基于PFC2D的花岗岩新生裂隙萌生扩展过程分析

岩石中新生裂隙萌生扩展过程是分析岩石工程性质的重要基础,可以使用颗粒流方法进行研究。本文使用花岗岩室内单轴压缩试验结果标定细观力学性质参数,使用图像处理技术确定花岗岩细观组分的实际分布,通过编制颗粒流代码来确定新生裂隙的类型和萌生扩展过程,分析了岩石变形破坏过程中不同类型新生裂隙变化的阶段性。结果表明,在单轴压缩条件下,花岗岩中新生裂隙萌生时首先出现剪切裂隙、然后出现拉伸裂隙,新生裂隙类型以拉伸裂隙为主;第一条新生剪切裂隙和拉伸裂隙分别出现在石英-黑云母、长石-石英之间;新生拉伸裂隙和剪切裂隙走向分别以331°～340°和以341°～350°为主;在峰值荷载前后新生裂隙扩展表现出明显不同的特点,在峰值荷载之前新生裂隙经受了压密、剪切裂隙增长、拉伸裂隙和剪切裂隙共同增长等阶段,在峰值荷载之后新生裂隙主要经受了拉伸裂隙和剪切裂隙快速增长阶段。本文结果对分析岩石变形破坏机理具有一定的参考价值。     

岩体裂隙扩展过程的数值模拟

本文根据断裂力学理论及节理岩体的等效连续模型,探讨了岩体裂隙扩展过程的数值模拟方法,文末算例说明了方法的有效性。     

莫高窟壁画裂隙萌生特性试验研究

敦煌莫高窟壁画长期受到外界微振动环境的影响,部分出现了裂隙病害,为此有必要深入开展壁画裂隙萌生特性的研究,为壁画的保护和修复工作提供科学依据和技术支持。针对莫高窟壁画地仗试件开展了四点弯曲抗拉试验,并利用数字图像相关技术实时测量了试件表面的全场应变变化与分布情况,分析了地仗在承受拉力破坏时裂隙萌生特性,给出了壁画所能承受的极限应变。同时,研究了试件缺陷、加载方式、疲劳次数等对试件的承载力与极限应变的影响。研究发现,莫高窟壁画裂隙萌生甚至一定程度的扩展并不会影响洞壁结构的承载力;壁画地仗的固有缺陷会导致壁画在缺陷处及其附近出现裂隙病害的风险加大;壁画地仗中添加的麻等纤维使其具有非常好的塑性和抗疲劳性能,只要在疲劳极限内就不会萌生裂隙。为了避免壁画产生裂隙病害,壁画地仗的最大正应变要严格控制在0.40%以内。     

岩石中三维单裂隙扩展过程的试验研究和数值模拟

自然岩体内部断续结构面的产状差异导致裂隙间岩桥破裂贯通成为一个复杂的三维岩石力学问题。为了探究裂隙的走向差异与尺寸效应对岩体力学行为的影响,开展了不同裂隙延伸尺寸下含走向异面断续裂隙类岩石材料的单轴压缩试验,结合CT扫描技术,分析了岩石走向差异角和尺寸效应对裂隙岩体破坏模式、岩桥贯通机制及力学特性的影响。研究表明：试样岩桥以外的端部主要由翼裂纹或反翼裂纹沿着荷载的方向扩展而引起破裂,岩桥区域的破裂模式受裂隙延伸尺寸和走向差异角综合影响,裂隙延伸尺寸的增加会使岩桥区域受拉因素变多,走向差异角在很大程度上减小岩桥的贯通率,并使岩桥区域贯通面的翘曲程度增大。随着裂隙延伸方向尺寸的增大,试样的峰值强度和弹性模量会随之减小然后趋于平缓,但是当裂隙增加了走向差异角后,峰值强度与弹性模量的变化规律更加复杂。走向差异角与裂隙延伸共同造成试样的整体三维不对称性,进一步影响了试样的峰值强度、弹性模量以及塑性。研究成果可为岩桥控制型复杂裂隙岩体工程的破坏机制和稳定性评价提供理论支撑。     

