页岩微纳观尺度雁列式断续裂纹的原位观测

在自然界岩石和工程围岩中,宏观尺度雁列式断续破裂现象较为常见。然而,在微纳观尺度上,是否也存在类似的雁列式断续破裂特征?其扩展与演化过程又是怎样的?这是非常值得探讨的基础科学问题。采用带切槽的弧形页岩试样,在扫描电子显微镜（SEM）下开展原位拉伸与微纳观破裂过程观测试验。基于原位观测分析,获得如下几点认识:（1）在微纳米尺度,依然存在断续破裂现象,呈“S”形或反“S”形雁列式展布,是由裂纹尖端部位拉张-剪切复合应力场及矿物非均质性共同诱导形成的。（2）微纳观雁列式断续裂纹总体展布特征和扩展、演化过程是有规律可循的,相邻微裂纹近似平行展布。对单个裂纹来讲,中间的拉张裂纹近似平行于最大拉张应力方向,而两端翼裂纹转向朝着最大剪切应力方向扩展。（3）微纳观雁列式断续裂纹仅代表着主裂纹路径形成过程中的暂时的剪切变形状态或某个中间扩展阶段。（4）雁列式断续裂纹在微米至纳米尺度上具有多尺度分层级结构特征:相对较大尺度的雁列式断续裂纹包含着更小尺度的次级雁列式断续裂纹。该研究提供了微纳观尺度页岩雁列式破裂的观测证据,为分析页岩多尺度裂纹演化过程提供了重要支撑。     

Mode of development of sigmoidal en echelon fractures

The solution of stress distribution for a multicrack system and model experiments confirm that en echelon cracks mutually interact with each other during their growth. Such a mechanical interaction deviates the crack-tip stress axes orientations from that of the bulk stress field and leads to a continuous change in propagation direction of tension cracks, initially at a right angle to the bulk tension direction. The sigmoidal shape of en echelon fractures evolve through rotation and crack length increments with changing orientations. The theoretical analysis shows that the instantaneous fracture-tip stress orientation is a function of initial crack spacing, orientation of crack array with respect to the principal axes of far-field stress.     

非贯通单节理岩体裂纹扩展方向研究

裂纹扩展方向的确定对分析岩桥破坏机制和岩体抗剪强度参数具有重要意义。首先以断裂力学观点推导了复杂应力条件下裂纹尖端应力分布函数的表达式,以节理岩体尖端的扩展裂纹可分为张拉裂纹和剪切裂纹为前提,基于Griffith破坏判据,提出了张拉裂纹扩展方向(张裂角)的计算公式;基于Mohr-Coulomb判据,提出了剪裂纹扩展方向(剪裂角)的计算公式。通过新判据与试验和其他判据的结果对比表明,该判据能准确判断张拉裂纹扩展方向,而剪裂角的扩展方向有待进一步试验验证。分析表明:在单向拉应力作用下,张裂纹扩展方向均有偏向于最大主应力方向的趋势,张裂纹与最大主应力夹角小于15°;双向拉应力作用下,随着微裂纹倾角变大,张裂纹有远离最大主应力方向的趋势;单轴压缩作用下,张裂角随裂纹倾角的增加而减小,而两者的和为先减小后增加。      

Analogue modeling of overlapping spreading centers: insights into their propagation and coalescence

The propagation and segmentation of mid-ocean ridges is studied using centrifuged analogue models built with non-linear materials. The deformation of the brittle-ductile model is controlled by diapiric uprise of buoyant analogous asthenospheric material induced by a centrifugal body force. This linear upwelling laterally stretches the model mantle that, in turn, induces failure in the upper layer simulating the brittle crust. Arrays of fractures initiate within zones of high stress concentration above the diapir. Fractures propagate laterally in a direction perpendicular to the maximum tensile (the minimum principal) stress. Secondary tension cracks initiate in the vicinity of parent fracture tips. Through-going fractures that crosscut the model surface develop by short fractures propagating toward each other and coalescing in different types of patterns. The overlap, overstep and inclination of fractures developed at the initial stage of extension control their subsequent growth and coalescence. Non-overlapping sub-parallel fractures propagate along nearly straight paths and coalesce to produce a single planar fracture. If overlapping fractures are parallel, they propagate towards each other along curved paths that enclose an intervening elliptical core of intact material. Fracture curvature in this case results from crack–crack interaction and is similar to that of overlapping spreading centers (OSCs) observed along mid-ocean ridges. Overlapping non-parallel fractures tend to coalesce by one of their tips propagating sub-parallel to the spreading direction toward the other fracture. Such offsets can serve as models for the development of the orthogonal ridge-transform fault patterns common along mid-ocean ridges.     

