Mechanical Behaviour of Reservoir Rock Under Brine Saturation

Acoustic emissions (AE) and stress–strain curve analysis are well accepted ways of analysing crack propagation and monitoring the various failure stages (such as crack closure, crack initiation level during rock failure under compression) of rocks and rock-like materials. This paper presents details and results of experimental investigations conducted for characterizing the brittle failure processes induced in a rock due to monocyclic uniaxial compression on loading of two types of sandstone core samples saturated in NaCl brines of varying concentration (0, 2, 5, 10 and 15 % NaCl by weight). The two types of sandstone samples were saturated under vacuum for more than 45 days with the respective pore fluid to allow them to interact with the rocks. It was observed that the uniaxial compressive strength and stress–strain behaviour of the rock specimens changed with increasing NaCl concentration in the saturating fluid. The acoustic emission patterns also varied considerably for increasing ionic strength of the saturating brines. These observations can be attributed to the deposition of NaCl crystals in the rock’s pore spaces as well some minor geo-chemical interactions between the rock minerals and the brine. The AE pattern variations could also be partly related to the higher conductivity of the ionic strength of the high-NaCl concentration brine as it is able to transfer more acoustic energy from the cracks to the AE sensors.     

基于超声检测的软岩单轴流变损伤试验

采用软岩相似材料进行了2种不同流变加载方式下的单轴流变损伤破坏试验,并通过超声检测技术获得了软岩在各级流变阶段的超声信号。试验结果表明:软岩相似材料具有强烈的黏、弹、塑性变形特征,选用西原模型对软岩各级流变过程进行模拟较为合适;当应力达到或超过软岩试件单轴抗压强度的80%时,纵波波速曲线才会在流变过程中持续下降,出现可检测损伤迹象,而首波振幅曲线波动大,检测结果不准确;以波速定义损伤变量,得到了软岩流变过程损伤演化曲线。试验结果能为软岩工程长期流变下的结构稳定性提供依据。      

Water effects on rock strength and stiffness degradation

Reduction in strength and stiffness in rocks attributed to an increase in water content has been extensively researched on a large variety of rock types over the past decades. Due to the considerable variations of texture and lithology, the extent of water-weakening effect is highly varied among different rock types, spanning from nearly negligible in quartzite to 90 % of uniaxial compressive strength reduction in shale. Readers, however, often face difficulties in comparing the data published in different sources due to the discrepancy of experimental procedures of obtaining the water saturation state and how the raw laboratory data is interpreted. In view of this, the present paper first reviews the terminologies commonly used to quantify the amount of water stored in rocks. The second part of the paper reviews the water-weakening effects on rock strengths, particularly focusing on uniaxial compressive strength and modulus, as well as tensile strength, under quasi-static loading and dynamic loading. The correlation relationships established among various parameters, including porosity, density and fabric of rocks, and external factors such as strain rate, surface tension and dielectric constant of the saturating liquid, absorption percentage and suction pressure, are reviewed and presented toward the end of the paper.     

Different bedding loaded coal mechanics properties and acoustic emission

Using TAW-2000KN electro-hydraulic servo rock press machines and the American Physical Acoustics Company’s SH-II acoustic emission systems, experimental studies began to address the mechanical properties in different beddings of loaded coal and the related acoustic emission characteristics, established based on the acoustic emission damage model, and verify the model. The results show that the mechanical properties of different coal sample beddings are distinctive, with maximum uniaxial compressive strength and elastic modulus of vertical stratification of coal samples and the minimum Poisson’s ratio. Thus, the minimum uniaxial compressive strength and elastic modulus of oblique bedding coal samples along with the maximum Poisson’s ratio in the processes of loading result in different bedding coal samples having different stress–strain curves, especially when different bedding coal samples experience the stages of fissure compression, elastic deformation, plastic deformation and instability and destruction. In addition, the displacement proportions of each stage of the loading process have relatively obvious differences: the loading times of vertical, parallel and oblique bedding coal are 495, 382 and 331 s, respectively, and their acoustic emission mutation points of peak stress are approximately 60, 41 and 33%, respectively. Thus, we can use the mutation point as precursor information to estimate the damage intensity in different bedding seams. The theoretical and experimental stress–strain curves obtained by the coal damage model are basically identical, verifying the reliability of the model and reflecting the feasibility of acoustic emission technology in the study of coal damage. The results can effectively forecast coal and gas outburst hazard in coal mines, especially highly gassy and outburst mines. It can also make comprehensive predictions for flooding accidents, roof fall accidents and other disasters, and provide valuable evacuation time for underground coal mine workers. The results are of great scientific significance in safeguarding the safety of coal mines.     

