A comparison of cross-hole electrical and seismic data in fractured rock

Cross‐hole anisotropic electrical and seismic tomograms of fractured metamorphic rock have been obtained at a test site where extensive hydrological data were available. A strong correlation between electrical resistivity anisotropy and seismic compressional‐wave velocity anisotropy has been observed. Analysis of core samples from the site reveal that the shale‐rich rocks have fabric‐related average velocity anisotropy of between 10% and 30%. The cross‐hole seismic data are consistent with these values, indicating that observed anisotropy might be principally due to the inherent rock fabric rather than to the aligned sets of open fractures. One region with velocity anisotropy greater than 30% has been modelled as aligned open fractures within an anisotropic rock matrix and this model is consistent with available fracture density and hydraulic transmissivity data from the boreholes and the cross‐hole resistivity tomography data. However, in general the study highlights the uncertainties that can arise, due to the relative influence of rock fabric and fluid‐filled fractures, when using geophysical techniques for hydrological investigations.     

Multiple scattering of seismic waves in fractured media: Velocity and effective attenuation of the coherent components ofP waves andS waves

We show that the multiple scattering by small fractures of seismic waves with wavelengths long compared to the fracture size and fracture spacing is indistinguishable from multiple-scattering effects produced by regular porosity, except for an orientation factor due to fracture alignment. The fractures reduce theP-wave andS-wave velocities and produce an effective attenuation of the coherent component of the seismic waves. The attenuation corresponds to 1000/Q of about unity for a Gaussian spectrum of fractures, and it varies with frequencyf asf ³. For a Kolmogorov spectrum of fractures of spectral index the attenuation is an order of magnitude or so larger and varies with frequency asf ^3-v The precise degree of attenuation depends upon the matrix properties, the fracture porosity, the degree of fracture anisotropy, the type of fluid filling the fractures, and the incidence angle of the wave.For fracture porosities less than about 15% theP-wave andS-wave velocities are decreased by the order of 5–10% with a lesser dependence on the type of fluid filling the fractures (gas, oil, or brine) and with a dependence on both the degree of anisotropy and the incident angle made by the wave. The tendency of fractures to occur perpendicularly to bedding suggests that the best way to measure seismically fractured rock behavior in situ is by using the travel-time delay and reflection amplitude. As both the offset and the azimuth of receivers vary from a shot, the travel-time delay and reflection amplitude should both show an elliptical pattern of behavior—the travel-time delay in response to the varying seismic speed, and the reflection amplitude in response to angular variations in the multiple scattering. Observations of attenuation at several frequencies should permit (a) determination of the spectrum of fractures (Gaussian versus Kolmogorovian) and (b) determination of the contribution of viscous damping to the effective attenuation.     

基于数字岩心的含裂隙储层砂岩介电性质研究

了解储层岩石的介电特性在石油工业的各个方面都有重要的应用.小尺度裂隙是影响岩石介电性质的地质因素之一,获得裂隙对含裂隙岩石介电性质影响的定量关系具有重要的理论和实践意义.以含裂隙人造砂岩的三维微观数字结构为基础,通过基于三维有限差分算法计算的岩石介电性质与实验数据的对比验证数值计算方法的有效性.在此基础上,通过理论模型获得不同孔隙度基质的介电性质,并在不含裂隙人造砂岩的三维微观数字结构中人为添加以裂隙密度和纵横比为定量表征参数的裂隙,应用验证后的数值算法模拟随频率变化的含裂隙砂岩的介电性质,分析和研究不同孔隙度基质中定向排列裂隙对砂岩介电性质的影响.结果表明,当裂隙孔隙度随裂隙纵横比或裂隙密度发生改变时,含裂隙砂岩的介电性质与裂隙密度以及裂隙纵横比呈正相关关系,而当裂隙孔隙度保持不变时,含裂隙砂岩的介电性质随裂隙纵横比的减小而增大;裂隙参数的改变对不同基质孔隙度的含裂隙砂岩的介电性质的影响趋势较为一致,但随着基质孔隙度的减小,裂隙对砂岩介电性质的影响逐渐增大.裂隙参数和基质孔隙度对含裂隙砂岩介电性质影响的研究结果为基于介电特性的裂缝性油气储层的定量表征提供了依据,在油气勘探开发中具有重要的应用前景.     

