Influence of frequency-dependent anisotropy on seismic amplitude-versus-offset signatures for fractured poroelastic rocks

Frequency-dependent amplitude variation with offset offers an effective method for hydrocarbon detections and analysis of fluid flow during production of oil and natural gas within a fractured reservoir. An appropriate representation for the frequency dependency of seismic amplitude variation with offset signatures should incorporate influences of dispersive and attenuating properties of a reservoir and the layered structure for either isotropic or anisotropic dispersion analysis. In this study, we use an equivalent medium permeated with aligned fractures that simulates frequency-dependent anisotropy, which is sensitive to the filled fluid of fractures. The model, where pores and fractures are filled with two different fluids, considers velocity dispersion and attenuation due to mesoscopic wave-induced fluid flow. We have introduced an improved scheme seamlessly linking rock physics modelling and calculations for frequency-dependent reflection coefficients based on the propagator matrix technique. The modelling scheme is performed in the frequency-slowness domain and can properly incorporate effects of both bedded structure of the reservoir and velocity dispersion quantified with frequency-dependent stiffness. Therefore, for a dispersive and attenuated layered model, seismic signatures represent a combined contribution of impedance contrast, layer thickness, anisotropic dispersion of the fractured media and tuning and interference of thin layers, which has been avoided by current conventional methods. Frequency-dependent amplitude variation with offset responses was studied via considering the influences of fracture fills, layer thicknesses and fracture weaknesses for three classes amplitude variation with offset reservoirs. Modelling results show the applicability of the introduced procedure for interpretations of frequency-dependent seismic anomalies associated with both layered structure and velocity dispersion of an equivalent anisotropic medium. The implications indicate that anisotropic velocity dispersion should be incorporated accurately to obtain enhanced amplitude variation with offset interpretations. The presented frequency-dependent amplitude variation with offset modelling procedure offers a useful tool for fracture fluid detections in an anisotropic dispersive reservoir with layered structures.     

Generalized anisotropy parameters and approximations of attenuations and velocities in viscoelastic media of arbitrary anisotropy type – theoretical and experimental aspects*

Seismic anisotropy in geological media is now widely accepted. Parametrizations and explicit approximations for the velocities in such media, considered as purely elastic and moderately anisotropic, are now standards and have even been extended to arbitrary types of anisotropy. In the case of attenuating media, some authors have also recently published different parametrizations and velocity and attenuation approximations in viscoelastic anisotropic media of particular symmetry type (e.g., transversely isotropic or orthorhombic). This paper extends such work to media of arbitrary anisotropy type, that is to say to triclinic media. In the case of homogeneous waves and using the so‐called ‘correspondence principle’, it is shown that the viscoelastic equations (for the phase velocities, phase slownesses, moduli, wavenumbers, etc.) are formally identical to the corresponding purely elastic equations available in the literature provided that all the corresponding quantities are complex (except the unit vector in the propagation direction that remains real). In contrast to previous work, the new parametrization uses complex anisotropy parameters and constitutes a simple extension to viscoelastic media of previous work dealing with non‐attenuating elastic media of arbitrary anisotropy type. We make the link between these new complex anisotropy parameters and measurable parameters, as well as with previously published anisotropy parameters, demonstrating the usefulness of the new parametrization. We compute the explicit complete directional dependence of the exact and of the approximate (first and higher‐order perturbation) complex phase velocities of the three body waves (qP, qS1 and qS2). The exact equations are successfully compared with the ultrasonic phase velocities and phase attenuations of the three body waves measured in a strongly attenuating water‐saturated sample of Vosges sandstone exhibiting moderate velocity anisotropy but very strong attenuation anisotropy. The approximate formulas are checked on experimental data. Compared to the exact solutions, the errors observed on the first‐order approximate velocities are small (<1%) for qP‐waves and moderate (<10%) for qS‐waves. The corresponding errors on the quality factor Q are moderate (<6%) for qP‐waves but critically large (up to 160%) for the qS‐waves. The use of higher‐order approximations substantially improves the accuracy, for instance typical maximum relative errors do not exceed 0.06% on all the velocities and 0.6% on all the quality factors Q, for third‐order approximations. All the results obtained on other rock samples confirm the results obtained on this rock. The simplicity of the derivations and the generality of the results are striking and particularly convenient for practical applications.     

