脆性岩石三轴压缩渐裂过程中的渗透性演化规律研究

岩石渐进破坏伴随着裂纹的开展发育,岩石的渗透性演化与裂纹的开展规律有着密切的联系。运用三轴渗流伺服装置对凝灰岩进行了不同围压和渗压下的渗流-应力耦合试验,分析了岩石在渐裂过程中不同裂纹开展阶段渗透率的演化规律。结果表明：在裂纹的稳定扩展阶段,渗透率的变化不明显,起裂强度对应的渗透率可用于确定最小渗透率;进入裂纹非稳定扩展阶段后,渗透率出现明显增大,增大过程可以分为两个阶段,裂纹环向应变能较好地反映渗透率增大的两个阶段,其拐点可用于确定起始渗透率的位置;在峰后软化阶段,渗透率出现下降并进入残余稳定阶段,裂纹环向应变率可以反映渗透率的下降阶段,并可用于确定峰值渗透率的位置。     

水力耦合作用下的砂岩声发射特性试验研究

水压会刺激岩石裂纹的产生和加速岩石破裂,对岩石的变形破坏特征和破坏机制有重要影响。利用MTS815 Flex Test GT 岩石力学试验系统和PCI-Ⅱ声发射仪开展了砂岩在不同围压下的水-力耦合试验。结果表明：在整个岩石破裂过程中,声发射活动随加载时间、应力变化表现出不同的特征;声发射活动在岩石的峰后阶段随着水压的增大更为集中,强度也更高,而随着围压的增大其集中程度和强度都有所降低;在相同围压下,声发射累计振铃计数和累计能量随着水压的增大而增大,在相同水压下,声发射累计振铃计数和累计能量则随着围压增大而有所减少;随着水压的增大,岩石最终失稳破坏时刻的声发射三维定位图中裂纹数量增多,裂纹的集中程度也更高,在宏观破坏形态上表现出破坏角减小。这些成果揭示了水-力耦合作用下岩石的破坏机制由压制剪切向压制张裂变化,岩石破裂的脆性破坏特征增强。     

考虑卸荷作用的裂隙岩体渗流应力耦合研究

大量试验证明,裂隙岩体在加载和卸荷的条件下其渗流与应力特性是不完全相同的。近年来,加载条件下的裂隙岩体渗流应力耦合研究得到了长足的发展,但卸荷作用下的水岩耦合分析还处于初始阶段。基于现有裂隙变形曲线的研究结果,建立了渗透系数与卸荷应力、应变间的本构关系,在此基础上,根据算例分析了各种工况下的渗流和应力特征,其计算结果表明,考虑卸荷后应力场的改变增加了临近坡面的岩体渗透系数,降低了地下水浸润线,增加了边坡的稳定性;考虑渗流后,边坡的位移场和应力场有较大改变,但考虑耦合后位移和应力分布和大小与不耦合时的相差不大;边坡开挖后应力场改变对渗流的影响远大于渗流对位移、应力等的影响。     

不同围压下岩石抗压强度与变形参数尺寸效应的数值模拟

岩石抗压强度和变形参数是岩石工程设计的重要指标。由于岩石是典型的非均质材料,其强度和变形特性随样品尺 寸的变化而不同。本文采用PFC2D程序模拟了不同围压下不同尺寸岩样的压缩试验。结果表明（1） 岩样具有明显的尺寸效 应。同一围压下,尺寸越大,岩石强度、峰值应变和压缩模量越小,尺寸的变化对岩样的破坏模式影响较小;（2） 岩样具 有明显的围压效应。同一尺寸的岩样,随着围压的增大,岩石强度、峰值应变和压缩模量均增加,其中强度和峰值应变随 围压的增加呈线性增加。同时,随着围压的增大,岩石破裂模式由轴向劈裂破坏向剪切破坏变化;（3） 围压的存在会影响 岩样的尺寸效应。不同尺寸岩样的强度和峰值应变在相同围压区间内的增量基本相同,同时随着围压的增大,其强度和峰 值应变增加,进而使岩石强度和峰值应变的尺寸效应弱化;而不同尺寸岩样的压缩模量在相同围压区间内的增长率大致相 同,因而造成围压对压缩模量尺寸效应的影响较小     

Relationship Between Permeability and Strain of Sandstone During the Process of Deformation and Failure