多场耦合作用下岩体裂隙扩展演化关键问题研究

裂隙岩体热-水-应力（THM）耦合是目前研究的热点和难点。首先总结了裂隙岩体多场耦合的机制、模型、方法及研究内容,并通过分析裂隙对THM耦合的重要控制作用,提出了在THM耦合中考虑裂隙网络扩展演化及模拟的关键问题,同时指出了研究的3个关键点：（1）建立考虑裂隙网络演化的耦合模型;（2）裂隙扩展的数值模拟方法;（3）THM耦合及岩体变形、失稳全过程的数值模拟算法。随后通过对模拟多场耦合和裂隙扩展数值方法的归类比较,重点论述了目前适用于模拟多场耦合下裂隙扩展模拟的各种数值方法（包括有限单元法、无单元法、单位分解法、离散单元法、岩石破裂过程分析方法和数值流形方法）的优缺点,并通过对比研究,推荐采用数值流形方法（NMM）来实现对关键问题的模拟研究。最后,对研究思路和难点进行了初步探讨。     

水平井中多条裂缝同步扩展时裂缝竞争机制

水平井中多条水力裂缝间的应力干扰行为,造成了压裂液排量的非均匀分配,影响了水力裂缝的几何形态。采用边界元法研究岩体在压裂液作用下的变形程度,以幂律流体泊肃叶平板流动方程来研究水力裂缝内部的压裂液流场,考虑了多条裂缝间应力干扰和压裂液流量分配,建立了流-固耦合的水平井多条水力裂缝同步扩展模型。模型可模拟水平井多条水力裂缝几何形态、应力干扰情况和压裂液排量的分配情况,可解释水力裂缝之间的竞争机制。多条裂缝同步扩展时,压裂液排量并非均等地分配到各个裂缝之中,进入到内部裂缝的压裂液流量最小,内部裂缝宽度最小;内部的水力裂缝增长一定长度后,停止增长,并且在应力干扰下逐渐闭合。     

扫描电子显微镜对花岗岩中主要矿物裂隙作用的研究

微裂隙对于岩石的物理性质有重要影响.通过光学显微镜对显微裂隙进行的观察,难以将薄片的制作过程中产生的裂隙同自然裂隙区分;其次,观测的裂隙大小限制在0.1mm或以上数量级,一些重要细节观察不到.扫描电镜的高放大倍数和三维分辨率使其成为适合于裂隙研究的理想工具.利用扫描电镜观测了浙江花岗岩在室温下由51.6MPa压力产生的裂隙的发育过程.观察花岗岩的表面以研究其微裂隙和矿物的解理、晶形及破裂作用.微裂隙分为3种类型:晶体内裂隙(完全发育在颗粒内部),晶体间裂隙(穿过颗粒边界进入其他颗粒中),颗粒边界裂隙(沿颗粒边界发育或与边界重合).本研究中的花岗岩为中-粗粒(1~6mm),经过蚀变,约含40%斜长石,25%钾长石,25%石英,10%镁铁矿物(主要为黑云母和白云母).     

岩石中三维双裂隙组扩展和贯通过程的试验研究和弹脆性模拟

最新研制了一种完全透明、较低温度下拉压比可达1/6.6的树脂,其与岩石特性的相似性较前人的研究有大幅度提高。制作了一组含三维内置平行双裂隙组的试件,详细描述并分析了单轴加载下试件的裂隙扩展与贯通过程,研究其产生的条件和机制。试验结果表明：试件的破坏大致经历4个阶段,三维裂隙组的断裂比二维情况复杂得多,产生了多种不同形态的裂纹。在FLAC3D软件框架内建立了一种新的弹脆性本构关系,并采用超细单元和合理形态的网格建模。将数值模拟结果与相关试验进行对比,相符程度良好,表明新数值模拟方法的有效性。当双轴加载时,模拟结果也与前人类似试验结果有很好一致性。     

岩体裂隙模拟的扩展有限元法应用研究

岩体中普遍存在着断层﹑节理和裂隙等结构面,这些结构面的存在和发展对岩体的整体强度﹑变形及稳定性有极大的影响。因此,研究岩体中原生结构面的萌生﹑发展以及贯通演化过程对评估岩体工程安全性和可靠性具有非常重要的理论与现实意义。扩展有限元法（XFEM）作为一种求解不连续问题的有效数值方法,模拟裂隙时独立于网格,因此,在模拟岩体裂隙扩展﹑水力劈裂等方面具有独特优势。针对扩展有限元法的基本理论及其在岩体裂隙扩展模拟中的应用展开了研究,建立了扩展有限元法求解岩体裂隙摩擦接触、岩体裂隙破坏等问题的数值模型,并将计算模型应用于岩质边坡稳定性分析和重力坝坝基断裂破坏等工程问题。     

