Microcracks In Rock Research Articles

Testing the frackability of granite using supercritical carbon dioxide: Insights into geothermal energy systems

Supercritical carbon dioxide (ScCO2) fracturing is an attractive method which has favourable physical and chemical characteristics for the enhancement of underground reservoir permeability, and as a replacement for the current water-based fracturing in reservoir stimulation. However, the behaviour of ScCO2 as a fracturing fluid and the reservoir response to the injection of ScCO2 remain unclear in the geothermal environment. Therefore, in this study, a series of ScCO2 fracturing tests were conducted on Harcourt granite rock specimens under different confining pressures (5–40 MPa) and temperatures (50–300 °C) to better understand the applicability of ScCO2 as a fracturing fluid and its influence on breakdown pressure under geothermal reservoir conditions. According to the results, the breakdown pressure under ScCO2 fracturing is lower than that for water-based fracturing. This is because ScCO2, being a low viscous fluid, has the ability to percolate into the micro-cracks in the reservoir rock, and the pressurisation of these micro-cracks increases the pore-pressure inside the rock matrix, reducing the effective stress required for fracture initiation. Our results reveal that the breakdown pressure increases linearly with increasing confining pressure. High-resolution CT scanning results show that the average density of the induced fracture tends to increase with the increase of confining pressure due to the sudden release of a large amount of energy under higher negative effective stress conditions (26% increase in fracture density with increase of confining pressure from 30 MPa to 40 MPa at 50 °C). In addition, regardless of the confining pressure, the breakdown pressure exhibits a decreasing trend with increasing temperature, which is due to the thermally-induced microstructural and mechanical deterioration of the rock specimen. The temperature increase significantly alters the fracture morphology, resulting in narrow fractures with multiple secondary branches under high temperature conditions. The results of the tortuosities of the induced fractures provide further evidence of this phenomenon. Importantly, ScCO2 has the ability to penetrate into the thermally-induced micro-cracks in the rock and link these micro-cracks to the main diametrical fracture, resulting in interconnected and complex fracture patterns under high temperature conditions. Also, the fractures tend to propagate mainly along the grain boundaries of quartz and feldspar minerals, while biotite grains exhibit a comparative resilience to fracture propagation.

Nanotopographic characterization of microfractures in rocks by Atomic Force Microscopy

The study of microfractures is one of the keys to understand a variety of geological issues as diverse as seismic rupturing mechanisms, process zone characterization and evolution toward large-scale fracturing, characterization of reservoirs of geological fluids, or identification of microhabitats outside Earth. In earlier works, microcrack initiation and propagation in rocks has been observed at microscale. We test Atomic Force Microscopy (AFM) as a tool to retrieve nanoscale digital elevation models (nDTM) of rock surfaces. Basalt samples from two different geological environments are studied. In a basalt from Siberia, the topography of the granular material infilling of the crack can be accurately characterised, and the pristine crack shape retrieved, opening access to an estimate of the volume of the infilling material. In a basalt from Ethiopia, mixed-mode fracturing (opening-shear) is identified from nanofracture topography, as well as a textbook example of along-strike variations of fracture displacement. These observations demonstrate that fractures already initiate and propagate at nanoscale, and AFM is a powerful tool to conduct quantitative structural geology analyses with a resolution of ∼1 nm.

Dynamic Fracture Test of Laurentian Granite Subjected to Hydrostatic Pressure

Dynamic fracture failure of rocks subjected to static hydrostatic pressure is commonly encountered in deep underground rock engineering. The static fracture behavior of rocks under hydrostatic stress has been well studied in the literature. However, it is desirable to investigate the dynamic fracture failure of rocks under various hydrostatic pressures. In this study, a triaxial split Hopkinson pressure bar (SHPB) system is used to measure the dynamic fracture toughness of rocks under five hydrostatic pressures. The results show that dynamic fracture toughness under a certain hydrostatic pressure enhances with the increase of the loading rate, and the dynamic fracture toughness at the similar loading rate increases with the hydrostatic pressure due to the closure of microcracks in rocks. An empirical formula is proposed to describe the influences of the loading rate and the hydrostatic pressure on the rock dynamic fracture toughness.