Influence of subsurface injection on time‐lapse seismic: laboratory studies at seismic and ultrasonic frequencies

Seismic monitoring of reservoir and overburden performance during subsurface CO₂ storage plays a key role in ensuring efficiency and safety. Proper interpretation of monitoring data requires knowledge about the rock physical phenomena occurring in the subsurface formations. This work focuses on rock stiffness and elastic velocity changes of a shale overburden formation caused by both reservoir inflation induced stress changes and leakage of CO₂ into the overburden. In laboratory experiments, Pierre shale I core plugs were loaded along the stress path representative for the in situ stress changes experienced by caprock during reservoir inflation. Tests were carried out in a triaxial compaction cell combining three measurement techniques and permitting for determination of (i) ultrasonic velocities, (ii) quasistatic rock deformations, and (iii) dynamic elastic stiffness at seismic frequencies within a single test, which allowed to quantify effects of seismic dispersion. In addition, fluid substitution effects connected with possible CO₂ leakage into the caprock formation were modelled by the modified anisotropic Gassmann model. Results of this work indicate that (i) stress sensitivity of Pierre shale I is frequency dependent; (ii) reservoir inflation leads to the increase of the overburden Young's modulus and Poisson's ratio; (iii) in situ stress changes mostly affect the P‐wave velocities; (iv) small leakage of the CO₂ into the overburden may lead to the velocity changes, which are comparable with one associated with geomechanical influence; (v) non‐elastic effects increase stress sensitivity of an acoustic waves; (iv) and both geomechanical and fluid substitution effects would create significant time shifts, which should be detectable by time‐lapse seismic.     

地震频段弹性模量测试系统改进与升级

获取地震频段弹性模量对于地震数据定量解释、研究地震波传播特征及油气勘探开发具有重要意义.前期建立的测试系统基于应力应变法可以获得地震频段内岩心的杨氏模量、泊松比和衰减.该系统仅适用于ϕ38 mm岩心, 而测量物性参数(孔隙度、渗透率等)、测量超声纵横波速度时一般使用ϕ25 mm岩心.为将ϕ25 mm岩心应用于地震频段弹性模量的测试, 统一各项岩石物理测试的数据, 本文结合有限元数值模拟和实验测试对地震频段弹性模量测试系统的机械结构与应变采集系统(惠斯通电桥)做了改进与升级.通过数值模拟与标定实验(铝和有机玻璃)论证了在ϕ25 mm岩心上进行低频实验的可靠性和准确性.利用改进后的测试系统对常规砂岩在干燥(空气)、充气(N₂)与完全饱和流体(白油)条件开展了高频(MHz)超声和低频(1~3 kHz)应力应变测量实验, 其结果表明: 干燥岩样和充气岩样的弹性参数在低频和超声频段一致, 以此验证了改进后测量系统的可靠性; 而饱和白油的砂岩弹性参数在1~3000 Hz频段范围内表现出明显的频散特性.改进后的地震频段弹性模量测试系统可以为定量研究含流体岩石的弹性频散特征提供有效的实验测量工具, 其实验结果不仅可应用于生产实践, 也可为检验、修正岩石物理模型的提供重要的实验依据.

     

Study the elastic properties and the anisotropy of rocks using different machine learning methods

This paper aims to demonstrate that the elastic stiffnesses and the anisotropic parameters of rocks can be accurately predicted from geophysical features such as the porosity, the density, the compression stress, the pore pressure and the burial depth using relevant machine learning methods. It also suggests that the extreme gradient boosting method is the best method for this purpose. It is more accurate, extremely faster to train and more robust than the artificial neural networks and the support vector machine methods. Very high R-squared scores was obtained for the predicted elastic stiffnesses of a relevant dataset that is available in the literature. This dataset contains different types of rocks, and the values of the features are in large ranges. An optimal set of parameters was obtained by considering an appropriate sensitivity analysis. The optimized model is very easy to implement in Python for practical applications.     

