Showing papers on "Fracture (geology) published in 1998"

On the micromechanics of crushable aggregates

Abstract: This paper presents a study of the micromechanical behaviour of crushable soils. For a single grain loaded diametrically between flat platens, data are presented for the tensile strengths of particles of different size and mineralogy. These data are shown to be consistent with Weibull statistics of brittle fracture. Triaxial tests on different soils of equal relative density show that the dilatational component of internal angle of friction reduces logarithmically with mean effective stress normalized by grain tensile strength. The tensile strength of grains is also shown to govern normal compression. For a sample of uniform grains under uniaxial compression, the yield stress is related to the average grain tensile strength. If particles fracture such that the smallest particles are in geometrically self-similar configurations under increasing geotoscopic stress, with a constant probability of fracture, a fractal geometry evolves with the successive fracture of the smallest grains, in agreement with the a...

Progressive Delamination Using Interface Elements

Abstract: The paper describes a method for modelling progressive mixed-mode delamination in fibre composites. The procedure, which is incorporated within the non-linear finite element method, is based on the use of interface elements in conjunction with softening relationships between the stresses and the relative displacements. Fracture mechanics is indirectly introduced by relating the areas under the stress/displacement curves to the critical fracture energies.

Test environments and mechanical properties of Zr-base bulk amorphous alloys

Abstract: The mechanical properties of two Zr-base bulk amorphous alloys (BAA), Zr-10Al-30Cu-5Ni (BAA-10) and Zr-10Al-5Ti-17.9Cu-14.6Ni (BAA-11), were studied by both tensile and compressive tests at room temperature in various test environments. The BAA ingots up to 7 mm in diameter were successfully produced by both arc melting and drop casting and induction melting and injection casting. The BAA specimens deformed mainly elastically, followed by catastrophic failure along shear bands. Examination of the fracture region revealed ductile fracture features resulting from a substantial increase in temperature, which was attributable to the conversion of the stored elastic strain energy to heat. Surprisingly, “liquid droplets” located at major shear-band cracks adjacent to the fracture section were observed, indicating the occurrence of local melting during fracture. The angle orientation of shear bands, shear-band cracks, and fracture surfaces relative to the stress axis is quite different for BAA specimens tested in tension and compression. This suggests that both shear stress and normal stress may play a role in developing shear bands during plastic deformation. The tensile properties of BAAs were found to be insensitive to the test environment at room temperature. However, the reaction of BAAs with distilled water and heavy water was detected by laser desorption mass spectrometry (LDMS). These results suggest that moisture-induced hydrogen embrittlement in BAAs may be masked by catastrophic fracture following shear bands.

Flow in rock fractures : the local cubic law assumption reexamined

Abstract: We investigate the validity of applying the “local cubic law” (LCL) to flow in a fracture bounded by impermeable rock surfaces. A two-dimensional order-of-magnitude analysis of the Navier-Stokes equations yields three conditions for the applicability of LCL flow, as a leading-order approximation in a local fracture segment with parallel or nonparallel walls. These conditions demonstrate that the “cubic law” aperture should not be measured on a point-by-point basis but rather as an average over a certain length. Extending to the third dimension, in addition to defining apertures over segment lengths, we find that the geometry of the contact regions influences flow paths more significantly than might be expected from consideration of only the nominal area fraction of these contacts. Moreover, this latter effect is enhanced by the presence of non-LCL regions around these contacts. While contact ratios of 0.1–0.2 are usually assumed to have a negligible effect, our calculations suggest that contact ratios as low as 0.03–0.05 can be significant. Analysis of computer-generated fractures with self-affine walls demonstrates a nonlinear increase in contact area and a faster-than-cubic decrease in the overall hydraulic conductivity, with decreasing fracture aperture; these results are in accordance with existing experimental data on flow in fractures. Finally, our analysis of fractures with self-affine walls indicates that the aperture distribution is not lognormal or gamma as is frequently assumed but rather truncated-normal initially and increasingly skewed with fracture closure.

An active fracture model for unsaturated flow and transport in fractured rocks

Abstract: The unsaturated zone at Yucca Mountain, a potential repository site of high-level nuclear waste, is a complex hydrologic system in which a variety of important flow and transport processes is involved. To quantify these processes as accurately as possible is a theoretically challenging and practically important issue. In this study, we propose a new formulation for modeling flow and transport in unsaturated fractured rocks. The formulation is mainly based on a hypothesis that only a portion of connected fractures are active in conducting water. Analysis of the relevant data with the new formulation suggests that about 18–27% of the connected fractures in the Topopah Spring welded (TSw) unit (the potential repository unit) of Yucca Mountain are active under ambient conditions. The relatively high percentage of active fractures is consistent with field observations from a variety of sources. Sensitivity analyses are performed to investigate effects of the “activity” of connected fractures on flow and transport behavior in unsaturated rocks.