基于数字岩心的含裂隙储层砂岩介电性质研究

了解储层岩石的介电特性在石油工业的各个方面都有重要的应用.小尺度裂隙是影响岩石介电性质的地质因素之一,获得裂隙对含裂隙岩石介电性质影响的定量关系具有重要的理论和实践意义.以含裂隙人造砂岩的三维微观数字结构为基础,通过基于三维有限差分算法计算的岩石介电性质与实验数据的对比验证数值计算方法的有效性.在此基础上,通过理论模型获得不同孔隙度基质的介电性质,并在不含裂隙人造砂岩的三维微观数字结构中人为添加以裂隙密度和纵横比为定量表征参数的裂隙,应用验证后的数值算法模拟随频率变化的含裂隙砂岩的介电性质,分析和研究不同孔隙度基质中定向排列裂隙对砂岩介电性质的影响.结果表明,当裂隙孔隙度随裂隙纵横比或裂隙密度发生改变时,含裂隙砂岩的介电性质与裂隙密度以及裂隙纵横比呈正相关关系,而当裂隙孔隙度保持不变时,含裂隙砂岩的介电性质随裂隙纵横比的减小而增大;裂隙参数的改变对不同基质孔隙度的含裂隙砂岩的介电性质的影响趋势较为一致,但随着基质孔隙度的减小,裂隙对砂岩介电性质的影响逐渐增大.裂隙参数和基质孔隙度对含裂隙砂岩介电性质影响的研究结果为基于介电特性的裂缝性油气储层的定量表征提供了依据,在油气勘探开发中具有重要的应用前景.

     

Studies on seismic waves

The development of seismic wave study in China in the past four years is reviewed. The discussion is divided into several aspects, including seismic wave propagation in laterally homogeneous media, laterally heterogeneous media, anisotropic and porous media, surface wave and seismic wave inversion, and seismic wave study in prospecting and logging problems. Important projects in the current studies on seismic wave is suggested as the development of high efficient numerical methods, and applying them to the studies of excitation and propagation of seismic waves in complex media and strong ground motion, which will form a foundation for refined earthquake hazard analysis and prediction.     

Studies on seismic waves

The development of seismic wave study in China in the past four years is reviewed. The discussion is divided into several aspects, including seismic wave propagation in laterally homogeneous media, laterally heterogeneous me-dia, anisotropie and porous media, surface wave and seismic wave inversion, and seismic wave study in prospect-ing and logging problems. Important projects in the current studies on seismic wave is suggested as the develop-ment of high efficient numerical methods, and applying them to the studies of excitation and propagation of seis-mic waves in complex media and strong ground motion, which will form a foundation for refined earthquake haz-ard analysis and prediction.     

Effect of topography and subsurface inhomogeneities on seismic SV waves

The effect of topography and subsurface inhomogeneity on surface motion is investigated in the case of incident SV waves. Several types of topography, such as a cliff, a cliff with a soft layer and filled land, are considered. Computations are made using a new method combining a particle model with a finite element method. The accuracy of this method is discussed through comparisons with Wong's solutions, which are in good agreement. It is found that the surface displacement is very much influenced by surface irregularities when the incident wavelengths are comparable to the size of the topographic features. Rayleigh waves are strongly produced in the neighbourhoods of the slopes of a cliff and a cliff with a soft layer, the latter being a cliff adjacent to and covered at its foot by a soft layer. Thus, a zone of large amplification takes place near a slope, combining incident SV waves and Rayleigh waves. A large displacement also occurs at the upper corner of the slope. In filled land, vertical and horizontal displacements are produced, which are 3 times larger than those at a distance. The present results are considered to be significant from the viewpoint of engineering seismology.     

Effect of topography and subsurface inhomogeneity on seismic rayleigh waves

The effect of topography and subsurface inhomogeneity on surface motion is investigated in the case of Rayleigh waves. In the previous paper, the same effect was investigated in the case of SV waves. Several types of topography, such as cliffs both with and without a soft layer at the foot of the slope, are considered. Computations are made using a new hybrid method combining a particle model with a finite element method. In cases of harmonic Rayleigh waves, surface motions with amplitudes as large as 1.5 to 5 times the horizontal surface displacement of the incident Rayleigh waves are produced near the slope and the sloping interface. When a Rayleigh wave propagating through a hard single-layered ground encounters a sloping interface where hard ground and soft ground make contact with each other, Rayleigh waves having two different, phase velocities are produced and they correspond to the fundamental mode, and the first mode determined by Haskell's method. In addition, the transient response when Rayleigh waves propagate through the cliff is also simulated. Assuming the vertical component of the Tokachi-oki Earthquake (1968) measured on the surface to be a Rayleigh wave, the incident Rayleigh wave can be obtained by a Fourier synthesis of eigenfunctions of Rayleigh waves.     