Repair of subsurface micro-cracks in rock using resin pre-coating technique

Subsurface micro-cracks in rock, detrimental to long-term structural integrity due to potential moisture or water penetration and stress concentration, should be carefully assessed and repaired in service. In this study, we propose a resin pre-coating (RPC) technique for sealing the sub-surface micro-cracks that normally cannot be accessed by adhesive bonding. The subsurface micro-cracks with narrow openings are penetrated by a specially prepared solution of resin (without hardener) and acetone through capillary action. After evaporation of acetone, the resin is left to fill the micro-cracks. The normal resin and hardener mixture is then applied. The diffusion process will grow the adhesive joint deep into those micro-cracks leading to stronger adhesive bonding. Notched three-point-bending (3-p-b) granite specimens were first tested to fracture, and then bonded together using epoxy adhesive after RPC. Short Aramid fibers (SAF) were also used to strengthen the brittle epoxy adhesive joint. After removing potential premature failure sites by sealing those sub-surface micro-cracks, the peak loads of repaired samples were 35% higher than those measured from the original granite samples, and the total fracture energy consumption was 118% higher than the original value.

Thermal damage constitutive model for rock considering damage threshold and residual strength

With the gradual depletion of mineral resources in the shallow part of the earth, resource exploitation continues to move deeper into the earth, it becomes a hot topic to simulate the whole process of rock strain softening, deformation and failure in deep environment, especially under high temperature and high pressure. On the basis of Lemaitre’s strain-equivalent principle, combined with statistics and damage theory, a statistical constitutive model of rock thermal damage under triaxial compression condition is established. At the same time, taking into account the existing damage model is difficult to reflect residual strength after rock failure, the residual strength is considered in this paper by introducing correction factor of damage variable, the model rationality is also verified by experiments. Analysis of results indicates that the damage evolution curve reflects the whole process of rock micro-cracks enclosure, initiation, expansion, penetration, and the formation of macro-cracks under coupled effect of temperature and confining pressure. Rock thermal damage shows logistic growth function with the increase of temperature. Under the same strain condition, rock total damage decreases with the rise of confining pressure. By studying the electron microscope images (SEM) of rock fracture, it is inferred that 35.40 MPa is the critical confining pressure of brittle to plastic transition for this granite. The model parameter F reflects the average strength of rock, and M reflects the morphological characteristics of rock stress–strain curves. The physical meanings of model parameters are clear and the model is suitable for complex stress states, which provides valuable references for the study of rock deformation and stability in deep engineering.

Lattice Boltzmann Simulation of Fluid Flow Characteristics in a Rock Micro-Fracture Based on the Pseudo-Potential Model

Slip boundary has an important influence on fluid flow, which is non-negligible in rock micro-fractures. In this paper, an improved pseudo-potential multi-relaxation-time (MRT) lattice Boltzmann method (LBM), which can achieve a large density ratio, is introduced to simulate the fluid flow in a micro-fracture. The model is tested to satisfy thermodynamic consistency and simulate Poiseuille flow in the case of large liquid-gas density ratio. The slip length is used as an index for evaluating the flow characteristics, and the effects of wall wettability, micro-fracture width, driving pressure and liquid-gas density ratio on the slip length are discussed. The results demonstrate that the slip length increases significantly with the increase of the wall contact angle in rock micro-fracture. And the liquid-gas density ratio has an important impact on the slip length, especially for the hydrophobic wall. Moreover, under the laminar flow regime the driving pressure and the micro-fracture width has little effect on the slip length.

Microcrack Evolution and Associated Deformation and Strength Properties of Sandstone Samples Subjected to Various Strain Rates

The evolution of micro-cracks in rocks under different strain rates is of great importance for a better understanding of the mechanical properties of rocks under complex stress states. In the present study, a series of tests were carried out under various strain rates, ranging from creep tests to intermediate strain rate tests, so as to observe the evolution of micro-cracks in rock and to investigate the influence of the strain rate on the deformation and strength properties of rocks. Thin sections from rock samples at pre- and post-failure were compared and analyzed at the microscale using an optical microscope. The results demonstrate that the main crack propagation in the rock is intergranular at a creep strain rate and transgranular at a higher strain rate. However, intergranular cracks appear mainly around the quartz and most of the punctured grains are quartz. Furthermore, the intergranular and transgranular cracks exhibit large differences in the different loading directions. In addition, uniaxial compressive tests were conducted on the unbroken rock samples in the creep tests. A comparison of the stress–strain curves of the creep tests and the intermediate strain rate tests indicate that Young’s modulus and the peak strength increase with the strain rate. In addition, more deformation energy is released by the generation of the transgranular cracks than the generation of the intergranular cracks. This study illustrates that the conspicuous crack evolution under different strain rates helps to understand the crack development on a microscale, and explains the relationship between the micro- and macro-behaviors of rock before the collapse under different strain rates.

