在围压条件下岩体模型应力应变关系的实验研究

含切口岩体模型在差应力作用下的应力应变关系表明:岩体不同部位的应力应变曲线按斜率变化特征可以划分为四个发展阶段。根据对切口岩石破裂过程的对比,各阶段的变形机制分别与岩体内原微裂纹闭合、孔隙和颗粒弹性变形、宏观张裂剪裂以及剪切、张-剪主破裂相对应,但各阶段的发展进程在时间、空间上却不一致,这一差异均与切口分布和岩性变化相关。最后讨论了应力应变特征与发震断层孕震过程的关系,对地震预报可能有实际意义     

基于团簇的岩石动态CT图像可视化与结构变形分析

岩石的变形机制是研究岩石圈变形和局部构造活动的重要基础,岩石的微观结构是研究其变形机制的关键。微观层析成像（微观CT）技术可以无损地获取样品内部高精度三维数字化图像,为观测岩石内部结构提供了技术保证。新近发展起来的动态CT技术,使得观测岩石变形过程成为可能。本文利用一组莱塔石灰岩受差应力变形的三维动态CT图像数据,通过图像切割、图像分割等处理步骤,以团簇为基本表征单位,表征不同形状、大小孔隙的动态变化过程,分析对比全部孔隙、大团簇、小团簇和裂隙型小团簇的动态图像。结果表明:随着岩石的压缩变形,其大孔隙在不断地缩小;在变形的初始阶段,靠近施压活塞的样品上部出现了密集的新生小孔隙;当岩石所受差应力到达73 MPa时,其下半部出现了明显的剪切带,具体表现为大量的新生裂隙生成并且新生裂隙集中带与轴向成约45°交角。此前的研究观测到该样品中下部若干压缩带形成的过程;本文通过基于团簇的可视化技术,观测到了高孔隙度岩石变形过程中压缩带和剪切带共同形成的过程。同时,新生的沿剪切带分布的微小裂隙并没有显示与剪切带相同的方向,而是具有各方向均匀分布的特征。该特征可能与岩石内部复杂的局部孔隙结构有关,是值得进一步探讨的问题。      

GPS初步结果揭示的中国大陆水平应变场与构造变形

根据中国大陆不同来源的多个GPS区域监测网1991～1999年间的观测资料和“中国地壳运动观测网络”基本网1998～2000年的观测资料，联合处理得到中国大陆地壳水平运动速度场结果，通过最小二乘配置法建立中国大陆水平运动速度场模型，获得了基于连续介质假设的中国大陆水平应变场（或称为视应变场）初步结果. 分析了水平运动、应变场空间分布特征及其与强震的关系，并简要分析了2001年11月14日昆仑山口西8.1级大地震的区域构造变形背景. 结果表明：中国大陆中西部构造变形强烈，应变速率值高，又以青藏块体及其边缘和新疆西部最为显著. 除川滇、新疆西部外，大部分地区的近东西向断裂存在左旋剪切变形，近南北向的断裂存在右旋剪切变形. 而东部地区构造变形相对较弱. 强震通常发生在剪切应变率的高值区及其边缘，尤其是与构造变形背景相一致的剪应变率高值区. 昆仑山口西8.1级地震发生在最显著的东西向左旋剪切应变率高值区，从该区域的应变状态分析，具备近东西向断裂产生巨型走滑破裂错动的构造变形背景.     

