Damage Zones Around En Echelon Dike Segments In Porous Sandstone (original) (raw)
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International Journal of Rock Mechanics and Mining Sciences, 2000
Several aspects of fracture arrays are reviewed brie¯y and discussed. The terminology applied to progressive or multi-stage brittle deformation in rock masses is improved by noting fundamental mechanical dierences in fracture type and the kinematic coupling between dilatant mixed-mode crack displacements and wing cracks developed at the fracture tips. An array of initially mixed-mode (I±II) cracks will evolve under remote tensile least principal stress and with increasing strain to a dilatant, mode-I crack array oriented approximately perpendicular to the remote tensile stress. This progressive fracture growth thus defeats predictions of fracture-set orientation and displacement based only on a Mohr circle estimate of initial elastic stress (valid in the rock mass only at the earliest stages of fracture nucleation). Slow, subcritical crack growth in rock is associated with distinctive changes in fracture population geometry, as shown by published numerical simulations of fracture±network evolution. An increase in the stress corrosion index promotes joint clustering and signi®cant changes in joint length±frequency that may lead to characteristic dierences in the statistics of large-strain fracture populations. These geometric clues can be used to re®ne estimates of strength and deformability of rock masses and to infer classes of physico-chemical processes acting at the fracture tips during the development of the fracture population.
Many rock fractures are entirely driven open by fluids such as ground water, geothermal water, gas, oil, and magma. These are a subset of extension fractures (mode I cracks; e.g., dikes, mineral veins and joints) referred to as hydrofractures. Field measurements show that many hydrofractures have great variations in aperture. However, most analytical solutions for fracture displacement and stress fields assume the loading to be either constant or with a linear variation. While these solutions have been widely used, it is clear that a fracture hosted by heterogeneous and anisotropic rock is normally subject to loading that is neither constant nor with a linear variation. Here we present new general solutions for the displacement and stress fields around hydrofractures, modeled as two-dimensional elastic cracks, opened by irregular overpressure variations given by the Fourier cosine series. Each solution has two terms. The first term gives the displacement and stress fields due to the average overpressure acting inside the crack; it is given by the initial term of the Fourier coefficients expressing the overpressure variation. The second term gives the displacement and stress fields caused by the overpressure variation; it is given by general terms of the Fourier coefficients and solved through numerical integration. Our numerical examples show that the crack aperture variation closely reflects the overpressure variation. Also, that the general displacement and stress fields close to the crack follow the overpressure variation but tend to be more uniform far from the crack. The present solutions can be used to estimate the displacement and stress fields around any fluid-driven crack, that is, any hydrofracture, as well as its aperture, provided the variation in overpressure can be described by Fourier series. The solutions add to our understanding of local stresses, displacements, and fluid transport associated with hydrofractures in the crust.
Fracture initiation and propagation in intact rock e A review
The initiation and propagation of failure in intact rock are a matter of fundamental importance in rock engineering. At low confining pressures, tensile fracturing initiates in samples at 40%e60% of the uniaxial compressive strength and as loading continues, and these tensile fractures increase in density, ultimately coalescing and leading to strain localization and macro-scale shear failure of the samples. The Griffith theory of brittle failure provides a simplified model and a useful basis for discussion of this process. The HoekeBrown failure criterion provides an acceptable estimate of the peak strength for shear failure but a cutoff has been added for tensile conditions. However, neither of these criteria adequately explains the progressive coalition of tensile cracks and the final shearing of the specimens at higher confining stresses. Grain-based numerical models, in which the grain size distributions as well as the physical properties of the component grains of the rock are incorporated, have proved to be very useful in studying these more complex fracture processes.
Rock Fractures in Geological Processes
2009
Rock fractures largely control many of the Earth's dynamic processes. Examples include plate-boundary formation and development, tectonic earthquakes, volcanic eruptions, and fluid transport in the crust. How rock fractures form and develop is of fundamental importance in many theoretical and applied fields of earth sciences and engineering, such as volcanology, seismology, hydrogeology, petroleum geology, natural hazards, and engineering geology. An understanding of rock fractures is essential for effective exploitation of many of the Earth's natural resources including ground water, geothermal water, and petroleum. This book combines results from fracture mechanics, materials science, rock mechanics, structural geology, hydrogeology, and fluid mechanics to explore and explain fracture processes and fluid transport in the crust. Basic concepts are developed from first principles and are illustrated with numerous worked examples that link models of geological processes to real field observations and measurements. Calculations in the worked examples are presented in detail with simple steps that are easy to follow-providing the readers with the skills to formulate and quantitatively test their own models, and to practise their new skills using real data in a range of applications. Review questions and numerical exercises are given at the end of each chapter, and further homework problems are available at www.cambridge.org/gudmundsson. Solutions to all numerical exercises are available to instructors online. Rock Fractures in Geological Processes is designed for courses at the advancedundergraduate and beginning-graduate level, but also forms a vital resource for researchers and industry professionals concerned with fractures and fluid transport in the Earth's crust.