Numerical Study of Failure Mechanism of Serial and Parallel Rock Pillars

Using a numerical modelling code, rock failure process analysis, 2D, the progressive failure process and associated acoustic emission behaviour of serial and parallel rock samples were simulated. Both serial- and parallel sample models are presented for investigating the mechanism of rock pillar failure. As expected, the numerical results show that not only the stiffness, but also the uniaxial compressive strength of the rock plays an important role in pillar instability. For serial pillars, the elastic rebound of a rock pillar with higher uniaxial compressive strength can lead to the sudden failure of an adjacent rock pillar with lower uniaxial compressive strength. The failure zone forms and develops in the pillar with lower uniaxial compressive strength; however, the failure zone does not pass across the interface of the two pillars. In comparison, when two pillars have the same uniaxial compressive strengths but different elastic moduli, both serial pillars fail, and the failure zone in the two pillars can interact, passing across the interface and entering the other pillar. For parallel pillars, damage always develops in the pillar having the lower uniaxial compressive strength or lower elastic modulus. Furthermore, in accordance with the Kaiser effect, the stress-induced damage in a rock pillar is irreversible, and only when the previous stress state in the failed rock pillar is exceeded or the subsequent applied energy is larger than the energy released by the external loading will further damage continue to occur. In addition, the homogeneity index of rock also can affect the failure modes of parallel pillars, even though the uniaxial compressive strength and stiffness of each pillar are the same.     

万福煤矿不同含水率砂岩蠕变力学特性试验研究

为研究深部巷道围岩在地下水作用下的长期蠕变力学特性,采用自主研制的深部软岩五联流变试验系统,开展不同含水率(0%、0.8%、1.6%、2.4%、3.3%)下砂岩吸水软化单轴压缩试验及单轴蠕变试验。通过试验结果研究表明：砂岩的单轴抗压强度、弹性模量和蠕变破坏应力与含水率呈指数下降关系,蠕变破坏应力与单轴抗压强度的比值在0.76～0.84之间;砂岩衰减蠕变阶段时间随着含水率的增加而减少,随应力水平的增加而增加。径向应变比轴向应变先进入稳态蠕变阶段,破坏应力下径向应变的加速蠕变阶段开始时间要先于轴向应变;基于稳态蠕变速率曲线确定了砂岩的长期强度,径向稳态蠕变速率确定的值略小于轴向,长期强度与含水率之间满足负指数关系;将蠕变试验中径向应变与轴向应变之比定义为μ_c,提出了基于μ_c值的岩石长期强度确定方法且μ_c值与含水率无关,对于本次砂岩样品可以认为μ_c值大于0.3时样品会在一定时间内发生加速蠕变破坏;随着含水率的增加,样品破坏形态由单斜面剪切破坏逐渐演变至X状共轭斜面剪切破坏。研究结果为地下水作用下巷道长期稳...     

Investigation and modeling on fatigue damage evolution of rock as a function of logarithmic cycle

The fatigue damage behavior of granite under constant and variable amplitude loadings is studied. The experimental analysis reveals that there is a three‐stage law for the fatigue damage evolution as a function of absolute or relative cycle and the inverted‐S damage model proposed by the author, in this case, is capable of representing the damage behavior of rock. However, when the logarithmic cycle is considered, there are only two stages, i.e. steady and accelerated stages and the fatigue damage evolution greatly depends on the properties of rock and stress level. Accordingly, the fatigue damage evolution curves have been categorized into three types. Then, the effect of maximum stress, amplitude and fatigue initial damage on the damage evolution of rock is investigated. The analysis reveals that the damage evolution greatly depends on these influencing factors. The fatigue life decreases with the increase in the maximum stress, amplitude and fatigue initial damage due to the decrease in the proportion of the first stage to the whole fatigue process and the increase in the damage rate in the first stage. Meanwhile, a linear‐exponential formula is used to model the fatigue damage behavior of rock subjected to cyclic loading. This damage model is superior to the inverted‐S damage model in the convenience of establishment of critical instability point. The physical meanings of its constants have been illuminated and the applicability of this model to constant and variable amplitude cyclic loading explored. The fitting results for the test data show that this damage model can properly represent the fatigue damage behavior of rock. Copyright © 2010 John Wiley & Sons, Ltd.     