含折线型裂隙砂岩试件翼型裂纹起裂与扩展机制研究

岩体内部赋存的裂隙很多表现为折线型,为探究这类岩体的断裂机制,制备含折线型裂隙砂岩试件并对其进行单轴压缩试验。采用数字图像相关(DIC)方法计算加载过程中的变形场演化,根据新生裂纹两侧的位移差异识别裂纹类型;运用扩展有限元法(XFEM)模拟断裂过程,根据应力分布特征解释翼型裂纹起裂与扩展机制。DIC计算结果表明,新生裂纹处出现应变局部化带,裂纹两侧发生相对分离;含直线型和折线型裂隙砂岩试件的翼型裂纹分别萌生于预制裂隙端部以及折角处,这是因为裂隙几何形态会改变拉应力集中位置;含折线型裂隙砂岩试件的起裂应力小于含直线型裂隙砂岩试件,这是因为相同加载条件下前者的最大拉应力值更大;这2类试件的裂纹扩展均是由于裂纹尖端集中的拉应力引起的,裂纹依然呈张开状态;裂隙几何形态未改变试件的最终破坏模式,均表现为对角剪切破坏。     

The subsurface initiation of some minor granite landforms

Flared slopes originate as concave weathering fronts beneath the regolith, and several other minor granite landforms found in association with them (tafoni, Rillen) may be initiated subsurface also. Other features, particularly incipient gnammas and gutters, have been observed on surfaces that have only recently been cleared of granite debris in situ.     

Frictional crack initiation and propagation analysis using the numerical manifold method

By employing both a physical mesh and a mathematical mesh to formulate a physical problem, the numerical manifold method (NMM) can lead to a very simple meshing task, which allows directly capturing the discontinuities across the crack surfaces without further incorporating unknowns to the related nodes through enrichment functions. These features enable the NMM to handle complex crack problems. In this study, based on the contact technique of the NMM and the incorporation of the Mohr–Coulomb crack initiation criterion, the effects of the friction and cohesion on the crack growth from a closed flaw (crack) under compression were investigated. A limited number of comparisons between the numerical results and the physical experiments show that with the Mohr–Coulomb crack initiation criterion, the NMM can not only accurately predict the pure tensile or pure shear crack growth, but the NMM can also satisfactorily predict the development of mixed shear–tensile crack types. Using a parametric analysis, the effects of the confining stress, the flaw inclination angle, the flaw friction angle and the material strengths on the crack development (crack initiation stress, crack initiation angle, crack type developed) have been investigated.     

天然裂缝影响下的花岗岩水力裂缝扩展数值模拟

水力压裂技术是成功实现干热岩资源开发利用的重要手段之一,数值模拟技术能够精准预测水力裂缝扩展。针对典型花岗岩,借助黏性单元法,分别模拟了致密花岗岩和天然裂缝存在情况下的水力裂缝扩展特征,得出以下结论:致密花岗岩的水力裂缝形态单一,天然裂缝的存在增加了压裂后裂缝的复杂性;致密花岗岩水力裂缝拓展主要分为憋压和拓展两个交替往复的阶段,当存在天然裂隙时,水力压裂过程会变得复杂;天然裂缝存在时,水力裂缝的缝长和缝宽分别为致密花岗岩的5.7倍和1.7倍;缝网的形成需要借助复杂的压裂工艺实现。研究结果可以为增强型地热系统(EGS)储层水力刺激工作提供理论支持。     

Study of the initiation and propagation of excavation damaged zones around openings in argillaceous rock

The study of the creation and evolution of the excavation disturbed zone (EDZ) in argillaceous rocks is a major issue for the safety of nuclear wastes underground repositories. In this context, the argillaceous Tournemire site has provided a unique opportunity to study the evolution of the EDZ with time thanks to the existence of three openings of different ages. A thorough characterization of the EDZ has been conducted by different means such as visual observation, analysis of samples extracted from drilled boreholes, EDZ permeability measurements, etc. On the basis of these measurements, a conceptual model of the EDZ initiation and propagation at the Tournemire site has been proposed. In order to validate this model, numerical simulations of increasing complexity have been carried out. In a first attempt, the response of the rock mass to the excavation phase, followed by seasonal cyclic variations of temperature and relative humidity inside the opening, has been simulated by means of a purely mechanical analysis, using a simple elastic material model. The EDZ has been estimated by post-processing the calculated stress states, using a Mohr–Coulomb failure criterion. The results obtained show that no EDZ could be predicted unless adopting a low cohesion value for the rock mass. Moreover, the deferred nature of the EDZ formation in Tournemire could not be reproduced. These limitations have then been suppressed by using a coupled viscoplastic-damaging mechanical model, the parameters of which have been identified from different laboratory experiments. With this model, a time evolution of the EDZ could be predicted, but the EDZ pattern could not match the one observed in situ. Finally, in view of the importance of the hydraulic couplings, unsaturated hydro-mechanical calculations have been carried out to investigate the effect of the numerous seasonal variations cycles and the resulting shrinkage.     