Experimental observation of fluid flow channels in a single fracture

Abstract: A method for obtaining precise replicas of real fracture surfaces using transparent epoxy resins was developed, allowing detailed study of fluid flow paths within a fracture plane. A natural rock fracture was collected from the field and prepared for study. Silicon rubber molds of the fracture surfaces were used to make a transparent epoxy replica of the original fracture. Clear and dyed water were injected into the fracture pore space allowing examination of the flow field. Digitized optical images were used to observe wetting, saturated flow, and drying of the specimen. Nuclear magnetic resonance imaging was used for quantitative measurements of flow velocity. Both video imaging and nuclear magnetic resonance imaging techniques show distinct and strong channeling of the flow at the submillimeter to several-centimeter scale. Each phenomenon, including wetting, drying, dye transport, and velocity channeling, has its own distinct geometric structure and scale. We find that fluid velocities measured simultaneously at various locations in the fracture plane during steady state flow range over several orders of magnitude, with the maximum velocity a factor of 5 higher than the mean velocity. This suggests that flow channeling in fractured rock can cause the breakthrough velocity of contaminants to far exceed the mean flow.

An experimental test of the critical behaviour of fracture precursors

Abstract: The statistical properties and the localization of fracture precursors on heteregeneous materials is studied by recording their acoustic emission as a function of the applied load. It's found that the microcrack cluster together as the load increases and the instantaneous acoustic energy has an invariant power law distribution. The integrated acoustic energy presents a critical divergency close to the failure load for the sample. These result support the idea that fracture can be viewed as a critical phenomenon. Finally a measure of the concentration of microcraks, which allows us to predict the failure load, is introduced. These properties are studied also when a periodic load is applied to the sample. It's found that the Kaiser effect is not strictly satisfied in heteregeneous materials.

Delayed fracture of an inhomogeneous soft solid

Abstract: The spontaneous fracture of polymer gels was studied. Contrary to crystalline solids, where fracture usually happens instantaneously at a well-defined breaking strength, the fracture of a polymer gel can occur with a delay. When a constant force was applied, the cracks nucleated and started to propagate after a delay that can be as long as 15 minutes, depending on the force. This phenomenon can be understood by calculating the activation energy for crack nucleation in arbitrary dimension and accounting for the inhomogeneity of the gel network in terms of its fractal dimension.

Permeability of three-dimensional fracture networks

Abstract: The permeability of a three-dimensional network of polygonal fractures is determined by triangulating the network and solving the two-dimensional Darcy equation in each fracture. The general triangulation methodology and the numerical solution are presented. Networks of regular hexagonal fractures are detailed; finite-size scaling is used to analyze the data relative to the percolation threshold, but the conduction exponent $t$ is found close to its classical value in three dimensions; for large fracture densities, permeability is shown to tend towards the mean-field model of Snow [Water Resour. Res. 5, 1273 (1969)]. Finally, the influence of the shape of the fracture is studied and can be rationalized by means of the excluded volume.

Mechanical performance of quartz‐tempered ceramics: part i, strength and toughness*

Abstract: The effect of quartz temper on the physical and mechanical properties of clay ceramics and the elucidation of the underlying mechanisms that are responsible for these effects are presented here. Characteristics studied included bulk density, open and closed porosity, density of impervious portion and fracture morphology. Mechanical behaviour was studied by measuring energy dissipation during fracture, Young's modulus, initial fracture toughness and strength in flexure. The significant increase in toughness with quartz volume fraction is explained by the development of a model that accounts for the crack distribution around the grains. The archaeological implications of the work are discussed on the basis of all the parameters that might affect the potter's choices of raw materials.

Enhancing fluid removal from fractures deliberately introduced into the subsurface

Abstract: The present Invention relates to hydrocarbon well stimulation, and more particularly to methods and compositions to remove (or more generally to transfer) fluid introduced into a subsurface fracture, in which the methods involve creating then exploiting chemical potential gradients at the fracture face to induce fluid flow from the fracture into the formation; the methods and compositions involve creating, or relying upon an intrisic or previously created, a selectively permeable membrane that is permeable to water but that does not allow solutes to pass through.

Cell Model for Nonlinear Fracture Analysis – II. Fracture-Process Calibration and Verification

Abstract: A cell endowed with the micro-separation characteristics of the material is critical to formulating a predictive tool for nonlinear fracture mechanics analysis. This companion article addresses the second step in a two-step calibration scheme, namely, the calibration of the fracture-process parameters D and f0. The discrete, three-dimensional nature of a cell element enables it to capture important features of (i) interaction between the cell elements forming the fracture process zone and the crack including a single cavity-crack tip interaction and (ii) interaction between the fracture process and the plastic dissipation in the background material. The full three-dimensional computational model is applied to two crack geometries known to give rise to significantly different crack tip constraints and crack growth resistance behavior. Details of the load, displacement and crack growth histories in these specimens including the measured three-dimensional crack profiles are accurately predicted by the computational model. The comparison of model predictions and test data of surface cracked plates loaded by different combinations of tension and bending is reported in the third paper (Gao et al., 1997) in this series.