The permeability of the rock specimen during the process of deformation and failure is a dynamic evolution, and it relates with the growth and coalesces of micro-cracks within the rock. Although extensive researches have been conducted on the relationship between the stress–strain and the permeability, the deformation of the specimen should be taken into account for the evaluation of the permeability. In this study, experiments on the hydro-mechanical coupling of sandstone are carried out. Change of the permeability with the axial, lateral and volumetric strains is investigated. The experimental results show that the change of the permeability during the process of deformation and failure has apparent phases, and the evolution of the permeability is closely correlated with the strain. According to the experimental data, two kinds of empirical equations are fitted to better describe the relationship between the permeability and the strain. One is the polynomial equation used to simulate the phase of compression of rock specimens, the other is exponential equation used to simulate the rest of deformation phases. The obtained equations are just the empirical ones for the sandstone and are not for the universal application.     

Anisotropic mechanical properties and brittleness evaluation of layered phylliteEI北大核心

为了研究层状千枚岩的力学特性与各向异性特征,开展了不同层理倾角与不同围压下的千枚岩力学试验。对比分析了千枚岩试样强度、变形、脆性与破坏模式等各向异性特征。结果表明:(1)随着层理倾角β增加,岩样的强度、变形特征曲线形状呈U型;随围压增加,岩样强度及塑性增强,各向异性度逐渐减弱稳定。(2)采用多种强度准则描述岩样强度各向异性,其中Saeidi准则和改进Ramamurthy准则能很好地预测岩样在不同层理倾角下的抗压强度。(3)基于岩样峰前应力-应变曲线与能量特征提出了综合脆性评价指标,在层理倾角β=45°左右时,岩样脆性指标较低,更易发生剪切滑移破坏,得出脆性下降顺序为:沿层理面拉伸劈裂>穿层理面拉伸劈裂>沿层理面剪切>穿层理面剪切。(4)千枚岩的破坏模式与层理倾角和围压有关,单轴条件下,岩样劈裂破坏后易形成复杂裂纹网络;高围压下,岩样破裂后多形成单一的沿层理面或贯穿多层理面的剪切破坏。     

脆性岩石卸围压强度特性试验研究

岩样的承载能力由材料强度和围压共同确定,轴向压缩和卸围压二者都能导致岩样破坏,但由于应力路径不同,两种条件导致岩样的破坏过程并不相同。对取自位于雅砻江上的锦屏二级水电站辅助洞内的白山组大理岩进行了常规三轴试验和峰前、峰后卸围压试验,通过对试验曲线和岩样破坏特征的分析表明：无论是峰前还是峰后卸围压,岩样都表现出脆性破坏的特征,而岩样破坏表现出的脆性峰前卸围压比峰后卸围压更为强烈;岩样在加、卸载条件下的变形均随主应力差的增大而增大,但在相同的主应力差下,卸载产生的扩容量比加载的扩容量更大;峰前卸围压试验当围压卸到初始围压值的60 %左右时,岩样发生破坏;峰后卸围压试验当围压卸到初始围压值的80 %左右时,岩样发生破坏。结论对相关工程稳定性分析、设计和施工具有一定的指导意义和参考价值。     

Experimental Studies on Permeability of Intact and Singly Jointed Meta-Sedimentary Rocks Under Confining Pressure

Three different types of permeability tests were conducted on 23 intact and singly jointed rock specimens, which were cored from rock blocks collected from a rock cavern under construction in Singapore. The studied rock types belong to inter-bedded meta-sandstone and meta-siltstone with very low porosity and high uniaxial compressive strength. The transient pulse water flow method was employed to measure the permeability of intact meta-sandstone under a confining pressure up to 30 MPa. It showed that the magnitude order of meta-sandstone’s intrinsic permeability is about 10^?18 m². The steady-state gas flow method was used to measure the permeability of both intact meta-siltstone and meta-sandstone in a triaxial cell under different confining pressures spanning from 2.5 to 10 MPa. The measured permeability of both rock types ranged from 10^?21 to 10^?20 m². The influence of a single natural joint on the permeability of both rock types was studied by using the steady-state water flow method under different confining pressures spanning from 1.25 to 5.0 MPa, including loading and unloading phases. The measured permeability of both jointed rocks ranged from 10^?13 to 10^?11 m², where the permeability of jointed meta-siltstone was usually slightly lower than that of jointed meta-sandstone. The permeability of jointed rocks decreases with increasing confining pressure, which can be well fitted by an empirical power law relationship between the permeability and confining pressure or effective pressure. The permeability of partly open cracked specimens is lower than that of open cracked specimens, but it is higher than that of the specimen with a dominant vein for the meta-sandstone under the same confining pressure. The permeability of open cracked rock specimens will partially recover during the unloading confining pressure process. The equivalent crack (joint) aperture is as narrow as a magnitude order of 10^?6 m (1 μm) in the rock specimens under confining pressures spanning from 1.25 to 5.0 MPa, which represent the typical ground stress conditions in the cavern. The in situ hydraulic conductivity measurements conducted in six boreholes by the injection test showed that the in situ permeability of rock mass varies between 10^?18 and 10^?11 m². The lower bound of the in situ permeability is larger than that of the present laboratory-tested intact rock specimens, while the upper bound of the in situ permeability is less than that of the present laboratory-tested jointed rock specimens. The in situ permeability test results were thus compatible with our present laboratory permeability results of both intact and jointed rock specimens.     