单轴加载条件下页岩层理角度对水力压裂缝扩展规律影响研究

水力压裂作为一种改造储层渗透性、压裂增产的技术,对页岩气开采具有重要意义。为研究射孔附近水力压裂过程中页岩各向异性特征对破裂压力及裂缝扩展的影响规律,开展了单轴试验条件下不同层理角度的页岩水力压裂试验。研究表明:页岩的破裂压力存在明显的各向异性,破裂压力随层理角度的分布曲线呈U型分布,其中0°和90°破裂压力最大,30°最小;页岩的破裂形态主要有两种,一种为沿着最大主应力方向即竖直方向起裂并延伸,另一种模式为裂缝先沿着最大主应力方向起裂并延伸,延伸过程中直接穿过层理面,随后渐渐转向为沿层理面方向扩展;破裂机制则包括拉张破坏和拉张剪切混合破坏。研究结果对于深入了解页岩裂缝起裂和延伸机理、水力压裂施工设计等具有重要的意义。     

Displacement features associated with fault zones: a comparison between observed examples and experimental models

Fault zones commonly consist of discontinuous surfaces or are composed of different kinds of en echelon fractures (tension cracks, R or P-type fractures) which give the brittle shear zone a width. Translation along these faults and related features can occur by sliding on the elementary planes and/or by solution/deposition processes with opening of transtension zones and possible dilatation of the shear zone.In order to understand how fault planes develop and to investigate the mechanical conditions corresponding to natural configurations, displacement along an analogue fault model was examined under conditions of direct shear. The experimental fracture patterns were then compared with the natural features. The structural elements of certain faults created by testing, and showing transpression and transtension zones, were found to be similar to natural domino structures bounded by elementary shears, and could be compared with computed situations.It can be inferred that stresses are reoriented inside the shear zone; the angle between elementary fractures depends on their order of development; transpression and transtension zones occur systematically; and the shear zone undergoes dilatancy under low normal stresses.     

Development of a full 3D numerical model to investigate the hydraulic fracture propagation under the impact of orthogonal natural fractures

Although hydraulic fracturing has been massively studied and applied as a key technique to enhance the gas production from tight formations, some problems and uncertainties exist to accurately predict and analyze the fracture behavior in complex reservoirs, especially in the naturally fractured reservoirs like shale reservoirs. This paper presents a full 3D numerical model (FLAC3D) to study hydraulic fracturing behavior under the impact of preexisting orthogonal natural fractures. In this numerical model, the hydraulic fracture propagation direction is assumed perpendicular to the minimum principal stress and activated only by tensile failure, whereas the preexisting natural fractures can be activated by tensile or shear failure or a combination of them, and only tensile failure can open the natural fracture as well. The newly developed model was used to study the impact of preexisting orthogonal natural fractures on hydraulic fracturing behavior, based on a multistage hydraulic fracturing operation in a naturally fractured reservoir from the Barnett Shale formation, northwest of Texas in USA. In this multistage operation, two more representative stages, i.e., stage 1 with a relatively large horizontal stress anisotropy of 3.3 MPa and stage 4 with a comparatively small one of 1.3 MPa, were selected to conduct the simulation. Based on the numerical results, one can observe that the interaction between hydraulic and natural fracture is driven mainly by induced stress around fracture tip. Besides, the horizontal stress anisotropy plays a key role in opening the natural fracture. Thus, no significant opened fracture is activated on natural fracture in stage 1, while in stage 4 an opened fracture invades to about 90 m into the first natural fracture. Conversely, the hydraulic fracture length in stage 1 is much longer than in stage 4, as some fluid volume is stored in the opened natural fracture in stage 4. In this work, the shear failure on natural fractures is treated as the main factor for inducing the seismic events. And the simulated seismic events, i.e., shear failure on natural fractures, are very comparable with the measured seismic events.

