针对塔河油田碳酸盐岩储层中存在的高角度裂缝的检测问题,开展了基于倾斜对称轴的横向各向同性介质(TTI介质)的P波剩余时差裂缝检测技术研究。回顾了TTI介质及其方位NMO剩余时差的基础理论,计算了三维TTI介质中P波的相速度;开发了一套适用于叠前地震道集的P波剩余时差裂缝预测技术,详细论述了其技术流程和具体实现步骤;设计了TTI介质裂缝模型,利用有限差分法模拟计算模型的叠前方位地震道集,根据模拟数据提取的方位NMO剩余时差反演出裂缝发育强度和裂缝角度,与模型设计参数基本吻合,验证了基于TTI介质的P波剩余时差裂缝检测技术的有效性。

Transverse isotropy with a horizontal axis of symmetry (HTI) is the simplest azimuthally anisotropic model used to describe fractured reservoirs that contain parallel vertical cracks. Here, I present an exact equation for normal‐moveout (NMO) velocities from horizontal reflectors valid for pure modes in HTI media with any strength of anisotropy. The azimuthally dependent P‐wave NMO velocity, which can be obtained from 3-D surveys, is controlled by the principal direction of the anisotropy (crack orientation), the P‐wave vertical velocity, and an effective anisotropic parameter equivalent to Thomsen's coefficient δ. An important parameter of fracture systems that can be constrained by seismic data is the crack density, which is usually estimated through the shear‐wave splitting coefficient γ. The formalism developed here makes it possible to obtain the shear‐wave splitting parameter using the NMO velocities of P and shear waves from horizontal reflectors. Furthermore, γ can be estimated just from the P‐wave NMO velocity in the special case of the vanishing parameter ε, corresponding to thin cracks and negligible equant porosity. Also, P‐wave moveout alone is sufficient to constrain γ if either dipping events are available or the velocity in the symmetry direction is known. Determination of the splitting parameter from P‐wave data requires, however, an estimate of the ratio of the P‐to‐S vertical velocities (either of the split shear waves can be used). Velocities and polarizations in the vertical symmetry plane of HTI media, that contains the symmetry axis, are described by the known equations for vertical transverse isotropy (VTI). Time‐related 2-D P‐wave processing (NMO, DMO, time migration) in this plane is governed by the same two parameters (the NMO velocity from a horizontal reflector and coefficient ε) as in media with a vertical symmetry axis. The analogy between vertical and horizontal transverse isotropy makes it possible to introduce Thomsen parameters of the “equivalent” VTI model, which not only control the azimuthally dependent NMO velocity, but also can be used to reconstruct phase velocity and carry out seismic processing in off‐symmetry planes.

Nonhyperbolic (long‐spread) moveout provides essential information for a number of seismic inversion/processing applications, particularly for parameter estimation in anisotropic media. Here, we present an analytic expression for the quartic moveout coefficient A4that controls the magnitude of nonhyperbolic moveout of pure (nonconverted) modes. Our result takes into account reflection‐point dispersal on irregular interfaces and is valid for arbitrarily anisotropic, heterogeneous media. All quantities needed to compute A4can be evaluated during the tracing of the zero‐offset ray, so long‐spread moveout can be modeled without time‐consuming multioffset, multiazimuth ray tracing.

裂缝储层岩石物理参数的准确获得对地下裂缝预测具有重要意义,而叠前地震反演是获得裂缝岩石物理参数的有效手段.本文从裂缝岩石物理等效模型的构建出发,从测井数据上估测了裂缝岩石物理参数,通过推导含裂缝岩石物理参数的方位各向异性弹性阻抗公式,探讨了基于方位各向异性弹性阻抗的裂缝岩石物理参数地震反演方法.实际工区地震数据应用表明,基于方位各向异性弹性阻抗的裂缝岩石物理参数反演方法合理、可靠,可以降低裂缝岩石物理参数估测的不确定性,为地下裂缝预测提供有力的依据.