基于三维X射线显微镜的孔隙性砂岩中变形带微观结构解析

孔隙性砂岩中变形带对揭示其相关地质作用过程和工程应用具有重要的意义,而其微观结构是其相关科学与工程研究的关键点。常规的岩石微观结构分析手段对于全面解析岩石内部的微观结构具有一定的局限性,而近年发展起来的三维X射线显微成像技术可以无损地获得岩石内部微米尺度的三维精细数字化图像,是建立岩石微观结构并对其进行解析的有效手段。通过三维X射线显微镜对孔隙性砂岩中变形带扫面,建立了高分辨率三维微观结构图像,利用高密度切片图像,对其微观结构特征进行解析。根据变形带微观产出状带和颗粒破碎过程的解析,建立颗粒破碎-破碎颗粒与残余破碎颗粒重新排列-局部颗粒破碎带-弱颗粒破碎变形带-强颗粒破碎变形带-强弱颗粒破碎变形带相间排列-簇状变形带的完整时间发育序列。      

天然气水合物地层岩石物理模型构建

天然气水合物地震反射特征的识别和物性信息的定量提取存在多解性与不确定性,岩石物理模型是将水合物地层地球物理观测值地震信息定量转化为物性信息的桥梁,而传统的岩石物理模型少有对微观孔隙结构和孔隙充填水合物剪切性质的描述.文章针对孔隙充填和颗粒支撑两种微观分布模式的水合物地层,重点考虑水合物地层的微观孔隙结构以及水合物的剪切性质,基于等效介质理论定量描述地层矿物组分特征和孔隙连通性及形状,利用斑块饱和理论和广义Gassmann理论定量描述孔隙充填水合物的剪切性质,在此基础上构建了两种模式水合物地层的岩石物理模型,揭示了水合物地层宏观弹性性质与微观物理性质之间的定量关系.数值研究发现,纵横波速度比随孔隙度和水合物饱和度增大而减小;颗粒支撑模式地层的纵横波速度对水合物含量较为敏感,且孔隙越狭长,敏感性越高;孔隙充填模式地层的纵横波速度比在高水合物饱和度时对水合物组分剪切性质的敏感性更高.实验数据和神狐海域的实际资料应用结果表明,岩石物理模型可有效地计算水合物地层宏观弹性性质与微观物性特征之间的定量关系,提供常规测井中缺乏的横波速度信息,确定对水合物含量指示能力较强的弹性参数,可以根据实际数据求取水合物饱和度、孔隙纵横比等物性参数,为天然气水合物地层的定量解释和资源评价提供理论依据和数据支持.     

水对高应变率扭剪变形实验中大理岩强度的影响

利用 Kolsky 扭转棒作为实验装置,笔者在常温常压条件下对干燥的和潮湿的大理岩试样进行了一系列的动态简单剪切的实验变形研究,以期了解在高应变率(373-1-1736-1)时岩石中孔隙水对于岩石抗剪强度的影响.结果表明,孔隙水的存在不但没有降低,反而稍微提高了岩石的抗剪强度.结合前人在静态条件下的实验研究资料,笔者认为水对岩石变形的作用机制随应变率的变化而改变.在低应变率(例如:10-9-1)时,水对岩石变形的影响分别表现为压溶作用、应力侵蚀作用和降低有效应力的效应.压溶作用和应力侵蚀作用导致岩石强度的降低;而有效应力的降低则导致岩石强度的相对提高.      

高孔隙岩石局部压缩屈服与帽盖模型

对石灰岩不同围压的三轴压缩过程进行微结构观察,发现:当平均应力与体积应变曲线偏离静水压力线时,岩样微结构产生颗粒之间胶结物破坏,颗粒挤紧,孔隙减小,导致应变硬化,达到剪切增强的压实阶段.继续加载较软的颗粒被挤碎,导致崩塌变软,岩样出现宏观软化现象;同时孔洞表面产生大量平行于最大主应力方向的裂纹,汇聚成核.在低围压(40 MPa)时,含天然缺陷的岩石中,观测到剪切带的产生,也会在局部缺陷处发生崩塌;随着围压的增加(150MPa),局部体积崩塌将逐步引发大面积的垮塌.对应岩石试件的屈服行为也由以剪切破坏机制为主,逐渐转变为以体积崩塌压缩机制为主,引起纵向应变迅速增加.当剪切增强压缩继续发展到C*′直到峰值应力前时产生颗粒破裂、移动或旋转,曲线出现转向,岩样发生体积膨胀,同时引起密集声发射活动.峰值应力后,微裂纹串联成裂纹簇开始剪切诱导岩样体积膨胀,导致应力降引起应变软化,同时产生剪切局部化.在没有无序性影响的压缩带发展和传播的数值模拟中,展示了压缩带由样品的上下端面向样品中部逐渐扩展的过程,与Bentheim砂岩切片图显示和AE监测结果是吻合的.通过微观与宏观实验结合的描述方法清楚地展现了剪切带、压缩带、膨胀带的产生机理以及压缩带的发展过程,这里压缩屈服过程即为帽盖模型的形成过程,它是近似椭圆的帽盖模型中的帽子部分.     