Microcrack-based coupled damage and flow modeling of fracturing evolution in permeable brittle rocks

Microcracks in brittle rocks affect not only the local mechanical properties, but also the poroelastic behavior and permeability. A continuum coupled hydro-mechanical modeling approach is presented using a two-scale conceptual model representing realistic rock material containing micro-fractures. This approach combines a microcrack-based continuous damage model within generalized Biot poroelasticity, in which the tensors of macroscopic elastic stiffness, Biot effective stress coefficient and of overall permeability are directly related to microcrack growth. Heterogeneity in both mechanical and hydraulic properties evolves from an initially random distribution of damage to produce localized failure and fluid transmission. A significant advantage of the approach is the ability to accurately predict the evolution of realistic fracturing and associated fluid flow in permeable rocks where pre-existing fractures exert significant control. The model is validated for biaxial failure of rock in compression and replicates typical pre- and post-peak strength metrics of stress drop, AE event counts, permeability evolution and failure modes. The model is applied to the simulation of hydraulic fracturing in permeable rocks to examine the effects of heterogeneities, permeability and borehole pressurization rate on the initiation of fracturing. The results indicate that more homogenous rocks require higher hydraulic pressure to initiate fracturing and breakdown. Moreover, both the fracturing initiation pressure and breakdown pressure decrease with permeability but increase with borehole pressurization rate, and the upper and lower limit of the initiation pressure are seen to be given by the impermeable (Hubbert–Willis) and permeable (Haimson–Fairhurst) borehole wall solutions, respectively. The numerical results are shown to be in good agreement with the experimental observations and theoretical results. This coupled damage and flow modeling approach provides an alternative way to solve a variety of complicated hydro-mechanical problems in practical rock engineering with the process coupling strictly enforced.     

岩土材料不同应力路径下脆性变化的二元介质模拟

试验研究表明,岩土材料在加卸荷时表现出不同的变形破坏特性,卸荷时脆性增加。建议了一个可以描述材料脆性变化的脆性指数 ,并验证了B指标能较好地描述岩土材料在加卸荷时的脆性变化。应用二元介质模型,对岩样在卸载周围应力过程中的应力-应变关系进行了模拟,表明这一模型不但可以模拟岩样在低围压下的应变软化到高围压下的应变硬化现象,而且可以模拟岩样在卸载周围应力过程中的变形破坏过程,特别是对于表现为应变软化的岩样,可以反映出应力峰值前后卸围压时的变形破坏过程的差别。     

脆性岩石全应力-应变过程渗流特性试验研究

为了探讨岩石在整个变形过程中渗透性变化特点,取三峡永久船闸高边坡花岗岩与山西万家寨水电站高边坡灰岩试样分别进行不同围压条件下全应力-应变过程渗流试验。试验结果表明：岩石在破坏前后不同变形阶段渗流特性具有明显不同;岩石变形过程渗透性与变形破坏形式密切相关。在试验基础上进一步探讨了岩石全应力-应变过程渗流特点之机理,为研究水电工程节理裂隙岩体应力场与渗流场耦合问题提供有价值的支持。     