An innovative micromechanics-based three-dimensional long-term strength criterion for fracture assessment of rock materials

Rocks may exhibit time-dependent behaviors. Long-term strength criterion significantly dominates creep failure of rocks. Rocks contain many microcracks, which lead to degrade of long-term strength. In this paper, it is assumed that there exist three-dimensional penny-shaped microcracks in rocks. The mode II stress intensity factors at tips of three-dimensional penny-shaped microcracks in Burgers viscoelastic rock matrix is derived. A novel micromechanics-based three-dimensional long-term strength criterion is established to consider the effects of time and the intermediate principal stress on creep failure of rocks. By comparison with the previous experimental data, it is found that the novel micromechanics-based three-dimensional long-term strength criterion is in good agreement with the experimental data.

Progress of brittle microfracturing in crystalline rocks under cyclic loading conditions

Progress of brittle microfracturing in crystalline rocks under cyclic loading conditions

The 3D numerical simulation of damage localization of rocks using General Particle Dynamics

The damage localization of rocks is an open issue in rock engineering. Moreover, it is difficult to observe directly the micro-scale process of damage localization and damage evolution of rocks. Therefore, in this paper, the General Particle Dynamic method (GPD) is developed to simulate the damage localization and failure process of 3D rock samples. The effect of the intermediate principal stress on the strength, the stress-strain curve and the damage localization modes of a cubical rock specimen subjected to the true triaxial compressive loads is investigated. The different failure modes including the single shear damage localization mode, axial splitting damage mode, X-shaped conjugate shear damage localization mode and Cone-shaped damage localization are found. It is found that the numerical results obtained from GPD are in good agreement with experimental observations. It is implied that the formation of damage localization band induced by the initiation and propagation and coalescence of micro-cracks in rocks can be well simulated using GPD.

Numerical Investigation of Damage Evolution and Localized Fracturing of Brittle Rock in Compression

In the present paper, a numerical model for deformation and fracturing of brittle rocks is used to study the damage evolution and localized fracturing of brittle rocks based on statistical mesoscopic damage mechanics. In the model, material heterogeneity and the mesoscopic renormalization concept are introduced to consider the interaction among microcracks in rock. The temporal and spatial evolution of acoustic emissions in the stressed rock are also described. Correspondingly, numerical tests on rock specimens with different heterogeneities, porosities, and scales are performed to investigate the damage evolution and localized fracturing of brittle rocks. The localized damage and fracturing of brittle rocks induced by microstructural damage is investigated. The study shows that heterogeneity, porosities, and scales of rock specimen have significant impact on damage evolution and localized fracturing mode. In addition, the study reveals that the postpeak response of stress-strain curve is due to the deformation of the structural elements, and localization means that the descending branch of the stress-strain curve of the rock specimen is size dependent, and the stress-strain curve can therefore not be regarded as a pure material property.

Identification methods for anomalous stress region in coal roadways based on microseismic information and numerical simulation

It is believed that the microseismicity induced by mining effect and gas gradient disturbance stress is a precursor to the essential characteristics of roadway unstability. In order to effectively identify and evaluate the stability of coal roadways in the process of mine development and extraction, a microseismic monitoring system was deployed for the study of the stress evolution process, damage degree and distribution characteristics in the tailgate and headgate. The mine under study is the 62113 outburst working face of Xin Zhuangzi coalmine in Huainan mining area. The whole process of microfractures initiation, extension, interaction and coalescence mechanisms during the progressive failure processes of the coal rock within the delineated and typical event clusters were investigated by means of a two dimensional realistic failure process analysis code (RFPA2D-Flow). The results show that the microseismic events gradually create different-sized event clusters. The microseismicity of the tailgate is significantly higher than that of the headgate. The study indicates that the greater anomalous stress region matches the area where microfractures continuously develop and finally connect to each other and form a fissure zone. Due to the mine layout and stress concentration, the ruptured area is mainly located on the left shoulder of the tailgate roof. The potential anomalous stress region of the coal roadway obtained by numerical simulation is relatively in good agreement with the trend of spatial macro evolution of coal rock microfractures captured by the microseismic monitoring system. The research results can provide important basis for understanding instability failure mechanism of deep roadway and microseismic activity law in complex geologic conditions, and it ultimately can be used to guide the selection and optimization of reinforcement and protection scheme.