Multiple scattering of seismic waves in fractured media: Crosscorrelation as a probe of fracture intensity

This paper presents the theory which describes how the lateral correlation of vertical displacement is influenced by multiple scattering of seismic waves caused by the presence of fluid-filled fractures, along with an observational methodology to show how to extract fracture-related quantities (e.g. fracture porosity, degree of anisotropy of the fractures, change in orientation of the fractures, density of fluid filling the fractures) from seismic data using the theoretical development as an underpinning framework. A simple physical picture is given of the basic process, as well as some rough order of magnitude estimates to show that the parameters quantifying the size of the multiple scattering fall squarely in the seismic regime.These calculations serve to create a basis to determine, by seismic means ahead of the drill, (1) the degree of fracturing of reservoir rocks (such as the Monterey shale), and (2) the type of fluid filling the fractures.     

Effect of station conditions on amplitudes and periods of seismic waves

Summary Station corrections for Prhonice, relating to Praha, were determined for body and surface waves using observations of Kirnos seismographs. The time stability of the station corrections was investigated and the possible causes of observed instabilities were discussed. A systematic difference in periods recorded at the stations was found.Dedicated to Academician Alois Zátopek on His 65th Birthday     

流体饱和度对裂缝-孔隙岩石频变纵波速度及其各向异性的影响

流体饱和度会改变裂缝性储层的纵波速度,从而影响地层速度的频散特性及各向异性程度,导致储层地震响应特征复杂,储层预测多解性强,流体识别难度大.本文根据多相流体饱和裂缝性储层的特点,借助于Norris和KG模型,建立部分饱和裂缝-孔隙等效介质模型,给出频变地震波速度随流体饱和度变化的精确关系式.数值模拟结果表明,当气、水两相共存时,随着含水饱和度的增加,高频段纵波相速度逐渐增大,各向异性程度逐渐减小;低频段纵波相速度逐渐减小,各向异性程度不变;相速度频散及其各向异性程度逐渐增强.组合已有的孔隙弹性理论模型,对实验室人工裂缝-孔隙砂岩岩样的纵波速度进行拟合,计算得到的曲线与实验室测量散点值吻合度较高,表明组合模型在给定参数下的有效性.该研究能够为多相流体饱和裂缝性储层的地震响应特征分析奠定扎实的理论基础,为提高储层预测的确定性和流体识别的准确性提供可靠的理论依据.

     

Influence of piezometer construction on groundwater sampling in fractured rock

A numerical model for groundwater flow and solute transport was employed to examine the influence of the screen and sandpack on the collection of a representative geochemical sample from a piezometer monitoring well installation in a discretely fractured bedrock aquifer. The optimization of screen and sandpack combinations was explored for the potential to reduce purging times and volumes in practice. Simulations accounted for the location of the fractures along the well screen, fracture aperture, screen length, and the pumping rate. The variability in the required purging times (t(99)-the time required to achieve 99% fractional contribution from the formation to pump discharge) can be explained by: (1) the relative hydraulic conductivities of the components of the system (fracture, sandpack, and screen), (2) the truncation of the flow field from the fracture to the screen by the upper and/or lower boundary of the sandpack of the flow field from another fracture, and (3) time-dependent drawdown. During pumping, only a portion of the sandpack may actually become hydraulically active. The optimal configuration (shortest purging time) is achieved when the ratios of the screen, sandpack, and fracture hydraulic conductivities are close to 1:1:1. More importantly, the role of the fracture hydraulic conductivity in the ratios is not as crucial to reducing t(99) as having the hydraulic conductivities of the screen and sandpack as similar as possible. This study provides a better understanding of well dynamics during pumping for the purpose of obtaining representative groundwater samples.     