基于试验的花岗岩渐进破坏本构模型研究

对取自大渡河流域大岗山电站的花岗岩进行了系统的试验研究,得到了不同围压下的应力-应变曲线。根据岩样受力过程中内部微裂纹的活动状态,定义了4个特征应力,即闭合应力、起裂应力、破损应力、峰值应力,来反映花岗岩的渐进破坏过程,特征应力也反映了岩石内部的损伤程度。依据特征应力,将试验曲线分为4个阶段,并对每一阶段分别求取变形模量和泊松比。结果表明,变形模量不仅与围压有关,且与岩石的损伤程度有关,而泊松比只取决于岩石的损伤程度。特征应力可由体积应变曲线结合线性回归技术准确求取。在此基础上,提出了一个考虑花岗岩渐进破坏的本构模型。在该模型中,变形参数(弹性模量、泊松比)是应力状态的函数,强度参数（凝聚力、内摩擦角）是塑性变形的函数。将该模型嵌入大型有限元分析软件ABAQUS,对不同围压下的花岗岩室内压缩试验进行模拟,结果表明,模拟所得的应力-应变曲线与试验曲线的吻合程度较高,模型能反映花岗岩等脆性岩石的峰前非线性力学行为。     

小湾水电站坝基卸荷岩体抗剪特性研究

通过对小湾水电站松弛岩体抗剪特性以及更深入的现场和室内多种试验研究,以更细致深入的评价松弛岩体结构面的变形破坏机理及抗剪强度参数,为后续评价坝基固结灌浆效果以及复核拱坝坝基开挖和地基处理设计方案提供可靠的依据。     

砂岩三轴循环加卸载条件下的渗透率研究

渗透率是地下工程的流-固耦合分析中的一个关键因素。对多孔红砂岩进行了三轴压缩试验,在不同变形阶段实施了轴向应力循环加卸载,并在试验全过程中测量轴向渗透率,得到了试样破坏全过程的渗透率演化规律。从平均应力和循环加卸载对渗透率的影响等两方面进行了深入分析,结果表明,（1）随着轴向变形的增加,初始压密阶段和弹性变形试样渗透率均匀减小;进入塑性变形阶段,渗透率与轴向变形的曲线逐渐趋于水平,低围压条件下渗透率略有增加。（2）轴向加载使骨架颗粒被压缩,引起孔隙减小,造成渗透率减小;采用经验公式定量描述了渗透率和平均应力之间的关系。（3）轴向应力循环加卸载过程中,骨架颗粒的不可恢复变形引起渗透率产生不可恢复现象。（4）峰值后渗透率只发生少许突跳,说明对于多孔砂岩,孔隙和裂隙对渗透率的影响相当,且渗透率的突跳程度随着围压的升高而降低。     

砂岩峰后卸除围压过程的渗透性试验研究

为了探讨煤层开采引起的围岩卸除围压过程中砂岩渗透性的变化规律,本文用数控瞬态渗透法在电液伺服岩石力学试验系统MTS815.02上进行了砂岩试样的渗透特性试验。得出了试样峰前渗透系数-应变与应力-应变的关系曲线,以及在峰后保持轴向应变一定卸除围压过程中试样的渗透系数-围压与主应力差-围压的关系曲线;对试验砂岩在变形破坏过程中渗透性变化规律进行了总结,重点分析了其峰后卸除围压过程中渗透性的变化规律;并对试验砂岩峰后渗透系数与有效围压关系进行了拟合,得出了拟合方程式,为煤层开采引起的围岩体应力场与渗流场耦合问题提供参考。     

花岗岩三轴循环加卸载条件下的气体渗透率

为研究花岗岩在三轴循环加卸载条件下气体渗透率的演化规律,利用岩石多场耦合三轴试验仪分别对花岗岩进行三轴恒下限分级循环加卸载试验和气体渗透试验。试验结果表明：加载曲线与上一次循环卸载曲线形成塑性滞回环,随着循环次数的增加,每个循环应力下限的轴向应变也随之增加;前期几个循环试样体积压缩明显,以轴向压缩变形为主,后期轴向应力达到一定数值时体积由压缩转为扩容,裂纹发展方向偏向于轴向方向,均呈剪切脆性破坏;气体渗透率变化分为稳定下降阶段、缓慢增加阶段、急剧上升阶段3个阶段,试样脆性破坏后气体渗透率均上升2～3个数量级;体积应变曲线拐点与横向应变曲线结合起来可以作为研究岩石渗透率变化规律的一个重要参考因素。     