Rupture by damage accumulation in rocks
International journal of fracture, 2006
The deformation of rocks is associated with microcracks nucleation and propagation, i.e. damage. The accumulation of damage and its spatial localization lead to the creation of a macroscale discontinuity, a so-called "fault" in geological terms, and to the failure of the material, i.e., a dramatic decrease of the mechanical properties as strength and modulus. The damage process can be studied both statically by direct observation of thin sections and dynamically by recording acoustic waves emitted by crack propagation (acoustic emission). Here we first review such observations concerning geological objects over scales ranging from the laboratory sample scale (dm) to seismically active faults (km), including cliffs and rock masses (Dm, hm). These observations reveal complex patterns in both space (fractal properties of damage structures as roughness and gouge), time (clustering, particular trends when the failure approaches) and energy domains (power-law distributions of energy release bursts). We use a numerical model based on progressive damage within an elastic interaction framework which allows us to simulate these observations. This study shows that the failure in rocks can be the result of damage accumulation.
Limits to crack density: The state of fractures in crustal rocks
SEG Technical Program Expanded Abstracts 1993, 1993
Shear-wave splitting in sedimentary basins and above small earthquakes in a wide range of geological and tectonic domains typically displays evidence for azimuthal shear-wave velocity anisotropy of between 1% and 5%. Interpreted as the effects of parallel vertical fractures, microcracks, and preferentially oriented pore-space, these percentages of anisotropy are equivalent to crack densities of = 0.01 and 0.05 with normalized mean crack diameters of 0.43 and 0.74, respectively. The only exceptions are percentages of anisotropy exceeding 10% > 0.1) observed in near-surface rocks where there is pronounced jointing.
Tectonophysics, 2014
Fracture lineaments in crystalline and metamorphic rocks of southern Norway can be subdivided into joint swarms, fracture corridors and faults, depending on displacement, the fracture mode and patterns, and the presence of fault rocks. Their physical appearance as lineaments seen by remote sensing is not discernible, as they define km-long and narrow tabular zones of high fracture intensity. Intrinsically, fracture zonation becomes better expressed from joint swarms to fracture corridors and especially faults as a consequence of increasing accumulate strain. Joint swarms and fracture corridors commonly reveal a symmetric fracture zonation on both sides of its core, whereas inclined extensional faults tend to have asymmetric patterns with enhanced strain and a wider damage zone in the hanging wall. Fracture lineament can be mapped in subzones A-B (core), which are typically some cm up to some tens of meters wide. Common structural elements are fault rocks/shear zones, lenses, and a network of fractures often with very high fracture frequency. Secondary minerals are common. Outside this, subzones C-D (damage zone) are commonly 20-50-m\ wide with lower fracture intensity of lineament-parallel fracturing, defining the topographic boundary of the lineament. Mineralisation is rarer. The transitional subzone E of multi-orientation fractures defines the transition to the background fracture system. We propose a model for the classification and development of fracture lineaments, applying their architecture (intrinsic geometry, spatial fracture pattern and spatial distribution of fault rocks) as tools for the systematic description. This links fault growth processes and mechanisms that can be ascribed to strain hardening and softening scenarios in a model of fault architecture.
Displacement and stress fields around rock fractures opened by irregular overpressure variations
Frontiers in Earth Science, 2014
Many rock fractures are entirely driven open by fluids such as ground water, geothermal water, gas, oil, and magma. These are a subset of extension fractures (mode I cracks; e.g., dikes, mineral veins and joints) referred to as hydrofractures. Field measurements show that many hydrofractures have great variations in aperture. However, most analytical solutions for fracture displacement and stress fields assume the loading to be either constant or with a linear variation. While these solutions have been widely used, it is clear that a fracture hosted by heterogeneous and anisotropic rock is normally subject to loading that is neither constant nor with a linear variation. Here we present new general solutions for the displacement and stress fields around hydrofractures, modeled as two-dimensional elastic cracks, opened by irregular overpressure variations given by the Fourier cosine series. Each solution has two terms. The first term gives the displacement and stress fields due to the average overpressure acting inside the crack; it is given by the initial term of the Fourier coefficients expressing the overpressure variation. The second term gives the displacement and stress fields caused by the overpressure variation; it is given by general terms of the Fourier coefficients and solved through numerical integration. Our numerical examples show that the crack aperture variation closely reflects the overpressure variation. Also, that the general displacement and stress fields close to the crack follow the overpressure variation but tend to be more uniform far from the crack. The present solutions can be used to estimate the displacement and stress fields around any fluid-driven crack, that is, any hydrofracture, as well as its aperture, provided the variation in overpressure can be described by Fourier series. The solutions add to our understanding of local stresses, displacements, and fluid transport associated with hydrofractures in the crust.
Natural Fracture Patterns in Layered Rocks: Initiation and Propagation Mechanisms
2000
Important contributors to reservoir permeability are fracture systems common in low porosity reservoirs. Although the conditions of fracturing in reservoirs on anticlines, in tectonically deformed areas, are relatively well identified, the formation of regional fractures in flat-lying reservoirs is still unclear. Such regional fractures have a great economic significance and are known to form at depth as a result of