龙马溪组页岩微观结构、地震岩石物理特征与建模

龙马溪组页岩是目前国内页岩气勘探的主要层位之一. 由于岩石物理实验结果具有区域性, 龙马溪组页岩的岩石特征与其地震弹性性质的响应规律需要开展相关的实验和理论研究工作予以明确. 本研究基于系统的微观结构观察(扫描电镜和CT成像技术)和岩石物理实验来分析龙马溪组页岩样品地震弹性性质的变化规律, 并依据微观结构特征建立相应的地震岩石物理表征模型. 研究结果表明, 石英含量对龙马溪组页岩的孔隙度以及有机碳(TOC)含量具有一定的控制作用, TOC和黄铁矿主要赋存于孔隙中; 岩石骨架组成亦受控于石英或粘土含量, 在石英含量大于40%(对应粘土含量小于30%)时, 以石英、粘土共同作为岩石骨架, 而粘土含量大于30%时, 则以粘土作为岩石的骨架. 因此, 岩石骨架组成矿物、TOC含量、孔隙度共同制约龙马溪组页岩的地震弹性性质, 富有机质储层岩石通常表现出低泊松比、低阻抗和低杨氏模量的特征, 但由于支撑矿物的转换, 某些富有机质页岩亦可表现为高阻抗特征. 粘土矿物的定向排列仍然是造成页岩样品表现出各向异性的主要原因, 各向异性参数与粘土含量具有指数关系. 基于龙马溪组页岩的岩性特征及微观结构特征, 可以利用自洽模型(SCA)、微分等效模量模型(DEM)和Backus平均模型的有效组合较为准确地建立龙马溪组页岩的地震岩石物理模型, 实验结果和测井数据验证了模型的准确性.研究结果可为龙马溪组页岩气储层的测井解释和地震"甜点"预测提供依据.     

成岩过程对五峰-龙马溪组页岩地震岩石物理特征的影响

五峰-龙马溪组页岩是目前国内页岩气勘探的首选层位,而其地震岩石物理特征是利用地震方法进行"甜点"预测的重要基础之一,但对五峰-龙马溪组页岩地震弹性特征变化规律的研究并未考虑沉积、成岩过程的影响,致使相应的规律性认识缺乏地质意义.在对五峰-龙马溪组页岩样品系统声学测量基础上,分析了页岩样品地震弹性性质的变化规律.利用X射线衍射分析、扫描电镜（SEM）、阴极发光（CL）与能谱分析确定了五峰-龙马溪组页岩在不同沉积环境下的成岩过程,并讨论了成岩过程与地震弹性性质变化规律的因果关系.研究结果表明,页岩中有机质（TOC）受高热演化程度的影响,其密度通常高于1.4 g·cm^-3,并接近于有机碳密度上限1.6 g·cm^-3（石墨密度）.五峰-龙马溪组页岩地震弹性性质变化规律整体受沉积环境控制,沉积环境的差异形成不同的成岩过程,致使地震弹性特征也表现出不同的变化规律.表现在五峰-龙马溪页岩样品动态岩石物理特征主要受岩石结构控制（支撑颗粒弹性性质）,而孔隙度、TOC含量以及孔隙形状则为对地震弹性特征影响的次一级因素.五峰-龙马溪组页岩上段为浅水陆棚相,机械压实与化学压实（硅质胶结）为先后两个过程,造成样品表现出高的速度-孔隙度变化率、高速度比（泊松比）、高各向异性以及低TOC含量的特征.五峰-龙马溪组页岩下段为深水陆棚相,机械压实过程中同时伴有生物成因的硅质胶结,造成岩石样品表现出较高TOC含量与孔隙度、各向异性较弱以及较小的速度-孔隙度变化率.研究结果可为五峰-龙马溪页气储层的测井解释和地震"甜点"预测提供依据.     

Effect of lateral inhomogeneity on seismic waves,II. Observations and analyses

Observation has been conducted on soft surface ground with a sloping basement to clarify the effects of lateral inhomogeneity on ground motions and strains induced by seismic waves and to examine the frequency characteristics of the ground surface motions. The scattered Rayleigh wave from the vicinity of the sloping basement has been detected on seismograms and is observed to have a large influence on the ground motions and strains. In this paper the response of the ground is simulated using a hybrid method which combines a particle model and finite element method (FEM). The computed seismograms are in good agreement with observed ones. It is found from the calculations that large ground strains are produced by the scattered Rayleigh waves as well as by the incident shear waves in the surface ground overlying the sloping basement. In addition, the effects of lateral inhomogeneity on an embedded pipeline are examined. The present results are considered to be significant from the viewpoint of earthquake engineering.     