工程岩爆灾害判别的RBF-AR耦合模型

岩爆是深部高地应力区地下岩体工程中的主要工程地质灾害之一,其发生及烈度预测是一个复杂的不确定系统问题。为了有效预测和判别深部工程岩爆灾害,在总体考虑岩爆各影响因素的基础上,选取地下工程中岩体完整性指数、岩石单轴抗压强度、岩石单轴抗拉强度、围岩最大切向应力、围岩抗压强度与其抗拉强度的比值、围岩切向应力与围岩抗压强度比值、弹性能量指数、岩爆倾向性指数作为岩爆预测的评判指标,提出了一种基于非线性参数优化的RBF-AR岩爆预测模型。在终南山隧道竖井岩爆判别中,利用RBF-AR法进行计算,计算结果与实际情况完全一致,表明该模型在岩爆预测中的可行性和有效性。     

Fatigue damage evolution of coal under cyclic loading

The experimental study of fatigue damage to coal under cyclic loading is important for guiding the design of pillars in underground coal mines where the pillars may be affected by repeated mining activity. In this paper, the strength, deformation, energy dissipation, and fatigue of samples of coal from a mine in China are studied using cyclic loading with a servo-controlled rock mechanical test system. The results indicate that coal is more likely to suffer fatigue damage than other, harder, rock lithologies. Under uniaxial cyclic loading, the fatigue failure “threshold value” for the coal samples studied is less than 78% of its uniaxial strength, but there is also a certain amount of fatigue damage when the cyclic loading/unloading experiments are carried out below the threshold value for fatigue failure. Axial deformation during the tests can be divided into three stages: initial deformation, constant steady deformation, and accelerated deformation. Transversal deformation can be divided into two stages: stable deformation and accelerated deformation. During cyclic loading experiments, imminent sample failure is signaled when transversal deformation increases significantly and quickly and the deformation recovers little when the load is removed. With an increasing number of loading/unloading cycles, a graph of energy dissipation per unit volume versus number of cycles presents an L-shaped curve when the coal samples do not suffer fatigue failure. However, for the coal samples that do rupture due to fatigue, the curve is U-shaped. Under cyclic loading, the evolution of compaction, strain hardening, strain softening, and failure of coal can be revealed in great detail by fatigue damage experiments.     

盐岩三轴间隔疲劳特性试验研究

为研究地下盐穴储气库受间歇性周期注、采循环载荷作用下围岩的非连续疲劳破坏演化过程,对取自巴基斯坦的深层盐岩进行了室内三轴间隔疲劳试验研究,并分析了三轴状态下不同围压、不同应力等级对盐岩间隔疲劳的影响。试验结果表明：（1）与单轴间隔疲劳相比,围压的存在不仅提高了盐岩的抗压强度,而且增加了盐岩的疲劳寿命,围压越高增加的幅度越明显。（2）三轴间隔疲劳试验中,盐岩间隔后循环中的残余应变大于间隔前的残余应变。这与单轴间隔疲劳得到的结果一致,但围压的上升会导致盐岩的残余变形积累速度以及间隔前后循环残余的变形差值都减小。（3）随着应力等级的升高,残余应变以及时间间隔前后的残余应变差值都呈现增大的趋势。     

A new rock mass failure criterion for biaxial loading conditions

To simulate brittle rocks, a mixture of glastone, sand and water was used as a model material. Thin galvanized sheets of thickness 0.254 mm were used to create joints in blocks made out of the model material. To investigate the failure modes and strength, both the intact material blocks as well as jointed model material blocks of size 35.6 × 17.8 × 2.5 cm having different joint geometry configurations were subjected to uniaxial and biaxial compressive loadings. A new intact rock failure criterion is proposed at the 3-D level. This criterion is validated for biaxial loading through laboratory experimental results obtained on intact model material blocks. Results obtained from both the intact and jointed model material blocks are used to develop a strongly non-linear new rock mass failure criterion for biaxial loading. In this failure criterion, the fracture tensor component is used to incorporate the directional effect of fracture geometry system on jointed block strength. The failure criterion shows the important role, the intermediate principal stress plays on rock mass strength.     

A method for estimating crack-initiation stress of rock materials by porosity

Crack-initiation stress of a rock under compression is the stress level that marks the initiation of the rock microfracturing process or in other words, the onset of new damage to the rock. This paper proposed a simple methodology with justifications to explore the feasibility of using total and effective porosities as estimators of crack-initiation stress of brittle crystalline rock materials under uniaxial compression. The validity/applicability of the proposed method was examined by an experimental study of granitic materials from Malanjkhand, Madhya Pradesh. It was found that effective porosity depicts better correlation with crack-initiation stress than with uniaxial compressive strength of the granitic materials. On the other hand, total porosity does not show any perceptible correlation with uniaxial compressive strength and crack-initiation stress. Plausible reasons for the nature of the obtained results were also explained in view of rock failure process under compression. It is concluded that following the proposed method, effective porosity can be used as a physical index to obtain a quick estimate of crack-initiation stress of the investigated rocks empirically.     