鄂尔多斯块体北缘与西缘地区地壳各向异性特征

本研究使用内蒙古自治区数字测震台网2010年1月至2017年10月区域小地震的波形记录资料,采用SAM方法,进行了地壳剪切波分裂的分析,得到鄂尔多斯块体北缘与西缘地区地壳介质地震各向异性的初步研究结果.根据15个台站161个有效地震记录的分析,鄂尔多斯块体北缘与西缘地区的快剪切波平均偏振方向为NE44.4°±38.4°,慢剪切波平均时间延迟为1.7±1.6 ms·km^-1.研究区域的快剪切波偏振显示出两个优势方向,一个是NE方向,另一个是近NS方向.区内的逆冲凸起与走滑正倾断层构造对剪切波分裂产生了直接的影响,造成了剪切波分裂参数的复杂分布,反映了剪切波分裂参数受到区域应力和构造共同作用的影响.鄂尔多斯块体北缘的快波偏振特征有NE和近NS两个优势偏振方向,其东区与西区的快剪切波偏振表现出明显不同的特征.东区的第一快剪切波优势偏振方向为NE,第二快剪切波优势偏振方向为近NS;西区的第一快剪切波优势偏振方向为近EW,第二快剪切波优势偏振方向为近NS.鄂尔多斯块体北缘的区域背景主压应力方向可能总体上为近NS方向,但空间分布有差异,东区NE方向的优势偏振与西区近EW方向的优势偏振更可能反映了断裂与构造的影响.鄂尔多斯块体西缘的快剪切波偏振特征显示出非常清楚的NE向的优势偏振方向,近NS向的优势偏振方向则不太明显,反映出该地区复杂构造对各向异性分布的影响.慢波时间延迟呈现出西低东高的特点,时间延迟的高值出现在鄂尔多斯块体北缘的东部,时间延迟的这种西低东高的各向异性强度变化,可能反映了区域构造活动西强东弱的特性.

     

A strategy for automated analysis of passive microseismic data to image seismic anisotropy and fracture characteristics

Monitoring of induced seismicity is gaining importance in a broad range of industrial operations from hydrocarbon reservoirs to mining to geothermal fields. Such passive seismic monitoring mainly aims at identifying fractures, which is of special interest for safety and productivity reasons. By analysing shear‐wave splitting it is possible to determine the anisotropy of the rock, which may be caused by sedimentary layering and/or aligned fractures, which in turn offers insight into the state of stress in the reservoir. We present a workflow strategy for automatic and effective processing of passive microseismic data sets, which are ever increasing in size. The automation provides an objective quality control of the shear‐wave splitting measurements and is based on characteristic differences between the two independent eigenvalue and cross‐correlation splitting techniques. These differences are summarized in a quality index for each measurement, allowing identification of an appropriate quality threshold. Measurements above this threshold are considered to be of good quality and are used in further interpretation. We suggest an automated inversion scheme using rock physics theory to test for best correlation of the data with various combinations of fracture density, its strike and the background anisotropy. This fully automatic workflow is then tested on a synthetic and a real microseismic data set.     

Effects of petrophysical parameters on attenuation and dispersion of seismic waves in the simplified poroelastic theory: Comparison between the Biot's theory and viscosity‐extended theory