     

Extensional aspects of earthquake induced ruptures determined by an analysis of fracture bifurcation

Numerical and experimental investigation of symmetric fracture bifurcation (Kalthoff, 1972), has shown that for forks with small branch angles α<α_c, where α_c is approximately 14°, the propagation of the branches tends to enlarge the angle. For forks with larger branch angles, α>α_c, the propagation of the branches tends to diminish the angle. Forks with the critical angle α_c will propagate in their original direction. Kalthoff theorized that the branch angle changes as a function of K_I/K_II, where K_I and K_II are the stress intensity factors for tensile and shear (sliding) modes, respectively, and K_I is considerably larger than K_II. In this study I test the hypothesis that this fracture mechanic theory applies to the analysis of fault bifurcation in the crust, particularly in cases of rapid fracture.Fractures produced during the 1968 earthquake at the Coyote Creek fault in California are intensively branched and an example of rapid rupture. The angular behaviour of the branching ruptures in eight forks follows Kalthoffs theory unusually well. This implies that fracture at the surface was dominated by the tensile mode. Additional observations that support this implication are: series of prominent ruptures which show openings (of 20–30 mm per rupture), the symmetrical and bilateral forking, the high-intensity and angular shapes of individual branches, the opening of grabens associated with several bifurcations, lack of bifurcation in the southern break of the Coyote Creek fault, and the patterns of en echelon fractures which reflect mixed mode surfacial rupture.Hence, contrary to previous interpretations, according to field evidence and fracture mechanic theory, the fault bifurcation and opening along the Coyote Creek fault are not compatible with local tension caused by the primary shear. Fracture probably occurred by different mechanical modes at depth and at the surface. While faulting may have originated by shear at depth, rupture at the surface was dominated by far-field tension associated with NE-SW extension in South California. The present model predicts the directions of fracture propagation along the fault.     

Transitional–tensile fracture propagation: a status report

One model for the development of hybrid shear fractures is transitional–tensile fracture propagation, a process described as the in-plane propagation of a crack subject to a shear traction while held open by a tensile normal stress. Presumably, such propagation leads to a brittle structure that is the hybrid of a joint and a shear fracture. Crack–seal veins with oblique fibers are possible candidates. While these veins clearly show shear offset, this is not conclusive evidence that a shear traction was present at the time of initial crack propagation. Many recent structural geology textbooks use a parabolic Coulomb–Mohr failure envelope to explain the mechanics of transitional–tensile fracturing. However, the laboratory experiments cited as demonstrating transitional–tensile behavior fail to produce the fracture orientation predicted by a parabolic failure envelope. Additional attempts at verification include field examples of conjugate joint sets with small acute angles, but these conjugate joints form neither simultaneously nor in the stress field required by the transitional–tensile model. Finally, linear elastic fracture mechanics provides strong theoretical grounds for rejecting the notion that individual cracks propagate in their own plane when subject to a shear traction. These observations suggest that transitional–tensile fracture propagation is unlikely to occur in homogeneous, isotropic rock, and that it is not explained by a parabolic Coulomb–Mohr failure envelope as several recent structural geology textbooks have suggested.     

青海玛多“5·22”MS7.4级地震的同震地表破裂特征、成因及意义

2021年5月22日2时4分在青海省果洛藏族州玛多县境内发生M_S7.4级地震,此次玛多M_S7.4级地震是2008年汶川M_S8.0级大地震之后中国震级最大的一次地震,及时查明其同震地表破裂展布及特征,对于正确认识发震构造和区域防震减灾具有重要意义。根据震后现场调查,结合高分辨率卫星遥感图像的解译分析、余震数据和典型地震地表破裂的无人机低空摄影测量等结果,初步获得了此次地震6处典型地震地表破裂的特征。结果发现:此次玛多地震的地表破裂主要沿已知的东昆仑断裂带的南侧分支断裂昆仑山口-江错断裂的东南段分布,分析表明其中的江错断裂应是此次地震的发震断层;同震破裂的西段总体走向275°~300°,主要表现为挤压鼓包和雁列式张裂隙的斜列组合,其中江错贡麻段至江多村段出现了明显的1.4~0.8 m的垂直位移,指示该段可能具有较明显的正断层成分;中部黄河乡段主要由一系列呈左阶斜列的北西向P剪切裂缝和右阶雁行排列的北东向张裂隙构成,走滑位移较小;而东段地表破裂出现了多个分支,其中北支昌马河段主要由一系列雁行排列的张裂隙组成,总体走向为260°,与断裂西段的走向明显不同;地震造成的最大左旋位移出现在西段的错尔加拉破裂段,约2.8 m,指示此次地震地表破裂带的走滑位移主要呈从西向东的单侧扩展-衰减特征。考虑到此次玛多地震出现在东昆仑主干断裂南侧的巴颜喀拉地块内部,表明该地块内部具有发生7级以上大地震的能力,因此,巴颜喀拉地块内部强震活动的孕震条件和机理应该是未来需要进一步关注的科学问题。      

