Showing papers on "Fracture (geology) published in 2019"
TL;DR: In this article, an equivalent Burgers model for rheological fracture of rock cracks under different hydraulic pressure and far field stresses has been proposed, which can be used to reveal the rheology behavior of fractured rock mass.
212 citations
TL;DR: In this article, the authors carried out fully coupled multiscale numerical analysis to investigate some key coupled processes of fluid-driven fracture propagation in naturally fractured rock mass and demonstrated that with an advanced numerical modeling tool, the complex fracturing associated with hydraulic fracturing in naturally-frained rock mass can be qualitatively analyzed and the extent of various uncertainties can be assessed.
Abstract: Naturally fractured rock mass is highly inhomogeneous and contains geological discontinuities at various length scales. Hydraulic fracture stimulation in such a medium could result in complex fracture systems instead of simple planar fractures. In this study, we carried out fully coupled multiscale numerical analysis to investigate some key coupled processes of fluid-driven fracture propagation in naturally fractured rock mass. The numerical analysis follows the concept of the synthetic rock mass (SRM) method initially developed in the discrete element method (DEM). We introduce a total of five case study examples, including fracture initiation and near wellbore tortuosity, hydraulic fracture interaction with natural fractures, multi-stage hydraulic fracturing with discrete fracture network (DFN), in-fill well fracturing and frac hits after depletion-induced stress change, and induced seismicity associated with fault reactivation. Through those case studies, we demonstrate that with an advanced numerical modeling tool, the complex fracturing associated with hydraulic fracturing in naturally fractured rock mass can be qualitatively analyzed and the extent of various uncertainties can be assessed.
160 citations
TL;DR: In this paper, a series of uniaxial compression tests were conducted on prismatic sandstone containing a prefabricated hole filled with different types of inclusions using a servo-hydraulic machine.
141 citations
TL;DR: In this article, the authors analyzed the effect of water saturation and loading rate on the fracture behavior of sandstone materials under different loading rates using a modified split Hopkinson pressure bar (SHPB) setup.
133 citations
TL;DR: In this paper, a phase-field/gradient damage formulation for cohesive fracture is extended to the dynamic case, and the model is characterized by a regularized fracture energy that is linear in the damage field, as well as non-polynomial degradation functions.
132 citations
TL;DR: In this article, a 3D crack growth inside the transparent 3D resin samples in real-time for the first time was studied with the aid of high-speed cameras, and the results indicated that flaw geometry has remarkable influence on the mechanical and fracture behaviors of the flawed samples.
122 citations
TL;DR: A mathematical model for advective-diffusive-reactive solute transport due to non-Newtonian fluid flows in a fracture surrounded by a tight porous medium is presented in this article.
114 citations
TL;DR: In this paper, the fracture properties of Marcellus shale were obtained through size effect tests, and the results showed that the nominal strength decreases with increasing specimen size and it can be fitted well by Bažant's size effect law.
Abstract: The fracture characterization of shale rocks requires understanding the scaling of the measured properties to enable the extrapolation from small-scale laboratory tests to field applications. In this study, the fracture properties of Marcellus shale were obtained through size effect tests. Fracture tests were conducted on three-point-bending specimens with increasing size. The test results show that the nominal strength decreases with increasing specimen size and it can be fitted well by Bažant’s size effect law. This demonstrates that shale fracture behavior deviates from classical linear elastic fracture mechanics (LEFM), and it has quasi-brittle characteristics. This implies, in turn, that the fracture toughness (or fracture energy) computed according to LEFM is size-dependent and, in general, cannot be considered a material property. Furthermore, the size effect analysis allows one to accurately identify the quasi-brittle fracture properties, namely the initial fracture energy and the effective fracture process zone length. A significant anisotropy was observed in the fracture properties determined with three principal notch orientations.
105 citations
TL;DR: In this paper, a series of experimental studies on mechanical properties are conducted using gypsum-type coal-like materials with different numbers and configurations of pressure relief boreholes, and the change in parameters, such as the strength and energy dissipation index of bored coallike materials under a uniaxial compression, is studied, and fracture propagation characteristics in the rock surrounding the boreholes in the experiments are analyzed.