川滇块体东边界主要断裂带运动特性及动力学机制研究

作为青藏高原东南缘的川滇块体,变形剧烈,地震频发.川滇块体的侧向挤出滑移造成了东边界的左旋剪切变形,不同的地震学者利用不同的方法得出了重要的定量结果.大地测量技术特别是GPS技术的快速发展,为断裂带形变场研究提供了高精度的观测数据,也给研究断裂带动态变化特征及其动力学机制提供了动态观测资料约束.因此,有必要综合利用块体运动模型、断裂带本身的构造变形定量分析及数值模拟方法,给出鲜水河—安宁河—则木河—小江断裂带的运动特征及应变积累特性的定量结果.     

关于岩土孔隙压力参数问题

孔隙压力参数是用来表述饱和岩土受力变形时所受力与超孔隙压力关系的参数,是地震地下水前兆机理研究中的重要问题。本文从室内试验、现场试验与野外观测的某些结果,分析了不同岩土、不同受力条件下孔隙压力参数的若干特征     

含不同随机缺陷数目岩样的破坏过程、前兆及全部变形特征

利用拉格朗日元法模拟了平面应变单轴压缩条件下具有初始随机材料缺陷的岩石试样的破坏过程、前兆和宏观力学响应.利用若干FISH函数于岩样内部规定初始缺陷并计算全部变形特征.比密实岩石弱的缺陷在破坏之后经历理想塑性行为.密实岩石在破坏之后先是经历线性应变软化行为,然后是理想塑性行为.随着缺陷数目的增加,剪切带间距缩小;岩样的强度下降.在应变软化阶段,最大不平衡力的明显的突增是由于被剪切带切割的试样块体发生了沿剪切带方向的运动.试样内部的缺陷数目越少,应变软化阶段最大不平衡力的峰值越高.无论从单元破坏角度,还是从侧向应变-轴向应变曲线、体积应变-轴向应变曲线、计算得到的泊松比-轴向应变曲线角度,都得到了相同的结论（即,缺陷越多,破坏前兆越明显）.采用两条倾斜的扫描线对岩样最终的破坏形态进行了扫描,发现当扫描线间距等于半个至1个单元长度时,初始随机缺陷位置与岩样最终破坏形态具有很好的相关性.     

基于非线性屈服准则及主应力判据的圆形巷道围岩岩爆过程的数值模拟

鉴于考虑拉伸截断的线性莫尔-库仑屈服准则不能考虑岩石在高应力条件下的非线性屈服特征,根据虎克-布朗本构模型,将上述屈服准则在受压区的线性屈服函数修改为非线性形式。采用3种岩爆的主应力判据,判别圆形巷道开挖之后围岩中各种级别岩爆的分布及演变规律。计算采用＂先加载,后挖洞＂的方式,岩石服从弹—脆—塑性本构模型。研究发现,发生高级别岩爆的单元数少于发生低级别岩爆的单元数;轻微岩爆区的形态更接近于塑性区及剪切应变增量的高值区。根据巴顿判据,发生重岩爆的单元数较多,而且主要发生的是拉伸岩爆;根据陶振宇判据,发生轻微及中等岩爆的单元数均多于根据谷明成-陶振宇判据判别的结果。上述两种判据判别的结果均表明,在不考虑拉伸岩爆的条件下,只有位于围岩内部的单元才有可能发生高级别的岩爆,而位于巷道表面的单元一般仅发生轻微岩爆。     