围岩强度和变形参数的分布特征及可靠性分析

从康家湾铅锌金矿Ⅲ-1号矿体上盘围岩取大量岩样,分别加工制作了50个压缩和拉伸试验的试样。利用RMT-150B伺服试验系统对试样进行单轴抗压、抗拉试验,各获得了50个试验结果。采用假设检验法,分别对50个单轴抗压强度和50个抗拉强度进行检验,结果表明,它们分别服从正态分布和对数正态分布;对50个 、 和50个C、 ,进行不放回抽样,组成50组E、 、C、 。利用FLAC计算软件,对硐室围岩中的应力进行了计算,分别获得了50个最大主应力和50个最小主应力;采用同样假设检验法,证明它们分别服从对数正态和正态分布;根据单轴抗压、抗拉强度及围岩中的最大主应力、最小主应力概率密度函数,计算了硐室围岩不发生拉伸破坏和压缩破坏的可靠度;并对硐室围岩抗剪强度的校核,得出了该地下硐室围岩稳定的结论。     

Study of mechanical properties of Vindhayan shaly rocks at elevated temperature

Shale is an important rock due to its suitability for different engineering and scientific applications. Elevated temperature may cause major deformation or damage in shale rock and it may be of irreversible in nature. Such damage have adverse effect on the physicomechnical properties of shale rock. The uniaxial compressive strength and tensile strength of two shales (upper Vindhyan basin, India) have been estimated at elevated temperature using point load strength index method. The rock samples have been analyzed at various temperatures starting from room temperature to 900 °C. The effect of elevated temperature on the physicomechanical properties and their influence on the uniaxial compressive strength has been studied in detail. Damage induced, in both shale have been estimated using compressional wave velocity. The analysis of the experimental result shows that the uniaxial compressive strength decreases from 63.45 to 18.45 MPa and 60.94 to 22.22 MPa, for Jhiri and Ganurgarh shale respectively. Tensile strength of shales have been also estimated. The value of tensile strength decreases from 3.65 to 1.05 MPa and 3.46 to 1.26 MPa respectively for Jhiri and Ganurgarh shale. Multivariate regression analysis has been carried out to obtain the correlation between physicomechanical properties and uniaxial compressive strength of Jhiri and Ganurgarh shale.     

Investigating the Effect of Cyclic Loading on the Indirect Tensile Strength of Rocks

This paper presents the results of laboratory experiments during the investigation of the stress–strain characteristics of Brisbane tuff disc specimens under diametral compressive cyclic loading. Two different cyclic loading methods were used: namely, sinusoidal cyclic loading and cyclic loading with increasing mean level. The first method applied the S–N curve approach to the indirect tensile strength (ITS) of rock specimens for the first time in the literature, and the second method investigated the effect of increasing cyclic loading on the ITS of rock specimens. The ITS of Brisbane tuff disc specimens was measured using the Brazilian tensile strength test. The reduction in ITS was found to be 33% with sinusoidal loading tests, whereas increasing cyclic loading caused a maximum reduction of 37%. It is believed that the fracturing under cyclic loading starts at contact points between strong grains and weak matrices, and that contact points at grain boundaries are the regions of stress concentration (i.e., indenters). Transgranular cracks emanate from these regions and intergranular cracks sometimes pass through the contact points. Once cracking begins, there is a steady progression of damage and a general ‘loosening’ of the rock, which is a precursor to the formation of intergranular cracks.     

A study of progressive failure of rock under cyclic loading by ultrasonic and AE monitoring techniques

Summary New experiments were conducted to expand the existing information on AE and pulse velocity changes. In this study, a few Hyderabad granites were subjected to uniaxial compressive cyclic loading conditions at room temperature. The laboratory tests were carried out by loading the rock to a constant stress maximum ranging between 40% and 80% of failure stress in the initial cycles, and then by progressively increasing the stress maximum, and also increasing the time gap between the cycles. Compressional wave velocity and amplitude changes were monitored in directions perpendicular to the applied stress, and acoustic emission event count data were recorded continuously until the fatigue failure of the rock.Rock specimens loaded cyclically to high stress levels, were in general found to fail after 10 to 12 cycles, at stresses corresponding to nearly 75% of the failure stress of intact rock. Deviations from the Kaiser effect were noticed when the stress maximum became comparable to the dilatant strength of the rock. The pulse amplitude and AE event counts were found to be sensitive parameters, to study the influence of cyclic stress, as well as the effect of the time gap between the cycles, on the microcrack development and progressive failure of rock.     