102 citations
TL;DR: An overview of the available approaches in determining the fracture energy of concrete and effective parameters is presented and includes experimental methods which form the basis for measuring the fracture energies obtained from work of fracture and size effect models and thus provides a comparison between different models.
100 citations
TL;DR: In this paper, the authors constructed a fracture-toughness model for covalent and ionic crystals and introduced an enhancement factor, which is determined by the density of states at the Fermi level and atomic electronegativities.
Abstract: Fracture toughness K I C plays an important role in materials design. Along with numerous experimental methods to measure the fracture toughness of materials, its understanding and theoretical prediction are very important. However, theoretical prediction of fracture toughness is challenging. By investigating the correlation between fracture toughness and the elastic properties of materials, we have constructed a fracture toughness model for covalent and ionic crystals. Furthermore, by introducing an enhancement factor, which is determined by the density of states at the Fermi level and atomic electronegativities, we have constructed a universal model of fracture toughness for covalent and ionic crystals, metals, and intermetallics. The predicted fracture toughnesses are in good agreement with experimental values for a series of materials. All the ingredients of the proposed model of fracture toughness can be obtained from first-principles calculations or from experiments, which makes it suitable for practical applications.
TL;DR: In this article, the fractal curves of the pores and fractures in coal samples with different particle sizes determined from MIP were all divided into two parts by an inflection point.
TL;DR: Wang et al. as discussed by the authors explored the dynamic mechanical properties and crack propagation law of rock under high strain rate impact loading, an experimental investigation with 12 sets of Brazilian disk (BD) rock specimens under Split Hopkinson Pressure Bar (SHPB) loading was undertaken.
TL;DR: In this article, the authors investigated the influence of geometric characteristics of deformable rough fractures under confining stresses on the behaviors of non-Darcy flow, and proposed a quantitative model to represent the relationship between the fracture geometric characteristics and the nonlinear coefficient β.
TL;DR: In this article, the effects of rock inherent heterogeneity and grain size on hydraulic fracture initiation and propagation for different propagation regimes were investigated through two dimensional discrete element modeling, where random particle assembly was used to mimic rock heterogeneity in the numerical model while regular particle assembly is used as the reference.
TL;DR: In this paper, the role of fissure angle and heat treatment temperature on the mechanical properties and deformation failure behavior of brittle granite was investigated using uniaxial compression tests.
Abstract: To investigate the role of fissure angle and heat treatment temperature on the mechanical properties and deformation failure behavior, uniaxial compression tests were carried out on granite specimens containing a single fissure. Using stress–strain curves, the peak strength, peak strain, and elastic modulus of the one-fissured granite specimens were analyzed in detail. The mechanical parameters are closely related to the fissure angle and the high temperature. As the fissure angle increases from 0° to 90°, the peak strength and elastic modulus first decrease and then increase, while the peak strain increases slowly. However, the peak strength and elastic modulus first increase and then decrease, while the peak strain first decreases and then increases with increasing treatment temperature. During the experiments, the crack evolution process and acoustic emission (AE) counts were obtained using real-time photography and the AE monitoring technique. In the granite specimens containing a pre-existing fissure, large AE counts are clearly observed before the peak strength, which indicates crack initiation and propagation. The accumulated AE count first increases slowly, but is followed by a sharp increase, with increasing deformation. The AE events in the one-fissured specimen also depend on the heat treatment temperature. As the temperature increases, the rate of increase of the accumulated AE count curve is reduced. Finally, using a digital image correlation method, the full fields of surface deformation were obtained for the entire testing process. In addition, the local strain around the pre-existing fissure was measured using strain gauges. The full strain field and local strain concentration are discussed to describe the fracture mechanism of brittle granite.
TL;DR: In this paper, the deformation and fracture behaviors of as-cast AZ31B Mg alloy were studied by uniaxial compression experiments and finite element simulations with wide ranges of temperature and strain rate.
