裂隙参数对衰减各向异性影响的数值模拟

理论、观测和实验均证实,在地壳和上地幔存在对应力变化敏感的直立裂隙,在整体上呈现方位各向异性.在Hudson裂隙理论基础上,系统全面地数值模拟了裂隙介质几何、物性和弹性波频率等参数对各向异性衰减的影响.结果显示,裂隙密度、裂隙纵横比、泊松比、裂缝填充物、弹性波频率和未破裂岩石母体纵横波速度等对各向异性衰减有着显著影响,...     

背景为各向异性的含裂缝岩石频散和衰减计算方法研究

裂缝广泛分布于各类储层岩石中,并且会显著提高储层的渗流能力.因此,裂缝的评价和表征对于提高油气产能具有重要意义.由于裂缝与背景介质之间的波致流会显著影响地震波的频散和衰减特性,所以地震勘探是评价裂缝性储层的有效手段.裂缝地震定量表征的前提是要基于含裂缝岩石中波致流对频散和衰减的影响建立含裂缝岩石物理特性与地震性质的关系.然而,目前相关的理论研究大部分基于各向同性背景这一假设,难以有效应用于常见的各向异性储层.本文针对背景为各向异性的含裂缝岩石提出了频散和衰减的计算方法.该方法首先将含裂缝岩石中的各向异性背景介质等效为层状背景介质;然后,通过分析不同频率下层状含裂缝岩石中的流体压力分布,理论计算了两个特定的中间频率并求解得到两个中间频率下的弹性参数;进一步,以计算得到的两个特定中间频率以及高低频极限下的弹性参数为基础,应用数值方法求解得到弛豫函数中的未知参数,最终实现了背景为各向异性含裂缝岩石中频散和衰减的理论模拟.通过将理论预测结果与实验测量和数值模拟结果进行对比,验证了该方法在背景为各向异性含不同分布裂缝岩石中的有效性.本文提出的方法考虑了常见的各向异性背景对含裂缝岩石频散和衰减的影...     

应力诱导饱和流体岩石弹性波各向异性

各向同性岩石中裂隙随机分布,施加单轴方向的压应力后,与压应力方向垂直的缝隙会优先闭合.应力作用下裂隙的闭合导致岩石呈现出横向各向异性的特征,进而影响弹性波的传播特征.前人对干燥岩石中应力引起的弹性波速度各向异性开展了大量研究,但未考虑充填流体对于应力诱导各向异性和弹性波传播的影响.弹性波通过应力作用的岩石时会诱导微孔隙结构上的"挤喷流"效应,从而造成弹性波显著的频散和衰减.在本文中,我们分析应力诱导各向异性岩石中挤喷流对弹性波传播的影响.利用硬币型裂隙模型,同时考虑挤喷流和应力诱导各向异性的影响,我们建立了应力作用下含流体岩石各向异性模型.基于本文的模型,我们研究了裂隙介质中单轴应力和充填流体对弹性波传播的影响.我们的模型成功预测了弹性波速度随应力的增加,同时表明了挤喷流造成纵波的各向异性频散更显著.     

周期性层状含孔隙、裂隙介质模型纵波衰减特征

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.     

孔、裂隙介质声学理论及其在油气识别应用中的新进展

近年来发展的孔隙、裂隙介质弹性波动理论是孔隙介质声学的重要进展,提高了声学参数对岩石流体识别的灵敏度.岩石中裂隙的存在增大了岩石速度的频散程度.目前发展的孔、裂隙弹性波理论可较好地模拟实验室观测到的岩石频散和衰减特征.将此理论应用于非常规油气储层的油气识别可大大提高解释的可靠性.本文综述了孔隙介质声学及孔裂隙理论的研究现状,阐述了孔、裂隙介质弹性波的传播机理;通过模拟实验室超声测量的岩石声速验证了孔、裂隙弹性波理论的可靠性.将此理论应用于测井实际数据,反演得到了地层的流体性质,反演结果显示明显提高了识别流体性质的可靠性.     

裂缝、裂隙介质衰减频散研究

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.     