Comprehensive evaluation of the stability of the left-bank slope at the Baihetan hydropower station in southwest China

The stability of the left-bank slope is a crucial geological engineering problem at the Baihetan hydropower station, China. Due to continuous excavations on the rock slope, different regions of the surrounding rock mass undergo varying degrees of unloading deformation. It is important to assess the stability of the rock slope from a macroscopic viewpoint by investigating its deformation characteristics and mechanisms. Therefore, in this work, microseismic (MS) monitoring was first employed to detect the progressive rock mass damage in the rock slope subjected to excavation, including the initiation, propagation, coalescence, and interaction of rock microfractures. Numerical modeling was subsequently performed to understand the deformation and failure mechanism of the rock slope. Moreover, traditional surveying approaches (i.e., multiple-point extensometers and inclinometers) and field observations were also used to analyze the deformation and failure characteristics of the rock slope. The MS monitoring results showed that spatiotemporal regularities in the evolution of seismic source locations were indicators of deformation failure and potential sliding surfaces. MS event clustering can be used to delineate activated pre-existing geological structures (i.e., LS331 and LS337). The simulation results show that the deformation and failure characteristics of the rock slope are mainly controlled by pre-existing weak structural planes (i.e., the intraformational faulted zones LS3319, LS331, and LS337 and fault F17). These results agree well with the results of geological data and conventional monitoring data. Our study reveals that an integrated approach combining MS monitoring, numerical modeling, traditional surveying, and field observations leads to a better understanding of the behavior of the rock slope under the influence of excavation as well as greater control of the working faces, ensuring safety under complex geological and excavation conditions.

Environmentally sustainable mining through proper selection of explosives in blasting operation

Blasting is considered as the first step in the mineral supply chain in most of the mines. In addition to rock fragmentation, blasting has a significant role in exerting inner damage and reducing the strength of rock fragments. The strength reduction facilitates many of downstream mining processes including comminution and reduces energy consumption in crushing and grinding stages, which is an important achievement in today’s environment. This study aims to investigate the effect of changing the explosive type on post-blast strength of rock fragments and their microcracks content and energy consumption in subsequent processes. For this purpose, several experimental blastings have been carried out using three types of explosives, ANFO, Emulite and pentolite, while the applied energy and other parameter were kept constant. Then, the P-wave velocity and point load index of rock fragments have been measured for all blasts. In the next step, microcrack density of rock fragments was measured by image processing of fluorescent microscopic images and also the quantity of all kinds of microcracks was examined by some light microscopy techniques. The results showed that rock fragments strength is totally influenced by explosive types, so that blasting with pentolite causes a 16.5% reduction in rock strength. Besides, it was found that the rock fragments strength is highly correlated with explosives detonation velocity (VOD). Furthermore, microcracks studies results showed that different types of explosive have dissimilar influences on rock microcrack network, so that the use of pentolite instead of ANFO increases the microcracks density more than 300%.

Impacts des vagues et de la circulation d’eau sur la microfracturation des falaises rocheuses. Mise en place d’un suivi multiparamètre en Bretagne

In order to gain new insights into the agents and processes controlling hard rock coast erosion and retreat, a high frequency and multi-parameter monitoring field experiment was carried out from February until May 2017 in Porsmilin cliffs (Plougonvelin, Finistère, Brittany). This survey aimed to discriminate the relative contributions of subaerial (groundwater infiltration, watertable variations…) and marine (infragravity wave loading, wave impact...) agents and processes in the evolution of rock microfracturing. A better knowledge of their contributions will allow to improve the forecasting/ prevention of cliff failure.

The role of rock joint frictional strength in the containment of fracture propagation

The fracturing phenomenon within the reservoir environment is a complex process that is controlled by several factors and may occur either naturally or by artificial drivers. Even when deliberately induced, the fracturing behaviour is greatly influenced by the subsurface architecture and existing features. The presence of discontinuities such as joints, artificial and naturally occurring faults and interfaces between rock layers and microfractures plays an important role in the fracturing process and has been known to significantly alter the course of fracture growth. In this paper, an important property (joint friction) that governs the shear behaviour of discontinuities is considered. The applied numerical procedure entails the implementation of the discrete element method to enable a more dynamic monitoring of the fracturing process, where the joint frictional property is considered in isolation. Whereas fracture propagation is constrained by joints of low frictional resistance, in non-frictional joints, the unrestricted sliding of the joint plane increases the tendency for reinitiation and proliferation of fractures at other locations. The ability of a frictional joint to suppress fracture growth decreases as the frictional resistance increases; however, this phenomenon exacerbates the influence of other factors including in situ stresses and overburden conditions. The effect of the joint frictional property is not limited to the strength of rock formations; it also impacts on fracturing processes, which could be particularly evident in jointed rock masses or formations with prominent faults and/or discontinuities.