TL;DR: Wang et al. as discussed by the authors combined qualitative analysis and quantitative characterization to investigate rock meso-structure damage due to water invasion-water loss cycles by analyzing the variations of rock mesosstructures using a scanning electron microscope (SEM).
Abstract: Water-level variations of tailings ponds can result in slope rocks being in a state of water invasion-water loss which can lead to irreversible damage to the rock meso-structure. This study combines qualitative analysis and quantitative characterization to investigate rock meso-structure damage due to water invasion-water loss cycles by analyzing the variations of rock meso-structures using a scanning electron microscope (SEM). Results from this analysis identified four stages in the variations of rock meso-structure under the action of water invasion-water loss cycles: overall homogeneity and compactness stage, primary pore expansion stage, porous flocculation stage, and a pore and fracture development stage. According to the fractal dimension in SEM test results, we can define rock meso-damage variable Df (which attained a maximum of 33.57%), thus realizing the quantitative characterization of rock damage under the action of water invasion-water loss cycles. After demonstrating that the evolutionary relationship between fractal dimension/damage variable and cycle number conforms to exponential function change, we also explored rock meso-damage mechanisms under the action of water invasion-water loss cycles.
TL;DR: In this paper, the authors investigated the influence of pore-structure and mortar properties on the fracture behavior of geopolymer foamed concrete and found that a decrease in pore size results in higher compressive strength and this influence is more significant for materials with lower porosity.
TL;DR: Song et al. as discussed by the authors developed a thermal-hydraulic-mechanical (THM) coupling model to investigate the heat extraction performances of various DFNs, and provided significant suggestions for the fracturing operation of multilateral-well EGS.
TL;DR: In this article, the authors used the theoretical analysis method to deduce the calculation method of the fracture failure of hard and thick key strata in coal mine, based on the occurrence law of microseismic (MS) events and the characteristics of surface subsidence.
TL;DR: In this article, the authors proposed an alternating-fluid-injection hydraulic-fracturing treatment, which exploited the advantages of the two fracturing fluids to form a large complex fracture network in deep shale gas reservoirs.
Abstract:
Deep shale gas reservoirs are characterized by high in-situ stresses, a high horizontal-stress difference (12 MPa), development of bedding seams and natural fractures, and stronger plasticity than shallow shale. All of these factors hinder the extension of hydraulic fractures and the formation of complex fracture networks. Conventional hydraulic-fracturing techniques (that use a single fluid, such as guar fluid or slickwater) do not account for the initiation and propagation of primary fractures and the formation of secondary fractures induced by the primary fractures. For this reason, we proposed an alternating-fluid-injection hydraulic-fracturing treatment. True triaxial hydraulic-fracturing tests were conducted on shale outcrop specimens excavated from the Shallow Silurian Longmaxi Formation to study the initiation and propagation of hydraulic fractures while the specimens were subjected to an alternating fluid injection with guar fluid and slickwater. The initiation and propagation of fractures in the specimens were monitored using an acoustic-emission (AE) system connected to a visual display. The results revealed that the guar fluid and slickwater each played a different role in hydraulic fracturing. At a high in-situ stress difference, the guar fluid tended to open the transverse fractures, whereas the slickwater tended to activate the bedding planes as a result of the temporary blocking effect of the guar fluid. On the basis of the development of fractures around the initiation point, the initiation patterns were classified into three categories: (1) transverse-fracture initiation, (2) bedding-seam initiation, and (3) natural-fracture initiation. Each of these fracture-initiation patterns had a different propagation mode. The alternating-fluid-injection treatment exploited the advantages of the two fracturing fluids to form a large complex fracture network in deep shale gas reservoirs; therefore, we concluded that this method is an efficient way to enhance the stimulated reservoir volume compared with conventional hydraulic-fracturing technologies.
TL;DR: In this paper, the impact of cyclic heating and cooling treatments on mode I fracture characteristics of granite was examined, and fracture tests of semi-circular bend specimens were conducted, acoustic emission (AE) events were monitored, and morphology characteristics of fracture surfaces were analyzed.