压力及流体对岩石孔隙结构与黏弹性的影响规律研究:理论模型及实验观测

地质成因和构造/热应力导致地壳岩石中的孔隙结构（裂隙和粒间孔）的变化.影响岩石黏弹性的因素包括压力、孔隙度、孔隙中包含的流体和孔隙几何形状等.相对于岩石中的硬孔隙,岩石黏弹性（衰减和频散）受软孔隙（裂隙）的影响更大.本文选取三块白云岩样本,进行了不同围压和流体条件下的超声波实验测量.利用CPEM（Cracks and Pores Effective Medium,裂隙和孔隙有效介质）模型获得了岩石高、低频极限的弹性模量,并通过Zener体（标准线性体）模型将CPEM模型拓展到全频带而得到CPEM-Zener模型,用该模型拟合岩石松弛和非松弛状态下的实验数据,本文得到平均裂隙纵横比和裂隙孔隙度以及纵波速度和品质因子随频率的变化关系.结果表明,饱水岩石的平均裂隙纵横比和裂隙孔隙度均高于饱油岩石,随着压差（围压和孔隙压力的差值）的增加,饱油岩石中的裂隙首先闭合.并且压差在70 MPa以内时,随着压差增大,岩石的平均裂隙纵横比和裂隙孔隙度在饱水和饱油时的差值增大,此时流体类型对于岩石裂隙的影响越来越显著,此外,对饱水岩石,平均裂隙纵横比随压差增加而增大,这可能是由于岩石中纵横比较小的裂隙会随压差增大而逐渐趋于闭合.在饱水和饱油岩石中,裂隙孔隙度和裂隙密度都随着压差增加而减小.通过对裂隙密度和压差的关系进行指数拟合,本文获得压差趋于0时的裂隙密度,且裂隙密度随孔隙度增大而增大,增大速率随压差增加而降低.针对饱水和饱油的白云岩样本,CPEM-Zener模型预测的纵波频散随压差增大而减小,此变化趋势和实验测得的逆品质因子随压差的变化关系基本一致,由此进一步验证了模型的实用性.本研究对岩石的孔隙结构和黏弹性分析以及声波测井、地震勘探的现场应用有指导意义.     

相对渗透率对部分饱和裂隙岩石中横波频变各向异性的影响

地震波在含裂隙岩石内传播时，很大程度上会受到裂隙系统与其内的流体所影响.本文在Chapman提出的频变各向异性理论的基础上，研究气水两相流体受频率影响的程度.不同于许多局限于单一流体假设的频变各向异性理论研究，本文着重于两相流体部分饱和的情况.因此计算频变各向异性弹性常数尤为重要，本文考虑了气水两相的相对渗透率对横波速度及其衰减的影响，并引入了基于分形理论的相对渗透率模型.首先研究在不同分形维数的情况下，气水两相相对渗透率随含水饱和度的变化趋势，并且随着该变化对有效流体迁移率的影响.气体的相对渗透率与分形维数成正比例关系，而水的相对渗透率与分形维数成反比例关系.随后根据数值算例计算双相不混溶流体饱和裂隙岩石频变各向异性弹性模量的方程，得到横波速度及横波衰减，对比分析相对渗透率及分形维数的变化对横波速度和横波衰减的影响．根据算例结果可知，在不同频率下，相对渗透率对横波速度和横波衰减的影响程度不同，在中频时最大．引入不同的相对渗透率模型，能在中频时观察到其对横波速度和衰减的不同影响．     