Most bulk elastic media are weakly anisotropic. The equations governing weak anisotropy are much simpler than those governing strong anisotropy, and they are much easier to grasp intuitively. These equations indicate that a certain anisotropic parameter (denoted δ) controls most anisotropic phenomena of importance in exploration geophysics, some of which are nonnegligible even when the anisotropy is weak. The critical parameter δ is an awkward combination of elastic parameters, a combination which is totally independent of horizontal velocity and which may be either positive or negative in natural contexts.

A model for an imperfectly bonded interface between two elastic media is proposed. Displacement across this surface is not required to be continuous. The displacement discontinuity, or slip, is taken to be linearly related to the stress traction which is continuous across the interface. For isotropic interface behavior, there are two complex frequency dependent interface compliances, ηN and ηT, where the component of the slip normal to the interface is given by ηN times the normal stress and the component tangential to the interface is given by ηT times the shear stress and is in the same direction. Reflection and transmission coefficients for harmonic plane waves incident at arbitrary angles upon a plane linear slip interface are computed in terms of the interface compliances. These coefficients are frequency dependent even when the compliances are real and frequency independent. Examples of the effects of buried slip interfaces on reflection coefficient spectra and on Love-wave dispersion relations are presented.

Ultrasonic velocities were measured on a block composed of lucite plates with roughened surfaces pressed together with a static normal stress to simulate a fractured medium. The measurements, normal, parallel, and oblique to the fractures, show that for wavelengths much larger than the thickness of an individual plate, the block can be modeled as a particular type of transversely isotropic (TI) medium that depends on four parameters. This TI medium behavior is the same as that of an isotropic solid in which are embedded a set of parallel linear slip interfaces, specified by (1) the excess compliance tangential to the interfaces and (2) the excess compliance normal to the interfaces. At all static stress levels, we inverted the data for the background isotropic medium parameters and the excess compliances. The background parameters obtained were basically independent of stress level and agreed well with the bulk properties of the lucite. The excess compliances decreased with increasing static closing stress, implying that increasing static stress forces asperities on either side of a fracture into greater contact, gradually eliminating the excess compliance that gives rise to the anisotropy. A medium with such planes of excess compliance has been shown, theoretically, to describe the behavior of a medium with long parallel joints, as well as a medium with embedded parallel microcracks.

The simplest effective model of a formation containing a single fracture system is transversely isotropic with a horizontal symmetry axis (HTI). Reflection seismic signatures in HTI media, such as NMO velocity and amplitude variation with offset (AVO) gradient, can be conveniently described by the Thomsen‐type anisotropic parameters [Formula: see text] [Formula: see text] and [Formula: see text] Here, we use the linear slip theory of Schoenberg and co‐workers and the models developed by Hudson and Thomsen for pennyshaped cracks to relate the anisotropic parameters to the physical properties of the fracture network and to devise fracture characterization procedures based on surface seismic measurements. Concise expressions for [Formula: see text] [Formula: see text] and [Formula: see text] linearized in the crack density, show a substantial difference between the values of the anisotropic parameters for isolated fluid‐filled and dry (gas‐filled) penny‐sh aped cracks. While the dry‐crack model is close to elliptical with [Formula: see text] for thin fluid‐filled cracks [Formula: see text] and the absolute value of [Formula: see text] for typical [Formula: see text] ratios in the background is close to the crack density. The parameters [Formula: see text] and [Formula: see text] for models with partial saturation or hydraulically connected cracks and pores always lie between the values for dry and isolated fluid‐filled cracks. We also demonstrate that all possible pairs of [Formula: see text] and [Formula: see text] occupy a relatively narrow triangular area in the [Formula: see text] [Formula: see text]plane, which can be used to identify the fracture‐induced HTI model from seismic data. The parameter [Formula: see text] along with the fracture orientation, can be obtained from the P-wave NMO ellipse for a horizontal reflector. Given [Formula: see text] the NMO velocity of a dipping event or nonhyperbolic moveout can be inverted for [Formula: see text] The remaining anisotropic coefficient, [Formula: see text] can be determined from the constraint on the parameters of vertically fractured HTI media if an estimate of the [Formula: see text] ratio is available. Alternatively, it is possible to find [Formula: see text] by combining the NMO ellipse for horizontal events with the azimuthal variation of the P-wave AVO gradient. Also, we present a concise approximation for the AVO gradient of converted (PS) modes and show that all three relevant anisotropic coefficients of HTI media can be determined by the joint inversion of the AVO gradients or NMO velocities of P- and PS-waves. For purposes of evaluating the properties of the fractures, it is convenient to recalculate the anisotropic coefficients into the normal [Formula: see text] and tangential [Formula: see text] weaknesses of the fracture system. If the HTI model results from penny‐shaped cracks, [Formula: see text] gives a direct estimate of the crack density and the ratio [Formula: see text] is a sensitive indicator of fluid saturation. However, while there is a substantial difference between the values of [Formula: see text] for isolated fluid‐filled cracks and dry cracks, interpretation of intermediate values of [Formula: see text] for porous rocks requires accounting for the hydraulic interaction between cracks and pores.

Azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock. The symmetry of a rock with horizontal bedding that contains two or more non‐orthogonal sets of vertical fractures may be approximated as monoclinic with a horizontal plane of mirror symmetry. For offsets that are small compared with the depth of the reflector, the azimuthal variation in P‐wave AVO gradient for such a medium varies with azimuth as where φ is the azimuth measured with respect to the fast polarization direction for a vertically polarized shear wave. φ2 depends on both the normal compliance BN and the shear compliance BT of the fractures and may differ from zero if BNBT varies significantly between fracture sets. If BNBT is the same for all fractures, \n\n and the principal axes of the azimuthal variation in P‐wave AVO for fixed offset are determined by the polarization directions of a vertically propagating shear wave. At larger offsets, terms in \n\n and \n\n are required to describe the azimuthal variation in AVO accurately. φ4 and φ6 also depend on BNBT. For gas‐filled open fractures \n\n but a lower value of BNBT may result from the presence of a fluid with non‐zero bulk modulus.

在倾斜裂缝中,能不能利用普遍使用的裂缝发育强度近似公式进行裂缝密度评价,是一个值得讨论的问题。由TTI介质反射振幅方程出发,推导出与HTI介质裂缝密度计算公式相似的TTI介质的裂缝密度计算公式；通过对两个公式的对比分析,建立了裂缝倾角影响裂缝密度计算结果的理论基础；通过数值计算,绘制了裂缝密度与裂缝倾角的关系曲线,对裂缝倾角的影响程度进行了量化分析,得出在设计的代表性模型中,当裂缝倾角小于80°时,利用简化公式评价裂缝密度,将使结果产生较大偏差的结论。依据塔河油田裂缝型储层设计了TTI介质和HTI介质模型,正演模拟结果验证了上述结论的正确性,同时还发现裂缝倾角对裂缝密度评价的影响甚至大于数据噪声引起的误差。基于此,提出对于倾斜裂缝介质，不能直接利用简化公式计算裂缝密度的建议。

<FONT face=Verdana>Can we use common fracture growth intensity approximate formula for fracture density evaluation in dip fracture? It is worth for discussion.Starting from reflecting amplitude equation of TTI medium,the fracture density formula of TTI medium was gained,which is similar with that of HTI medium.Through comparison on the two formulas,the basic theory for fracture density calculations varying with its dip was built.Then,by numerical calculation,the relationship curve of fracture density and dip was plotted,and the impact of fracture dip was quantitatively analyzed.The results indicate that in the representative designed model,when fracture dip is smaller than 80°,large deviation will be obtained by evaluating fracture density through simplified approximate formula.The forward modeling results of TTI and HTI medium models designed in terms of fractured reservoirs of Tahe Oilfield prove the above conclusions.Meanwhile,it is found that the impact of fracture dip on its density evaluation is even larger than the error caused by data noise.So,we suggest that it couldnt directly use the simplified formula for fracture density calculation in dip fractured medium.</FONT>