TL;DR: In this paper, the heat mining performance of variably fractured HDR with different fracture network characteristics is compared with that of doublet-well pattern, and the results indicate that the increase of fracture density enhances flow around and retards thermal drawdown.
TL;DR: In this article, optical microscopy, CO2 and N2 adsorption, and X-ray CT were used for reconstructing the pore-fracture system of different marcolithotypes.
TL;DR: In this paper, the authors proposed a phase field formulation in which fracture of viscoelastic solids is driven by both elastic and viscous components of the energy and showed that at low strain rates viscous energy dissipation accelerates the fracture growth rate but essentially does not affect the crack path, while at high rates the effect of viscous dissipation is minor.
TL;DR: In this article, a study on the spatial distribution of biogrout in a rock fracture and its effect on permeability reduction is presented, where a series of experiments together with 3D scanning and 3D flow simulation were performed on rock fractures with various initial apertures treated by bio-grouting.
01 Dec 2019
TL;DR: In this article, the classification of defects of castings obtained by electric arc smelting is considered, and the defect is characterized by a clearly defined uniform surface over which the fracture occurs.
Abstract: In this research work the classification of defects of castings obtained by electric arc smelting is considered. Of particular interest to researchers is the rock-like and naphthalene fractures. A stone-like fracture is characterized by a clearly defined uniform surface over which the fracture occurs. Grain boundaries are partially soluble in the austenite phase, consisting of fine individual particles or films formed from molten eutectics. It is also worth noting that most often the stone-like fracture is observed at the grain boundaries.
TL;DR: In this paper, a novel open-type microwave-induced fracturing apparatus (OMWFA) was developed for fracturing hard rock. But the results of field test reveal that borehole fracturing can exhibit a favorable effect around boreholes.
Abstract: A new, high-efficiency technology for fracturing and breaking rocks is required. Due to various advantages including high efficiency, energy-saving, and having no secondary pollution, the technology of microwave-induced fracturing of hard rock has been considered as a potential method for rock fracturing and breaking. Aiming at the realisation of two engineering applications: microwave-assisted mechanical rock breaking and stress release from rock masses in deep underground engineering works to prevent geological disasters caused by high-stress concentrations such as rockbursts, a novel (open-type) microwave-induced fracturing apparatus (OMWFA) for fracturing hard rocks was developed. On this basis, the two modes of microwave-induced subsurface fracturing and microwave-induced borehole fracturing of hard rocks were proposed. Due to removal of the restraint of the microwave cavity, OMWFA can be used to fracture large-size rock samples and engineering-scale rock masses. Using the apparatus, the fracturing effects of the two fracturing modes on different dimensions of cuboidal basalt samples were investigated. By combining the microwave-induced fracturing apparatus with a press machine to explore the influence of unidirectional stress on the fracturing effect of microwave treatment on basalt. Moreover, field tests were carried out on rock masses encountered in underground engineering works at Baihetan Hydropower Station in Sichuan Province, China, and the fracturing effects were evaluated by applying a digital borehole televiewer and conducting acoustic wave testing. The results show that the apparatus had favourable fracturing effects on the subsurface and borehole samples of basalt. When no stress was applied, the cracks radially expanded from the approximate centre of the radiant surface and unidirectional stress promoted fracturing. The number and depth of cracks increased with prolonged microwave exposure. After microwave treatment, the P-wave velocity of the samples declined, and the longer the microwave exposure, the more significant the reduction in P-wave velocity was. The results of field test reveal that borehole fracturing can exhibit a favourable effect around boreholes. The sound velocity around the borehole and between the boreholes both declined to some extent. Microwave-induced hard rock fracturing offers guiding significance to those exploring and developing new rock breaking and tunnelling methods, and generally enhances construction safety in deep underground engineering works.
TL;DR: This study presents the implementation procedures of a staggered scheme for phase field fracture of elasto-plastic solids in commercial finite element software Abaqus using subroutines UEL and UMAT, which exhibit good agreement with the experimental observations and numerical results in literature.