地震波在粘弹性各向异性地壳介质中的传播及其应用研究

实际地球介质中普遍存在着多尺度、多取向、填充各种流体的孔隙、裂隙和裂缝,这种介质常常被称为双相介质或多相介质。裂隙及其填充流体的存在一方面使得介质的弹性模量(体模量和剪切模量)减小,改变流体输运性质,另一方面也导致地震波传播的各向异性、衰减和频散。介质的双相和各向异性特征是现代地球物理学的两个难点。一般各向异性弹性介质的弹性矩阵有21个独立参量,满足Christoffel方程;在高频近似条件下,满足射线追踪方程。然而直接求解波动方程往往十分困难,常常不能得到解析解。考虑到实际地球介质大多是弱各向异性,因此可以利用扰动理论近似描述,将相速度、偏振矢量线性化并表示为弱各向异性参数(Weakly Anisotropy,WA)的形式。我们给出了TTI介质中qP波的相速度和偏振矢量的弱各向异性参数线性化反演公式。20世纪70年代以来,科学家们通过地震波速度的方位各向异性、横波分裂、接收函数法、面波偏振等方法研究地球介质的各向异性。其中横波分裂方法是各向异性最直观的表现,具有相对简单、横向分辨率高、对裂隙密度及纵横比较敏感等优点,是研究介质各向异性性质的一种主要方法,也是研究大构造(如岩石圈等)的重要手段。本文以EDA(Extensive Dilatancy Anisotropy),APE(Anisotropic Poro-Elasticity),Chapman多尺度模型为例介绍了双相介质中的等效介质模型。EDA、APE理论的研究对象是微观尺度的单组微裂隙,其裂隙密度定义为张性扩容各向异性。EDA被认为是地壳各向异性的主要因素,当横波通过这种介质会分裂成快慢横波,且快横波的优势偏振方向往往与区域最大水平应力相同。基于SOC(Self Organized Criticality)理论双相介质的破裂特性,可以对由于应力作用处于临界状态下的微裂隙与流体的动态特征进行描述的APE模型表明,横波分裂对地壳介质的微裂隙的几何特征(如裂隙的纵横比)十分敏感,而微裂隙的几何特征变化对应力场的变化十分敏感。因此将横波分裂结果在地震前后的变化解释为横波对地应力场变化的响应,并提出可以利用这一特征进行应力检测。在不同地质构造时期,由于区域应力场的作用,可能导致实际地球介质中的微裂隙是多尺度的,并且有多个优选方向。本文介绍的Chapman多尺度模型将中等尺度断裂加入分析中,并考虑两种尺度的微裂隙和裂缝之间的相互作用,获得了频率相关的等效弹性矩阵。通过引入复数弹性张量,可考虑各向异性粘弹性介质的波动理论。由于弹性矩阵的虚数部分的引入,且是频率相关的,因此解析解的计算比一般各向异性弹性介质更加复杂。在高频近似时,可将一般各向异性弹性介质的射线追踪方程以及扰动理论应用到弱各向异性弱衰减(Weakly Anisotropy-Attenuation,WAA)介质中,此时其固体骨架由各向同性变为各向异性介质,且弹性矩阵的扰动由实数变为复数。本文给出了P波在一般WAA介质中随方位衰减的线性化反演公式。通过2个、3个、6个等方位间距的剖面组成的walkawayVSP模拟实验,使用TTI模型和一般各向异性模型对模拟数据进行了反演,验证了反演公式的正确性和可靠性。然后,对一个来自JavaSea的由三条剖面组成的walkaway VSP实际观测数据进行了反演计算,获得了钻井中不同深度检波器处的WA参数。中国地震台网中心测定,北京时间2008年5月12日14:28:04,在青藏高原东缘龙门山推覆构造带上发生了逆冲为主带有少量右旋、挤压型M S8.0大地震(31.01°N,103.42°E)。为提高时频分辨率,分析时频局部化信息,引入Meyer-Yamada正交小波基MYW。在粘弹性各向异性线性化理论的基础上,运用小波分析方法对以汶川地震为中心,200km为半径的范围内的地震进行了分析,获得了汶川震中区9个固定台站的P波小波波谱的平均衰减结果和方位各向异性衰减结果。该方法能够有效利用小震数据研究震源区介质各向异性随时空的变化规律,具有广阔的应用前景。     

含混合裂隙、孔隙介质的纵波衰减规律研究

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.     

Modelling frequency-dependent seismic anisotropy in fluid-saturated rock with aligned fractures: implication of fracture size estimation from anisotropic measurements

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.     