采动煤岩渗透率演化模型及数值模拟

为描述采动煤岩渗透率演化过程,引入强度退化指数,基于Hoek-Brown强度准则,建立了考虑围压影响的煤岩应变软化力学本构模型。给出了体积应变和渗透率的关系方程,结合应变软化模型建立了采动煤岩渗透率演化模型,并在FLAC下予以实现。通过数值模拟研究了不同围压下圆柱岩样的峰后应变软化力学行为和某煤矿工作面开采过程中煤岩的渗透率演化过程,结果表明：（1）该模型能较好地反映围压对煤岩峰后应变软化行为的影响;（2）随着工作面推进,越来越多的煤岩单元破坏,渗透率也不断增长,逐渐成为瓦斯等流体运移的主要通道。（3）模型能再现采动煤岩渗透率演化的动态过程,从而为煤与瓦斯共采、煤层瓦斯抽放和瓦斯灾害防治提供指导。     

Laboratory Investigations of the Hydro-Mechanical–Chemical Coupling Behaviour of Sandstone in CO2 Storage in Aquifers

This paper is devoted to experimental investigations of the hydro-mechanical–chemical coupling behaviour of sandstone in the context of CO₂ storage in aquifers. We focused on the evolution of creep strain, the transport properties and the elastic modulus of sandstone under the effect of CO₂–brine or CO₂ alone. A summary of previous laboratory results is first presented, including mechanical, poromechanical and hydro-mechanical–chemical coupling properties. Tests were then performed to investigate the evolution of the creep strain and permeability during the injection of CO₂–brine or CO₂ alone. After the injection of CO₂–brine or CO₂ alone, an instantaneous volumetric dilatancy was observed due to the decrease in the effective confining stress. However, CO₂ alone had a significant influence on the creep strain and permeability compared to the small influence of CO₂–brine. This phenomenon can be attributed to the acceleration of the CO₂–brine–rock reaction by the generation of carbonic acid induced by the dissolution of CO₂ into the brine. The original indentation tests on samples after the CO₂–brine–rock reaction were also performed and indicated that the elastic modulus decreased with an increasing reaction time. The present laboratory results can advance our knowledge of the hydro-mechanical–chemical coupling behaviour of sandstone in CO₂ storage in aquifers.     

小湾水电站低高程坝基开挖卸荷松弛机理试验研究

小湾电站低高程坝基开挖过程中,表层岩体卸荷松弛强烈,主要形式有沿已有裂隙地错动、张开和扩展,及因新生破裂而松弛。后者主要包括:葱皮现象、板裂现象和岩爆现象。试验测试表明,开挖面附近微新岩石为坚硬极坚硬岩石,但其初始损伤明显,其损伤发展启动应力在5.2~12.4MPa之间,抗拉强度在3~6MPa之间,这是坝基表层岩体卸荷松弛强烈的物质基础;坝基面爆破开挖的应力集中和爆炸作用影响是坝基岩体卸荷松弛强烈的岩石力学环境条件。     

Numerical Simulation of 3D Hydraulic Fracturing Based on an Improved Flow-Stress-Damage Model and a Parallel FEM Technique

The failure mechanism of hydraulic fractures in heterogeneous geological materials is an important topic in mining and petroleum engineering. A three-dimensional (3D) finite element model that considers the coupled effects of seepage, damage, and the stress field is introduced. This model is based on a previously developed two-dimensional (2D) version of the model (RFPA2D-Rock Failure Process Analysis). The RFPA3D-Parallel model is developed using a parallel finite element method with a message-passing interface library. The constitutive law of this model considers strength and stiffness degradation, stress-dependent permeability for the pre-peak stage, and deformation-dependent permeability for the post-peak stage. Using this model, 3D modelling of progressive failure and associated fluid flow in rock are conducted and used to investigate the hydro-mechanical response of rock samples at laboratory scale. The responses investigated are the axial stress–axial strain together with permeability evolution and fracture patterns at various stages of loading. Then, the hydraulic fracturing process inside a rock specimen is numerically simulated. Three coupled processes are considered: (1) mechanical deformation of the solid medium induced by the fluid pressure acting on the fracture surfaces and the rock skeleton, (2) fluid flow within the fracture, and (3) propagation of the fracture. The numerically simulated results show that the fractures from a vertical wellbore propagate in the maximum principal stress direction without branching, turning, and twisting in the case of a large difference in the magnitude of the far-field stresses. Otherwise, the fracture initiates in a non-preferred direction and plane then turns and twists during propagation to become aligned with the preferred direction and plane. This pattern of fracturing is common when the rock formation contains multiple layers with different material properties. In addition, local heterogeneity of the rock matrix and macro-scale stress fluctuations due to the variability of material properties can cause the branching, turning, and twisting of fractures.