The simplified macro‐equations of porous elastic media are presented based on Hickey's theory upon ignoring effects of thermomechanical coupling and fluctuations of porosity and density induced by passing waves. The macro‐equations with definite physical parameters predict two types of compressional waves (P wave) and two types of shear waves (S wave). The first types of P and S waves, similar to the fast P wave and S wave in Biot's theory, propagate with fast velocity and have relatively weak dispersion and attenuation, while the second types of waves behave as diffusive modes due to their distinct dispersion and strong attenuation. The second S wave resulting from the bulk and shear viscous loss within pore fluid is slower than the second P wave but with strong attenuation at lower frequencies. Based on the simplified porous elastic equations, the effects of petrophysical parameters (permeability, porosity, coupling density and fluid viscosity) on the velocity dispersion and attenuation of P and S waves are studied in brine‐saturated sandstone compared with the results of Biot's theory. The results show that the dispersion and attenuation of P waves in simplified theory are stronger than those of Biot's theory and appear at slightly lower frequencies because of the existence of bulk and shear viscous loss within pore fluid. The properties of the first S wave are almost consistent with the S wave in Biot's theory, while the second S wave not included in Biot's theory even dies off around its source due to its extremely strong attenuation. The permeability and porosity have an obvious impact on the velocity dispersion and attenuation of both P and S waves. Higher permeabilities make the peaks of attenuation shift towards lower frequencies. Higher porosities correspond to higher dispersion and attenuation. Moreover, the inertial coupling between fluid and solid induces weak velocity dispersion and attenuation of both P and S waves at higher frequencies, whereas the fluid viscosity dominates the dispersion and attenuation in a macroscopic porous medium. Besides, the heavy oil sand is used to investigate the influence of high viscous fluid on the dispersion and attenuation of both P and S waves. The dispersion and attenuation in heavy oil sand are stronger than those in brine‐saturated sandstone due to the considerable shear viscosity of heavy oil. Seismic properties are strongly influenced by the fluid viscosity; thus, viscosity should be included in fluid properties to explain solid–fluid combination behaviour properly.     

四川长宁页岩气开采区上地壳各向异性及其变化特征

四川长宁页岩气开采区2018年12月16日和2019年1月3日相继发生了M_L5.7和M_L5.3地震, 两次中强地震给附近建筑物带来了严重的破坏.本文收集了长宁地区2018年11月至2019年2月的地震波形数据资料, 利用剪切波分裂方法获得了研究区内14个台站的剪切波分裂参数快波偏振方向和慢波延迟时间, 分析了长宁地区地壳各向异性特征, 以及地壳应力场的变化, 初步探讨了两次M_L5.0地震孕育的应力环境.结果显示, 多数台站快波偏振方向有两个优势方向, 北东向和北西向.研究区位于四川盆地南缘盆山结合地带, 构造环境复杂, 剪切波分裂参数较为离散.北西向的快波偏振方向与区域主压应力方向、震源机制解结果一致, 北东向快波偏振方向与之正交.整个研究区域内慢波延迟时间普遍较高, 均值为6.16 ms·km^-1.慢波延迟时间最大值来源于位于两次M_L5.0地震震中之间的XIC台站.离散的剪切波分裂参数以及较高的各向异性强度, 可能与页岩气开采压裂作业有关.水力压裂施工, 页岩地层中注入高压流体, 从而改变了岩体和断层的应力状态, 引起应力环境的改变.慢波延迟时间在M_L5.3地震发生之前表现出明显增加的趋势, 表明地震发生之前地壳应力的积累.M_L5.3地震发生之后观测到慢波延迟时间的降低, 反映了震后地壳应力的突然释放.

     

Constraining the anisotropy structure of the crust by joint inversion of seismic reflection travel times and wave polarizations

In the context of wide-angle seismic profiling, the determination of the physical properties of the Earth crust, such as the elastic layer depth and seismic velocity, is often performed by inversion of P- and/or S-phases propagation data supplying the geometry of the medium (reflector depths) or any other structural parameter (P- or S-wave velocity, density...). Moreover, the inversion for velocity structure and interfaces is commonly performed using only seismic reflection travel times and/or crustal phase amplitudes in isotropic media. But it is very important to utilize more available information to constrain the non-uniqueness of the solution. In this paper, we present a simultaneous inversion method of seismic reflection travel times and polarizations data of transient elastic waves in stratified media to reconstruct not only layer depth and vertical P-wave velocity but also the anisotropy feature of the crust based on the estimation of the Thomsen’s parameters. We carry out a checking with synthetic data, comparing the inversion results obtained by anisotropic travel-time inversion to the results derived by joint inversion of seismic reflection travel times and polarizations data. The comparison proves that the first procedure leads to biased anisotropic models, while the second one fits nearly the real model. This makes the joint inversion method feasible. Finally, we investigate the geometry, P-wave velocity structure and anisotropy of the crust beneath Southeastern China by applying the proposed inversion method to previously acquired wide-angle seismic data. In this case, the anisotropy signature provides clear evidence that the Jiangshan-Shaoxing fault is the natural boundary between the Yangtze and Cathaysia blocks.     