Interaction of multiple courses of wave‐induced fluid flow in layered porous media

Different theoretical and laboratory studies on the propagation of elastic waves in layered hydrocarbon reservoir have shown characteristic velocity dispersion and attenuation of seismic waves. The wave‐induced fluid flow between mesoscopic‐scale heterogeneities (larger than the pore size but smaller than the predominant wavelengths) is the most important cause of attenuation for frequencies below 1 kHz. Most studies on mesoscopic wave‐induced fluid flow in the seismic frequency band are based on the representative elementary volume, which does not consider interaction of fluid flow due to the symmetrical structure of representative elementary volume. However, in strongly heterogeneous media with unsymmetrical structures, different courses of wave‐induced fluid flow may lead to the interaction of the fluid flux in the seismic band; this has not yet been explored. This paper analyses the interaction of different courses of wave‐induced fluid flow in layered porous media. We apply a one‐dimensional finite‐element numerical creep test based on Biot's theory of consolidation to obtain the fluid flux in the frequency domain. The characteristic frequency of the fluid flux and the strain rate tensor are introduced to characterise the interaction of different courses of fluid flux. We also compare the behaviours of characteristic frequencies and the strain rate tensor on two scales: the local scale and the global scale. It is shown that, at the local scale, the interaction between different courses of fluid flux is a dynamic process, and the weak fluid flux and corresponding characteristic frequencies contain detailed information about the interaction of the fluid flux. At the global scale, the averaged strain rate tensor can facilitate the identification of the interaction degree of the fluid flux for the porous medium with a random distribution of mesoscopic heterogeneities, and the characteristic frequency of the fluid flux is potentially related to that of the peak attenuation. The results are helpful for the prediction of the distribution of oil–gas patches based on the statistical properties of phase velocities and attenuation in layered porous media with random disorder.     

Frequency-dependent anisotropy due to meso-scale fractures in the presence of equant porosity

Fractured rock is often modelled under the assumption of perfect fluid pressure equalization between the fractures and equant porosity. This is consistent with laboratory estimates of the characteristic squirt-flow frequency. However, these laboratory measurements are carried out on rock samples which do not contain large fractures. We consider coupled fluid motion on two scales: the grain scale which controls behaviour in laboratory experiments and the fracture scale. Our approach reproduces generally accepted results in the low- and high-frequency limits. Even under the assumption of a high squirt-flow frequency, we find that frequency-dependent anisotropy can occur in the seismic frequency band when larger fractures are present. Shear-wave splitting becomes dependent on frequency, with the size of the fractures playing a controlling role in the relationship. Strong anisotropic attenuation can occur in the seismic frequency band. The magnitude of the frequency dependence is influenced strongly by the extent of equant porosity. With these results, it becomes possible in principle to distinguish between fracture- and microcrack-induced anisotropy, or more ambitiously to measure a characteristic fracture length from seismic data.     

Seismic attenuation and velocity dispersion in fractured rocks: The role played by fracture contact areas

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.     

介观尺度孔隙流体流动作用对纵波传播特征的影响研究——以周期性层状孔隙介质为例

介观尺度孔隙流体流动是地震频段岩石表现出较强速度频散与衰减的主要作用.利用周期性层状孔隙介质模型,基于准静态孔弹性理论给出了模型中孔隙压力、孔隙流体相对运动速度以及固体骨架位移等物理量的数学解析表达式,同时利用Biot理论将其扩展至全频段条件下,克服了传统White模型中介质分界面处流体压力不连续的假设. 在此基础上对准静态与全频段下模型介质中孔隙压力、孔隙流体相对运动速度变化形式及其对弹性波传播特征的影响进行了讨论,为更有效理解介观尺度下流体流动耗散和频散机制提供物理依据.研究结果表明,低频条件下快纵波孔压在介质层内近于定值,慢纵波通过流体扩散改变总孔隙压力, 随频率的增加慢波所形成的流体扩散作用逐渐减弱致使介质中总孔压逐渐接近于快纵波孔压,在较高频率下孔压与应力的二次耦合作用使总孔压超过快纵波孔压.介质中孔隙流体相对运动速度与慢纵波形成的流体相对运动速度变化形式一致;随频率的增加孔隙流体逐渐从排水的弛豫状态过渡到非弛豫状态,其纵波速度-含水饱和度变化形式也从符合孔隙流体均匀分布模式过渡到斑块分布模式,同时介质在不同含水饱和度下的衰减峰值与慢纵波所形成的孔隙流体相对流动速度具有明显的相关性.     