北疆侏罗系与白垩系泥质砂岩物理力学特性对比分析及其能量损伤演化机制研究

特殊的地理、气候条件及工程地质的复杂性决定了在北疆地区软岩地层中修建引水隧洞的设计施工难度较大。隧洞成拱效应与围岩自稳能力差,围岩渗透性强,遇水软化特性显著,极易产生软岩大变形甚至坍塌失稳灾害。为进一步研究北疆地区侏罗系与白垩系泥质砂岩的物理力学性质、遇水软化特性与能量损伤演化机制,开展了二者的单轴压缩、常规三轴与单轴蠕变试验。研究结果表明：两种岩石均富含黏土矿物,白垩系泥质砂岩的粒径分配更好,但其胶结程度较差,导致其强度稳定性与地层波速相对较低。低围压条件下,两者均以环向变形与体积扩容为主,但随着围压升高,其破坏模式由体积扩张过渡到体积压缩类型。高围压加载会造成岩石内部损伤,从而导致其抗压强度的降低。遇水后,两种岩石的延塑性与应变软化特性均明显增强,白垩系泥质砂岩的遇水软化特性更为显著。白垩系泥质砂岩的蠕变特性更为显著,两者的长期强度接近其单轴压缩损伤应力值。两种泥质砂岩的能量损伤演化过程均呈现S型变化规律,侏罗系泥质砂岩的能量硬化特性更为显著,白垩系泥质砂岩会更早地进入到能量硬化与能量软化阶段。     

基于统计损伤的岩爆预测

基于岩石单轴抗压强度的分布特征,对脆性岩石采用统计损伤的平行杆模型模拟,经岩石全过程曲线拟合,表明了 模型的合理性。进而提出采用平行杆模型模拟地下深埋洞室脆性围岩,推出了不同应力条件下岩体损伤的表达式,从统计损 伤的概念上,利用损伤变量来表征岩爆发生概率。工程实例分析表明该预测方法是有效的。     

基于Drucker-Prager准则的岩石弹塑性损伤本构模型研究

大多数岩石材料软化本构模型在硬化函数中引入塑性内变量来表示材料的硬化/软化性质,但并不能反映岩石微裂隙损伤对材料力学性能的影响及单轴拉伸和压缩所表现的初始屈服强度f0与屈服极限fu的差异。基于D-P准则同时考虑塑性软化及损伤软化,建立岩石类材料的弹塑性本构关系及其数值算法。塑性屈服函数采用Borja等的应力张量的硬化/软化函数,反映塑性内变量及应力状态对硬化函数的影响;由于岩石损伤软化是微裂隙扩展所导致的体积膨胀引起的,因此,提出用体积应变表征岩石损伤变量的演化,并用回映隐式积分算法编制了岩石的弹塑性损伤本构程序。对单轴压缩及拉伸荷载作用下的岩石材料试验进行数值模拟,结果表明,所提出的岩石弹塑性损伤本构模型可以较好地符合岩石材料的力学特性。     

拉-压应力状态下脆性岩石强度及破坏机制颗粒流模拟

高地应力区地下岩体工程开挖常形成围岩拉-压应力状态,发生岩体张性破坏灾害。本文针对传统PFC平行黏结模型不能模拟脆性岩石高单轴压缩与拉伸强度比的问题,建立双抗拉强度参数的平行黏结强度准则,开展岩石拉-压数值模拟试验,得到了与物理试验接近的拉-压强度,实现了岩石高压拉强度比的模拟,并深入分析了破坏机制。研究结果表明随着围压的增加,破裂面倾角逐渐增大,由拉伸破裂转化为拉-剪破裂,发现了拉-压应力状态下破裂面处的雁行裂纹。根据细观颗粒位移场揭示了破裂面力学性质,随着围压的增加(破裂面倾角逐渐增大),破裂面张性逐渐减弱而剪性增强。可将拉-压应力状态下岩石损伤演化过程大致分为弹性变形阶段、稳定破裂发展阶段、不稳定破裂发展阶段和整体破裂阶段(峰后应力跌落及残余阶段)。围压较大时弹性变形和稳定破裂发展阶段相对较短,不稳定破裂发展阶段相对较长较剧烈,峰后残余阶段破裂面摩擦更强、应力波动较大。