Discrete element method simulation of random Voronoi grain-based models

The generation of random Voronoi grain-based models for DEM simulation is proposed in this investigation. Voronoi diagram has important applications in rock engineering and geological engineering. However, the application of 3D Voronoi models has been limited due to the difficulty of generating 3D Voronoi domains. Little research has been done on the Voronoi tessellations of objects with irregular shapes. Voronoi tessellations of objects with regular and irregular shapes are investigated by using the Matlab function mpt_voronoi. The proposed models are then employed to simulate micro-fracturing and tension cracks in brittle rock. The 3D Voronoi grain-based slope model is used to investigate the dynamic response and failure mechanism of the rock slopes during earthquakes. The failures of rock slopes are actually due to the increasing and expanding of the tension cracks, the development of the tension surfaces and sliding of the blocks during earthquakes. The 2D Voronoi grain-based specimen composed of clusters is adopted to carry out rock fragmentation by TBM cutters. PFC simulation results reveal that the damaged rock undergoes some volume dilation that causes the expansion of the crushed zone and the propagation of the vertical crack. The rock fragmentations are actually caused by the increasing of the tension cracks. According to the simulation, it is believed that Voronoi grain-based models will contribute to an improved knowledge of DEM simulation.

A two-step homogenization-based permeability model for deformable fractured rocks with consideration of coupled damage and friction effects

The permeability variation induced by anisotropic damage of rock blocks and nonlinear deformation of fractures is modeled with a two-step homogenization technique for deformable fractured rocks containing arbitrarily-distributed penny-shaped microcracks in rock blocks separated by multiple sets of critically orientated fractures of much large scales. The proposed model accounts for the influences of the microstructural alteration of rock blocks and fractures at the laboratory scale and the development pattern of the fracture system as well as the rock-fracture interaction at the field scale on the permeability of a fractured rock under mechanical loading or excavation condition. The proposed model was first validated by laboratory data of compression tests with permeability measurements on the Beishan granite, coupled shear-flow tests with hydraulic conductivity measurements on a granite fracture and anisotropic strength tests on the Martinsburg slate, with good agreement between the model predictions and the laboratory measurements. Numerical simulations were finally performed to examine the excavation-induced permeability change in the surrounding granitic rock of the Stripa tunnel, demonstrating the importance in considering both the damage of rock blocks and the deformation of fractures for better understanding the excavation-induced permeability change in the surrounding rock near the excavation surface.

Weakening of gneiss surfaces colonized by endolithic lichens in the temperate climate area of northwest Italy

AbstractA role of lithobionts in geomorphological processes is increasingly argued, but the spatio‐temporal scale of their impact is largely unexplored in many ecosystems. This study first characterizes in the temperate zone (northwest Italy) the relationships between lithobiontic communities including endolithic lichens and the hardness of their siliceous rock substrate (Villarfocchiardo Gneiss). The communities are characterized, on humid and xeric quarry surfaces exposed for decades and natural outcrops exposed for centuries, in terms of lichen and microbial constituents, using a combined morphological and molecular approach, and with regard to their development on and within the gneiss.A lichen species belonging to Acarosporaceae (Polysporina‐Sarcogyne‐Acarospora group, needing taxonomic revision) chasmoendolithically colonizes both the humid and xeric quarry surfaces, on which epilithic cyanobacterial biofilms and epilithic pioneer lichens respectively occur. Light and electron microscopic observations show the development of the endolithic thalli within rock microcracks and the hyphal penetration along crystal boundaries down to depths of 1 to 3 mm, more pronounced within the humid surfaces. Such colonization patterns are likely related to biogeophysical deterioration, while no chemical alteration characterizes minerals contacted by the endolithic lichen. By contrast, on natural outcrops, where the endolithic colonization is negligible, a reddish rind below epilithic lichens indicates chemical weathering processes.Schmidt Hammer measurements highlight that the endolithic lichens deeply affect the hardness of the gneiss (down to −60% with respect to fresh controls and surfaces only colonized by cyanobacteria), exerting a significantly higher weakening effect with respect to the associated epilithic lithobionts. The phenomenon is more remarkable on humid than on xeric quarry surfaces and natural outcrops, where epilithic lichens are likely involved in long‐term hardening processes supporting surface stabilization.Endolithic lichens are thus active biogeomorphological agents at the upper millimetric layer of siliceous rocks in temperate areas, exerting their weakening action during the early decade‐scaled stages of surface exposure. Copyright © 2015 John Wiley & Sons, Ltd.

Formation of microcracks upon the dynamic fracturing of rocks

The results from experimental studies on the parameters of microcracks formed after the dynamic fracture of rocks, performed by means of electron microscopy, are presented. Some physical aspects and mechanisms of the formation of microcracks in rocks are discussed. It is noted that an important characteristic of microcracks is the magnitude of their opening. According to experiments, the minimum value of the opening of microcracks is ~0.1 μm.