煤岩水力压裂起裂压力和裂缝扩展机制实验研究

为了研究煤岩水力压裂的起裂压力和水力压裂裂缝扩展规律,采用型煤试样,利用自主研发的水力压裂实验系统,参照现场压裂施工制定了“施加三向应力-顶部注水”的煤岩水力压裂物理模拟实验方案并开展了水力压裂实验,分析了不同条件下泵注压力和水力压裂裂缝。实验结果表明：压裂液泵注排量越大,起裂压力越大。三向应力满足最大水平主应力σ_H > 垂向应力σ_v > 最小水平主应力σ_h,水力压裂裂缝沿着垂直于σ_h的方向扩展。σ_v和σ_h一定,随着σ_H的增大,煤岩起裂压力先增大后减小,水力压裂裂缝扩展路径越平直。当σ_H远大于σ_v和σ_h时,水力压裂裂缝扩展路径越复杂,分叉缝角度越大。研究结果可为煤岩水力压裂理论的完善提供一定的参考和借鉴。     

Investigation of hydraulic fracturing operation and fracture propagation simulation in reservoir rock

For many decades most oil wells in Iran have produced using their natural flow potential and haven’t needed to be fractured. As time goes by, the reservoir pressure depletes and the need for hydraulic fracturing as a stimulation practice arises. Nonetheless there is no record of successful hydraulic fracturing in Iran.

The Bangestan reservoir with a suitable amount of oil in place and good rock reservoirs, has been selected for the present research work. In this work, the in situ stress profile was calculated by using the available petrophysical data. This is achieved by using poroelastic theory for the stresses, and the Mohr–Coulomb criterion to predict failure. The model leads to easily computed expressions for calculating the pressure required to maintain hydraulic fracturing. Then the appropriate depth for treatment was determined. The results indicate that Ilam and Sarvak formations could be good candidates for hydraulic fracturing. Then, for two layers, a hydraulic fracture was designed and the production was predicted and the Net Present Value (NPV) resulting from the fracture of both layers was investigated.     

深源花岗岩中的碳氢化合物

利用石油地球化学的溶剂抽提和气相色谱技术研究了北京延庆碓臼峪花岗岩（深源花岗岩）样品中的可溶有机质，同时用高温处理过的石英砂进行空白对比实验。结果表明，所研究的两个花岗岩样品中均存在少量的高碳数碳氢化合物，且具有相同的成分特征。这为自然界存在深源的长链碳氢化合物提供了新的证据，也为解释花岗岩浆演化晚期水热流体中短链羧酸的成因提供了线索。     

Investigation of thermal-hydro-mechanical coupled fracture propagation considering rock damage

Thermal-hydro-mechanical (THM) coupled fracture propagation is common in underground engineering. Rock damage, as an inherent property of rock, significantly affects fracture propagation, but how it influences the THM coupled fracturing remains stubbornly unclear. A pore-scale THM coupling model is developed to study this problem, which combines the lattice Boltzmann method (LBM), the discrete element method (DEM), and rock damage development theory together for the first time. This model can more accurately calculate the exchanged THM information at the fluid-solid boundary and fluid conductivity dependent on fracture and rock damage. Based on the developed model, the synergistic effect of injected temperature difference (fluid temperature below rock temperature) and rock damage (characterized by the parameter “critical fracture energy”, abbreviated as “CFE”) on fracture propagation of shale are investigated particularly. It is found that: (1) the generation of branched cracks is closely related to the temperature response frontier, and the fracture process zone of single bond failure increases in higher CFE. (2) through the analysis of micro failure events, hydraulic fracturing is more pronounced in the low CFE, while thermal fracturing displays the opposite trend. The fluid conductivity of fractured rock increases with a higher injected temperature difference due to the more penetrated cracks and wider fracture aperture. However, this enhancement weakens when rock damage is significant. (3) in the multiple-layered rock with various CFEs, branched cracks propagating to adjacent layers are more difficult to form when the injection hole stays in the layer with significant rock damage than without rock damage.