Mapping Hydraulic Fracture Growth and Geometry Using Microseismic Events Detected by a Wireline Retrievable Accelerometer Array

Abstract: Technology has advanced to the point where microseismic monitoring of hydraulic fractures can provide critical information for fracture optimization. Important elements of a monitoring system include the receivers, the telemetry system, and automatic processing of the vast amounts of data. Procedures and additional data requirements are discussed and examples of the important results which can be obtained are illustrated. Hydraulic fracturing is a critical technology for the exploitation of natural gas and oil resources, but its optimization has been impeded by an inability to observe how the fracture propagates and what its overall dimensions are. Recent field experiments in which fractures have been exposed through coring or mineback have demonstrated that hydraulic fractures are not the ideal, symmetric, planar features that are currently envisioned. Instead, they appear to commonly have multiple strands, secondary fractures, height and length asymmetries, and other complexities which make a priori predictions difficult. It is clear that model validation, fluid selection, proppant loadings, problem identification and solution, field development, and many other aspects of fracture optimization have been encumbered by the absence of ground-truth information on fracture behavior in normal field settings. Technology is now becoming available, however, to provide extensive diagnostic information on fracture growth, final size and geometry. Multi-level wireline receiver arrays for downhole passive imaging of fracture behavior have become viable and are demonstrating that hydraulic fractures can be imaged, assessed, and eventually controlled. These receiver arrays require high-quality transducers, well-designed clamping systems, high-speed telemetry, real-time processing capabilities, and careful procedures to be effectively used. This paper discusses this technology, its application and validation, and examples of the value of fracture imaging. It concentrates on a 5-level, accelerometer-based, fiber-optictelemetry system currently being used for microseismic mapping.

Determination of fracture storativity in hard rocks using high‐pressure injection testing

Abstract: The determination of the storage capacity of fractures in crystalline rocks by means of hydraulic injection tests is studied by coupled hydromechanical finite element simulations. The results verify that the storage is related to the fracture opening, which is dependent on the combined stiffness of the fracture and the ambient rock mass. In most practical cases the storage is entirely controlled by the normal stiffness of the fractures. The strong coupling to the fracture opening implies that the storage capacity can be estimated from the pressure dependency of the fracture aperture in a high-pressure injection test. Such high-pressure injection tests can be conducted in addition to a conventional low-pressure test to independently determine the storativity of the fracture. This provides an additional validation of the evaluated storativity, which implies not only that the value is more accurately assessed but also that other hydraulic properties can be determined more unambiguously. The method of high-pressure injection testing is applied in field experiments to deep fractures in granitic rocks at two sites, and its usefulness is demonstrated in an analysis of the field data.

Fracture Width and Embedment Testing in Soft Reservoir Sandstone

Abstract: A new computer-controlled laboratory technique has been developed to measure propped fracture width and embedment in weakly consolidated cores or unconsolidated sands under simulated down-hole conditions. Previously laboratory studies on cores have determined embedment in hard rocks where embedment is normally limited to 50% of the proppant grain diameter. Several studies also indicated the importance of embedment with one monolayer or less of proppant coverage. The effects of water saturation and fracture fluid filtrate on formation softness and embedment have not been previously published. Consequently, the objectives of the current paper are to extend previous research results to include soft weakly-consolidated cores and unconsolidated sands with multiple proppant layers. The influence of water saturation and dynamic fluid leakoff on embedment are also investigated. The current investigations indicate that embedment becomes a problem when the Brinell hardness of the formation is less than about 20 kg/mm{sup 2} or when the static Young`s modulus of formations cores is less than about 2 million psi (13 GPa). Embedment has been determined for cores with static Young`s moduli between 0.1 and 1.4 million psi (0.7 to 9.6 GPa). In soft and wet sandstone, embedment can reduce fracture width up to 60% or more formore » proppant concentrations of 2 lb/ft{sup 2}. For unconsolidated sands, embedment is influenced by fracturing fluid type, water saturation and down-hole conditions. Cyclic loading conditions associated with well shut-ins also increase embedment in unconsolidated sands. Test data on formation cores from South Texas, New Mexico, the North Sea and the Gulf of Mexico arc reviewed and discussed. Current fracture design programs often neglect embedment problems in calculating fracture width and assume that most formations are hard as steel.« less