A seismic reflection from isotropic‐fractured fluid‐saturated layer

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.     

Seismic dispersion and attenuation in layered porous rocks with fractures of varying orientations

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.     

小波尺度域含气储层地震波衰减特征

黏弹性衰减因子Q的可靠估计可通过Q反褶积来提高地震资料的分辨率并有助于振幅分析. 本文从小波理论出发,结合地震波在黏弹性介质中的传播方程,推导出小波尺度域地震波能量衰减公式. 能量衰减公式具有下列性质：（1）Q值越大,能量衰减得越慢;Q值越小,能量衰减越严重;（2）尺度越小,信号中保留的能量越少;（3）对于脉冲源来说在理想的无衰减介质(即Q趋近于∞)中传播时,信号在不同尺度内的能量相同. 利用尺度能量公式,可从反射地震资料中直接估计品质因子Q（即衰减因子）,也可以提取不同尺度的能量衰减剖面作为储层描述的属性参数,用来进行岩性识别和指示气藏,与经典的谱比法相比,避免了谱比法所面临的双时窗问题以及进行谱估计的窗选择问题. 理论模型试验表明了本文方法的正确性和有效性.     

Joint elastic‐electrical properties of reservoir sandstones and their relationships with petrophysical parameters

We measured in the laboratory ultrasonic compressional and shear‐wave velocity and attenuation (0.7–1.0 MHz) and low‐frequency (2 Hz) electrical resistivity on 63 sandstone samples with a wide range of petrophysical properties to study the influence of reservoir porosity, permeability and clay content on the joint elastic‐electrical properties of reservoir sandstones. P‐ and S‐wave velocities were found to be linearly correlated with apparent electrical formation factor on a semi‐logarithmic scale for both clean and clay‐rich sandstones; P‐ and S‐wave attenuations showed a bell‐shaped correlation (partial for S‐waves) with apparent electrical formation factor. The joint elastic‐electrical properties provide a way to discriminate between sandstones with similar porosities but with different clay contents. The laboratory results can be used to estimate sandstone reservoir permeability from seismic velocity and apparent formation factor obtained from co‐located seismic and controlled source electromagnetic surveys.     

Frequency-dependent PP and PS reflection coefficients in fractured media

When a porous layer is permeated by mesoscale fractures, wave-induced fluid flow between pores and fractures can cause significant attenuation and dispersion of velocities and anisotropy parameters in the seismic frequency band. This intrinsic dispersion due to fracturing can create frequency-dependent reflection coefficients in the layered medium. In this study, we derive the frequency-dependent PP and PS reflection coefficients versus incidence angle in the fractured medium. We consider a two-layer vertical transverse isotropy model constituted by an elastic shale layer and an anelastic sand layer. Using Chapman's theory, we introduce the intrinsic dispersion due to fracturing in the sand layer. Based on the series coefficients that control the behaviour of velocity and anisotropy parameters in the fractured medium at low frequencies, we extend the conventional amplitude-versus-offset equations into frequency domain and derive frequency-dependent amplitude-versus-offset equations at the elastic–anelastic surface. Increase in fracture length or fracture density can enlarge the frequency dependence of amplitude-versus-offset attributes of PP and PS waves. Also, the frequency dependence of magnitude and phase angle of PP and PS reflection coefficients increases as fracture length or fracture density increases. Amplitude-versus-offset type of PP and PS reflection varies with fracture parameters and frequency. What is more, fracture length shows little impact on the frequency-dependent critical phase angle, while the frequency dependence of the critical phase angle increases with fracture density.