The geometry of en-echelon vein arrays

Two distinct types of en-echelon vein arrays are recognised, those which form in active ductile shear zones and those which form by the primary nucleation of fractures not connected with active shear zones. The theoretical relation between vein width and orientation in a zone undergoing progressive simple shear is examined and compared with similar data from natural arrays. It is found that many veins have undergone dilation prior to rotation during simple shear, showing that shearing occurred after the en-echelon array was established. Generally, all strongly sigmoidal veins occur in arrays in which pressure solution was also active. In non-sigmoidal arrays there is a clear relation between the amount of overlap of adjacent veins and the orientation of the veins relative to the zone containing them. Two different patterns of dilation and distortion of arrays in which pressure solution was not active are described. It is concluded that primary en-echelon veins originated as tensile fractures, whilst en-echelon veins formed in active shear zones originated as shear fractures.     

Prediction and analysis of the stimulated reservoir volume for shale gas reservoirs based on rock failure mechanism

The ultra-low-permeability shale gas reservoir has a lot of well-developed natural fractures. It has been proven that hydraulic fracture growth pattern is usually a complex network fracture rather than conventional single planar fractures by micro-seismic monitoring, which can be explained as the shear and tensile failure of natural fractures or creation of new cracks due to the increase in reservoir pore pressure caused by fluid injection during the process of hydraulic fracturing. In order to simulate the network fracture growth, a mathematical model was established based on full tensor permeability, continuum method and fluid mass conservation equation. Firstly, the governing equation of fluid diffusivity based on permeability tensor was solved to obtain the reservoir pressure distribution. Then Mohr–Coulomb shear failure criterion and tensile failure criterion were used to decide whether the rock failed or not in any block on the basis of the calculated reservoir pressure. The grid-block permeability was modified according to the change of fracture aperture once any type of rock failure criterion was met within a grid block. Finally, the stimulated reservoir volume (SRV) zone was represented by an enhancement permeability zone. After calibrating the numerical solution of the model with the field micro-seismic information, a sensitivity study was performed to analyze the effects of some factors including initial reservoir pressure, injection fluid volume, natural fracture azimuth angle and horizontal stress difference on the SRV (shape, size, bandwidth and length). The results show that the SRV size increases with the increasing initial pore reservoir and injection fluid volume, but decreases with the increase in the horizontal principal stress difference and natural fracture azimuth angle. The SRV shape is always similar for different initial pore reservoir and injection fluid volume. The SRV is observed to become shorter in length and wider in bandwidth with the decrease in natural fracture azimuth angle and horizontal principal stress difference.     

Influence of inclination angles and confining pressures on mechanical behavior of rock materials containing a preexisting crack

To deeply understand the cracking mechanical behavior of brittle rock materials, numerical simulations of a rock specimen containing a single preexisting crack were carried out by the expanded distinct element method (EDEM). Based on the analysis of crack tips and a comparison between stress- and strain-based methods, the strain strength criterion was adopted in the numerical models to simulate the crack initiation and propagation processes under uniaxial and biaxial compression. The simulation results indicated that the crack inclination angle and confining pressure had a great influence on the tensile and shear properties, peak strength, and failure behaviors, which also showed a good agreement with the experimental results. If the specimen was under uniaxial compression, it was found that the initiation stress and peak strength first decreased and then increased with an increasing inclination angle α. Regardless of the size of α, tensile cracks initiated prior to shear cracks. If α was small (such as α ≤ 30°), the tensile cracks dominated the specimen failure, the wing cracks propagated towards the direction of uniaxial compression, and the propagation of shear cracks was inhibited by the high concentration of tensile stress. In contrast, if α was large (such as α ≥ 45°), mixed cracks dominated the specimen failure, and the external loading favored the further propagation of shear cracks. Analyzing the numerical results of the specimen with a 45° inclination angle under biaxial compression, it was revealed that lateral confinement had a significant influence on the initiation sequence and the mechanical properties of new cracks.     

