有限差分方法,数值模拟了含有倾斜裂缝孔隙介质地层中点声源所激发的井孔声场问题.为满足薄裂缝计算需求,开发了不均匀网格有限差分算法,提高了计算精度及计算速度.利用将孔隙介质方程参数取为流体极限的办法来处理裂缝中的流体,实现了流体-孔隙介质界面处的差分方程统一,使界面处的计算更加灵活方便.在验证了方法正确性的基础上,分别考察了单裂缝宽度、裂缝带宽度、裂缝倾斜角度以及孔隙介质渗透率等参数的变化对井轴上阵列波形的影响并进行了分析.结果表明,声波经过裂缝时可能产生反射横波及斯通利波,后者随裂缝宽度的减小而减小,而前者随裂缝宽度的改变,变化不大,在裂缝很小(20μm)时依然存在;裂缝带的宽度、密度越大,反射斯通利波越强;当裂缝(裂缝带)倾斜时,反射横波消失,但反射斯通利波受裂缝倾斜角度的影响较小;渗透率的改变对斯通利波的衰减影响较为明显. 相似文献
18.
A model of wave propagation in fluid-saturated porous media is developed where the principal fluid/solid interaction mode affecting the propagation of the acoustic wave results from the conjunction of the Biot and the Squirt flow mechanism. The difference between the original Biot/Squirt (BISQ) flow theory and the new theory, which we call the reformulated BISQ, is that the average fluid pressure term appearing in the dynamic equation for a two component solid/fluid continuum is independent of squirt flow length. P-velocity and attenuation relate to measurable rock physical parameters: the Biot's poroelastic constants, porosity, permeability, pore fluid compressibility and viscosity. Modelling shows that velocity and attenuation dispersion obtained using the reformulated BISQ theory are of the same order of magnitude as those obtained using the original BISQ theory. Investigation on permeability effect on velocity and attenuation dispersion indicate that the transition zone in velocity and attenuation peak, occurring both at the relaxation frequency, shifts toward high frequency when permeability decreases. This behaviour agrees with Biot's theory prediction. 相似文献
19.
非常规油气藏（如致密性地层及蕴藏油气的页岩地层）的重要特征是低孔、低渗,但裂隙或裂缝比较发育.为满足非常规勘探的需求,本文将孔、裂隙介质弹性波传播理论应用于多极子声波测井的井孔声场模拟,重点研究了致密介质中裂隙发育时多极子声波的传播机理以及衰减特征.井孔声场的数值计算结果表明裂隙的存在明显改变了弹性波和井孔模式波的频散、衰减和激发强度,尤其是井壁临界折射纵波的激发谱的峰值随着频率的增加逐渐降低,这与应用经典的Biot理论下的计算结果相反,且裂隙的存在也使得饱含水和饱含气时临界折射纵波激发强度的差异变大.井孔模式波的衰减与地层横波衰减和井壁流体交换有关,井壁开孔边界下致密地层裂隙发育还使得井孔斯通利波和艾里相附近的弯曲波对孔隙流体的敏感性增强,在井壁闭孔边界条件下引起井孔模式波衰减的主要因素是裂隙引起的地层横波衰减造成的,且在截止频率附近弯曲波的衰减与地层的横波衰减一致.数值计算结果为解释非常规油气地层的声学响应特征提供了参考. 相似文献
20.
Propagation through stress-aligned fluid-filled cracks and other inclusions have been claimed to be the cause of azimuthal anisotropy observed in the crust and upper mantle.This paper examines the behavior of seismic waves attenuation caused by the internal structure of rock mass,and in particular,the internal geometry of the distribution of fluid-filled openings Systematic research on the effect of crack parameters,such as crack density,crack aspect ratio(the ratio of crack thickness to crack diameter),pore fluid properties(particularly pore fluid velocity),VP/VS ratio of the matrix material and seismic wave frequency on attenuation anisotropy has been conducted based on Hudson’s crack theory.The result shows that the crack density,aspect ratio,material filler,seismic wave frequency,and P-wave and shear wave velocity in the background of rock mass,and especially frequency has great effect on attenuation curves.Numerical research can help us know the effect of crack parameters and is a good supplement for laboratory modeling.However,attenuation is less well understood because of the great sensitivity of attenuation to details of the internal geometry.Some small changes in the characteristics of pore fluid viscosity,pore fluids containing gas and liquid phases and pore fluids containing clay can each alter attenuation coefficients by orders of magnitude.Some parameters controlling attenuation are therefore necessary to make reasonable estimations,and anisotropic attenuation is worth studying further. 相似文献