Amplitudes of explosion-generated seismic waves

Summary Records of underwater explosions in Iceland in 1959 and 1960, mainly performed for crustal studies, have been investigated with regard to amplitudes. The amplitudes ofP2 in the 1959 explosions and of the first arrivingP waves in the 1960 explosions were found to be proportional to the first power of the charge weight, whereas amplitudes ofP1 andS1 in the 1959 explosions were proportional to the 3/4-power of the charge up to 200 kg, but increased much slower for larger charges. The influence of water depth of shot point on the charge weights, required to obtain a certain amplitude, has been determined and it was found that the logarithm of the charge weight has a linear relation to water depth down to about 8 meters. The amplitudes of first arrivingP waves decrease as the inverse 2.2-power of the distance up to about 30 km. For greater distances an exponential decrease of the form (const./) exp (–) is valid. ForP2 waves with a frequency of 10 cps we found =0.027±0.003 km^–1 for a profile across central Iceland and =0.009±0.004 km^–1 for a profile in the western part of Iceland.

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Zusammenfassung In den Jahren 1959 und 1960 wurden auf Island Unterwasser-Explosionen ausgelöst, deren Hauptziel Untersuchung der Erdkruste waren. Die Aufzeichnungen der Explosionen werden hier in Hinsicht auf die Amplituden der Wellen untersucht. DieP2-Amplituden in den Explosionen von 1959 und die Amplituden der zuerst ankommendenP-Wellen in den Explosionen von 1960 waren proportional der ersten Potenz des Ladungsgewichtes, während diePl-undSl-Amplituden in den Explosionen von 1959 proportional der 3/4-Potenz des Ladungsgewichtes bis 200 kg waren. Für höhere Ladungsgewichte wuchsen die Amplituden langsamer an. Es wurde der Einfluss der Tiefe des Schusspunktes unterhalb des Wasserspiegels auf das für eine gewisse Amplitude benötigte Ladungsgewicht bestimmt. Es ergab sich dabei, dass der Logarithmus des Ladungsgewichtes in einer linearen Beziehung zur Tiefe, bis etwa 8 m, steht. Die Amplituden der zuerst ankommendenP-Wellen sind umgekehrt proportional der 2.2-Potenz der Entfernung, bis etwa 30 km. Für grössere Entfernungen ist die Abnahme der Amplituden exponential und kann durch (Konst./) exp (–) dargestellt werden. FürP2 mit einer Frequenz von 10 cps haben wir =0.027±0.003 km^–1 für ein Profil in Zentral-Island und =0.009 ± 0.004 km^–1 für ein Profil in West-Island gefunden.

     

裂缝介质岩石物理模型研究综述

随着我国油气勘探开发事业的快速发展,裂缝型油气藏将成为本世纪的重点勘探对象之一.由于裂缝模型对于预测裂缝型油气藏非常重要,因此,裂缝模型的研究备受关注.本文简单概述裂缝模型的发展历程,重点介绍Schoenberg模型、Hudson模型、Kachanov模型、Eshelby-Cheng模型、Thomsen模型、DEM模型...     

雷州半岛南部玄武岩岩石物性与激发地震波属性关系研究

雷州半岛南部地区近地表火成岩发育、表层结构复杂、岩性横向变化大,激发接收条件变化剧烈,造成地震资料品质差.针对雷州半岛南部近地表玄武岩,本文通过岩心样品力学性质测试和岩石结构特征分析,获得了岩样抗压强度、波速、模量、弹性应变能特性以及在不同岩石微相激发的地震子波属性特征,研究了不同岩石微相的弹性模量、变形特征规律,建立...     

双组份裂缝岩石非线性有效应力模型研究

为认识黏土矿物和裂缝对岩石有效应力的影响,提出了双组份裂缝岩石椭圆模型.同时,将双组份裂缝岩石椭圆模型等效为共聚焦的椭圆岩石环与椭圆黏土环的叠加;基于复变函数和保角变换,分别得到了黏土环内椭圆长半轴和短半轴与压力间的关系式.进而结合椭圆孔渗透率计算式和岩石有效应力表达式,计算与分析了岩石孔隙度、黏土矿物含量、硬度比以及椭圆孔截面纵横比等参数对有效应力系数的影响.结果表明,当岩石中不含黏土矿物时,有效应力系数随纵横比的减小而增大,且小于1.0,这与Bernabé的观点一致.当岩石中含有易于压缩的黏土矿物,且纵横比为1.0（孔隙截面为圆形）时,有效应力系数随硬度比和黏土矿物含量的增大而增大,甚至远大于1.0,这与Al-Wardy等的研究结论一致;当纵横比小于1.0时,有效应力系数表现出了明显的非线性特征,随硬度比的增大而增大——甚至远大于1.0,而随黏土矿物含量的增大而减小,纵横比对有效应力系数的影响受黏土矿物含量大小的影响.易于压缩组分和裂缝的存在使得岩石有效应力系数变化特征更为复杂.