孔隙-裂隙性质和黏土含量对页岩油层系致密岩石弹性波耗散规律的影响研究

作为一种非常规油气资源,页岩油储量丰富、分布范围广,具有巨大的勘探开发潜力,是近年来油气产业关注的重点与热点.然而,页岩油储层具有岩石矿物组分多样、低孔低渗、孔隙结构复杂、非均质性强等特征,与常规油气资源存在明显差异.本研究选取鄂尔多斯盆地中生界延长组长7油层组的10块致密砂岩样本,基于X射线衍射分析得到各样本的矿物组分,开展不同围压和流体条件下的超声波实验观测,进而获得样本的纵、横波速度和纵波衰减逆品质因子.基于实验测量获得变压力条件下的孔隙度,结合线性外推的方法,估算各样本的裂隙孔隙度,代入EIAS（Equivalent Inclusion-Average Stress,等效嵌入体应力平均）模型,求取对应的裂隙纵横比和裂隙密度,分析页岩油储层孔隙-裂隙性质对纵波衰减的影响.结果显示相对于衰减,致密砂岩总孔隙度、裂隙纵横比、裂隙密度和衰减变化量（不同围压下的衰减观测值与最大围压下的衰减观测值的差）之间的相关性更加明显.基于薄片分析,结果显示致密样本存在孔内黏土包体、微裂隙包体和粒间孔的三重孔隙结构,因此本文引入三重孔隙结构模型,定量估算各样本的孔内黏土含量,进而分析孔内黏土含量及总黏土含量和纵波衰减之间的关系.结果显示孔内黏土含量是主导页岩油储层纵波衰减大小的主要因素之一,而非总黏土含量.本研究可为页岩油储层衰减特征分析、岩石物理模型构建及地震勘探方法研究提供理论支撑.

     

Advances in the deep tectonics and seismic anisotropy of the Lijiang-Xiaojinhe fault zone in the Sichuan-Yunnan Block,Southwestern China

The Sichuan-Yunnan Block (SYB) is located at the SE margin of the Qinghai-Tibetan Plateau (TP). Under the influence of the southeastward movement of material originated from the TP, intense crustal deformation, frequent seismic activity, and complex geological structures are observed in the SYB. The Lijiang-Xiaojinhe fault (LXF) goes through the central part of the SYB, dividing it into two blocks from north to south, and forming an intersecting fault system with the surrounding faults. This paper firstly introduces the morphology and the nature of the LXF, the distribution of the regional surface displacements and the focal mechanisms, and then analyzes the medium deformation and the effects of faults. Moreover, according to the regional tectonics and geophysical patterns, the paper discusses the characteristics of the north-south blocks of the SYB and the abrupt change of deep structure along the LXF zone. Since seismic anisotropy is an essential property for detecting crustal stress, deep structures and dynamical mechanisms, this paper is dedicated to the advances in seismic anisotropy at different depths and different scales in the study area. There are noteworthy differences in the anisotropic features between the north part and the south part of the SYB, possibly associated with a clear boundary adjacent to the LXF. Such phenomenon suggests some close correlation between anisotropic zoning boundary and the LXF, although this boundary is not consistent with the LXF in strike. The results from the deformation of the crust and the upper mantle elucidate the distribution patterns of the crust-mantle coupling in the north part and the crust-mantle decoupling in the south part, even though this conclusion needs to be further verified by more studies. Presently, the scientific understanding of the deep tectonics and the media deformation around the “generalized” LXF i.e. the LXF with the Jinpingshan fault on its eastern side, is still insufficient, and related equivocal topics deserve more in-depth studies.