A semi-empirical velocity-porosity-clay model for petrophysical interpretation of P- and S-velocities

We design a velocity–porosity model for sand-shale environments with the emphasis on its application to petrophysical interpretation of compressional and shear velocities. In order to achieve this objective, we extend the velocity–porosity model proposed by Krief et al., to account for the effect of clay content in sandstones, using the published laboratory experiments on rocks and well log data in a wide range of porosities and clay contents. The model of Krief et al. works well for clean compacted rocks. It assumes that compressional and shear velocities in a porous fluid-saturated rock obey Gassmann formulae with the Biot compliance coefficient. In order to use this model for clay-rich rocks, we assume that the bulk and shear moduli of the grain material, and the dependence of the compliance on porosity, are functions of the clay content. Statistical analysis of published laboratory data shows that the moduli of the matrix grain material are best defined by low Hashin–Shtrikman bounds. The parameters of the model include the bulk and shear moduli of the sand and clay mineral components as well as coefficients which define the dependence of the bulk and shear compliance on porosity and clay content. The constants of the model are determined by a multivariate non-linear regression fit for P- and S-velocities as functions of porosity and clay content using the data acquired in the area of interest. In order to demonstrate the potential application of the proposed model to petrophysical interpretation, we design an inversion procedure, which allows us to estimate porosity, saturation and/or clay content from compressional and shear velocities. Testing of the model on laboratory data and a set of well logs from Carnarvon Basin, Australia, shows good agreement between predictions and measurements. This simple velocity-porosity-clay semi-empirical model could be used for more reliable petrophysical interpretation of compressional and shear velocities obtained from well logs or surface seismic data.     

Biot流动和喷射流动耦合作用下波传播的FCT紧致差分模拟

本文从含流体多孔隙介质中同时包含Biot流动和喷射流动两种力学机制的BISQ(Biot-Squirt)方程出发,利用FCT(Flux-Corrected Transport)紧致差分方法数值模拟了在Biot流和喷射流共同作用下的波在含流体多孔隙各向同性介质中的传播.通过与仅受Biot流动作用下的波场结果对比,我们研究了...     

An erosion criterion for gravel-bed rivers

A theoretically-based erosion criterion is developed for gravel-bed rivers which incorporates the effect of both grain geometry and turbulent velocity fluctuations. It is derived from a balance of instantaneous drag, lift, and gravity forces operating on individual grains and is calculated for spherical grains arranged in three distinct geometries. To accommodate the temporal variation in bed shear stress, the model includes a stochastic element based on the characteristics of turbulence derived from the flume evidence of McQuivey (1973a, b). In terms of the Shields parameter, results show reasonable agreement with the range of observations quoted from the field and with the experimental data of Fenton and Abbott (1977). Finally, the argument is generalized to cover applications in the wider context of field conditions including a range of grain sizes and flow conditions.     

Compaction and Failure in High Porosity Carbonates: Mechanical Data and Microstructural Observations

We investigated systematically the micromechanics of compaction in two carbonates of porosity above 30%, Majella grainstone and Saint Maximin limestone. The composition, grain size and pore surface area of these rocks were determined. Hydrostatic compression experiments were performed under dry and wet conditions beyond the onset of grain crushing. A significant water weakening effect was observed in both rocks. A set of conventional triaxial experiments was also performed on both rocks under dry conditions at confining pressures ranging from 3 to 31 MPa. Microstructural observations were carried out on the deformed samples. The mechanical behavior of these high porosity carbonates is dominated by shear-enhanced compaction associated in most cases with strain hardening. Stress-induced cracking and grain crushing are the dominant micromechanisms of deformation in both rocks. In Majella grainstone, compactive shear bands appeared at low confinement, in qualitative agreement with the deformation bands observed in the field. At higher confining pressures, compaction localization was inhibited and homogeneous cataclastic flow developed. In Saint Maximin limestone, compaction localization was observed at all confining pressures. An increasing number of compactive shear bands at various orientations appeared with increasing strain. These new data suggest that compaction localization is important in the mechanical compaction of high porosity carbonates.     