Dilation and linkage of echelon cracks

We investigate arrays of echelon cracks in rock (veins, joints, and dikes) formed by dilation. Individual cracks are interpreted as cross-sections of blade-like cracks at the leading edges of a parent fracture. Two morphological end members are distinguished as having straight or highly curved propagation paths as seen in such cross-sections. Bridges of rock between overlapping straight crack paths bend to accommodate crack dilation. Sigmoidal curvature of the bridges is achieved in a phase of increased dilation as the aspect ratio of bridges increases and their resistance to bending drops. Large bending strains within bridges may lead to cross fractures that link adjacent cracks. Good estimates of the dilation and shear displacement over arrays may be obtained directly from measurements of bridges. In contrast, the sigmoidal form of cracks of the other end member is caused by their curving propagation paths. These cracks may link as one crack tip intersects the adjacent crack wall. Some arrays of sigmoidal echelon cracks are associated with shear zones, but those analyzed here are not.     

Macro fracture systems of the granites controlled by the tectonism: A case study of the Dangjiasi pluton in the northeastern margin of the Gonghe BasinSCIEI北大核心CSCD

花岗岩中先存裂缝系统的识别、评价与建模,关系到干热岩热能提取的有效性、规模性、安全性,是地热能勘查、开采的难点与关键点。本文对共和盆地东北部干热岩勘察开采示范区紧邻的当家寺岩体开展了详细地质野外调查及综合分析,观测了花岗岩体内裂缝的产状、类型和样式,详细解剖了岩体裂缝系统组成及空间分布,探讨了构造作用对裂缝系统形成的时限、动力学成因的控制。研究发现其宏观裂缝系统以构造破裂缝为主,同时还发育少量的成岩缝。构造缝主要由小尺度断裂、火成岩脉、石英脉、方解石脉及多期节理缝共同构成;在岩体不同分区部位的断裂、脉体及节理等裂缝体系发育差异明显,脉体、节理受临近断层控制,其三者走向具有较好的相似性,且存在明显的多期次性。根据产状、交切关系及应力机制可以划分为5种构造破裂类型:单一应力场形成的节理系、叠加先期形成单向滑移的共轭节理组、持续走滑剪切形成的雁列石英脉与共轭节理组合、拉张形成的岩脉及脉内雁列节理组、多期次叠加形成的网状裂缝。宏观裂缝系统的形成与三叠纪末期碰撞后伸展、侏罗-白垩纪区域性隆升、渐新世-中新世中期走滑断裂活动、中新世晚期以来走滑-逆冲转换等有关。现存大量共轭剪节理形成应力场与现今最大主应力方向(NE)有差异,反映了古走滑剪切构造作用的影响。宏观裂缝系统的差异分布,不仅对花岗岩型干热岩热储层规模、质量、分布有约束,也控制着后期的建储与改造。     

基于PIV技术的贯通单裂隙岩体压-剪破坏特征试验

基于PIV技术,采用相似材料预制贯通单裂隙岩体试件并开展压-剪试验,探究了裂隙开度对贯通单裂隙试件的强度、变形及破坏过程的影响.结果表明:(1)试件发生初始断裂时不同裂隙开度试验组应力均明显下降,且随着开度的增大裂隙岩体胶结部位塑性变形增强,岩体试件的特征应力值减小;(2)裂隙开度影响到裂隙和胶结部位变形在试件总变形中...     

基于考虑动力本构的连续−非连续方法的单轴压缩岩样开裂过程模拟

动载作用下岩石的破坏规律研究对于众多地质灾害的机制分析和预防具有重要的理论及实际意义。鉴于数值模拟研究的优势,应大力发展适于岩石动力断裂过程模拟的数值方法。在自主开发的拉格朗日元与离散元耦合连续−非连续方法的基础上,采用朱−王−唐本构模型取代了广义胡克定律,发展了考虑动力本构的连续−非连续方法,其正确性通过模拟不同加载速度时砂岩试样的单轴压缩试验进行了验证。通过统计裂缝区段数目随着岩样的纵向应变的演化规律,并监测岩样左、右对称线上多个测点的最小主应力的演化规律,开展了不同加载速度时单轴压缩花岗岩试样的变形−开裂过程研究,阐明了岩样的开裂机制。研究发现,剪裂缝以雁列式展布,整体上形成剪切带。随着时步数目的增加,各测点的最小主应力均呈波动下降−震荡上升的变化趋势。震荡上升阶段对应岩样的应变软化阶段。测点分离后最小主应力的震荡幅度较大,这是由于节点分离和单元接触激发了较大的应力波。剪切带尖端的最小主应力集中会使测点发生剪切分离。当岩样的三角块向下楔入时,下方测点的应力状态类似于紧凑拉伸试验进而发生拉伸分离。     

东坪金矿床地质特征及构造控矿作用

东坪金矿床具有雁列式分布的矿体 ,石英脉型与钾蚀变岩型叠置的矿化 ,多期次成矿作用的贫硫富碲型矿石等 3大特点 ,其中尤以雁列式分布的矿体最为特征 ,其形成是由于燕山期该区在SN向应力场作用下 ,产生一对左旋力偶 ,形成了一组走向NNE ,倾向NW的左行雁列式压扭性断裂和一组走向NW ,倾向SW的右行雁列式张扭性断裂。而二者在空间相互追踪中 ,NNE向断裂又严格制约了NW向断裂发展 ,从而构成多字型断裂构造格式 ,控制了金矿成矿作用。     

Numerical models of hydraulic fracturing and the interpretation of syntectonic veins

The hydraulic fracture is modelled as an ellipse in an infinite elastic medium with an internal fluid pressure and loaded under biaxial stresses at infinity. The available stress function for this model has been evaluated numerically, and the magnitudes of the stresses generated around the crack calculated for a variety of loading conditions and crack orientations. Fracture initiation is predicted from the Griffith maximum tensile stress criterion. The location of the maximum tensile stress around the crack is recorded and it is found that for many conditions of applied stresses and crack fluid pressure, the hydraulic shear fracture has a symmetrically developed maximum tensile stress and fracture initiation will occur by growth along the direction of the crack. It is also predicted that fracture initiation will occur when the ratio of fluid pressure to applied least principal stress is considerably less than one. The elastic strain energy fields around elliptical hydraulic flaws have been calculated, and in particular, the change in strain energy upon introduction of a small flaw, and the change in strain energy upon growth of this flaw, have been investigated. The results allow an evaluation of the second part of the Griffith criterion-that fracture growth is accompanied by a decrease in strain energy-for hydraulic fractures. Changes in strain energy with small increases in fluid pressure provide a physical basis for dilatancy hardening and fracture instability. Quasi-static growth from a flaw is modelled by calculating changes in strain energy for unit increases in half length. The distinction between fractures which show an increasing and a decreasing rate of change in strain energy with increasing length, and between fractures which may only extend spontaneously for short distances and those which may show extensive spontaneous growth on the basis of the rate of change of strain energy with length, is made. A gradual drop in crack fluid pressure once the threshold for fracture initiation has been passed may promote the extent to which spontaneous crack growth occurs.The formation of syntectonic veins, particularly in rocks being deformed under low grade metamorphic conditions, is often the most abundant evidence of natural hydraulic fracturing in rocks. Commonly observed geometric features of syntectonic veins-length, simple tapering, symmetric and asymmetric forking, branching, irregular zig-zag traces, en échelon patterns—are discussed primarily with reference to the strain energy model for growth established, and the geometric variation is interpreted in terms of variation in applied stress and fluid pressure conditions and the rate of change of stored strain energy with crack growth. In particular, terminal branching arises when the minimum stress changes from a symmetric to an asymmetric location at the tip of a growing shear fracture, and terminal forking results when there is an increase in the energy release rate during crack growth, and may be symmetric or asymmetric depending on the location of the minimum stress at the crack tip at the time of forking.