Discontinuous Galerkin methods for advective transport in single-continuum models of fractured media

Accurate simulation of flow and transport processes in fractured rocks requires that flow in fractures and shear zones to be coupled with flow in the porous rock matrix. To this end, we will herein consider a single-continuum approach in which both fractures and the porous rock are represented as volumetric objects, i.e., as cells in an unstructured triangular grid with a permeability and a porosity value associated with each cell. Hence, from a numerical point of view, there is no distinction between flow in the fractures and the rock matrix. This enables modelling of realistic cases with very complex structures. To compute single-phase advective transport in such a model, we propose to use a family of higher-order discontinuous Galerkin methods. Single-phase transport equations are hyperbolic and have an inherent causality in the sense that information propagates along streamlines. This causality is preserved in our discontinuous Galerkin discretization. We can therefore use a simple topological sort of the graph of discrete fluxes to reorder the degrees-of-freedom such that the discretized linear system gets a lower block-triangular form, from which the solution can be computed very efficiently using a single-pass forward block substitution. The accuracy and utility of the resulting transport solver is illustrated through several numerical experiments.     

Effect and mechanism of stresses on rock permeability at different scales

1 Introduction Unlike general solids, rocks are porous materialswhich include different scales of pores, such as pores, cracks, fractures, capillary and disfigurement in the crystal, tiny pores and cracks between crystal grains at micro-scale, in which the fluid is water, oil or gas. Thedifferences between rocks and solids can be seen in two aspects, one is stresses bearing states. Solids are only subjected to external stresses, while rocks are subjected to external stresses σ ij (i, j=1,2,3)…     

基于三相孔隙弹性理论的火山岩气层岩石物理响应特征分析

Unlike previous theories with velocity and/or elastic modulus averaging, we use a three-phase porous rock physics model developed by Santos for analyzing the seismic response of two immiscible fluids in saturated porous media. Considering reservoir reference pressure and coupling drag of two fluids in pores, the effects of frequency, porosity, and gas saturation on the phase velocities of the P-and S-waves are discussed in detail under field conditions. The effects of porosity and gas saturation on Vp/Vs are also provided. The data for our numerical experiments are from a sample of deep volcanic rock from Daqing. The numerical results show that the frequency dispersion effect can be ignored for deep volcanic rocks with low porosity and low permeability. It is concluded that for deep volcanic rocks the effect of gas content in pores on Vp/Vs is negligible but the effect of porosity is significant when there is a certain amount of water contained in the pores. The accurate estimate of lithology and porosity in this case is relatively more important.     

Pore fluid pressure effects in anisotropic rocks: Mechanisms of induced seismicity and weak faults

Many observations and studies indicate that pore fluid pressure in the crustal rocks plays an important role in deformation, faulting, and earthquake processes. Conventional models of pore pressure effects often assume isotropic porous rocks and yield the nondeviatoric pressure effects which seem insufficient to explain diverse phenomena related to pore pressure variation, such as fluid-extraction induced seismicity and crustal weak faults. We derive the anisotropic effective stress law especially for transversely-isotropic and orthotropic rocks, and propose that the deviatoric effects of pore fluid pressure in anisotropic rocks not only affect rock effective strength but also cause variation of shear stresses. Such shear stress variations induced by either pore pressure buildup or pore pressure decline may lead to faulting instability and trigger earthquakes, and provide mechanisms for the failure of crustal weak faults with low level of shear stresses. We believe that the deviatoric effects of pore fluid pressure in anisotropic rocks are of wide application in studies of earthquake precursors and aftershocks, oil and gas reservoir characterization, enhanced oil recovery, and hydraulic fracturing.