Showing papers on "Fracture (geology) published in 1987"

Influence of Geologic Discontinuities on Hydraulic Fracture Propagation (includes associated papers 17011 and 17074 )

Abstract: Geologic discontinuities, such as joints, faults, and bedding planes, can significantly affect the overall geometry of hydraulic fractures. This can occur by arresting the growth of the fracture, increasing fluid leakoff, hindering proppant transport, and enhancing the creation of multiple fractures. Results from mineback experiments and laboratory tests and analyses of these data are integrated to describe this complex fracture behavior.

Fractal structure of spatial distribution of microfracturing in rock

Abstract: Summary. A constant stress fracture experiment of Oshima granite was carried out at the confining pressure of 40MPa. Hypocentres of 2064 acoustic emissions were located during the experiment. Using the ‘correlation integral’, we found that the spatial distribution of hypocentres of acoustic emission is a fractal, and that the fractal dimension decreases with the evolution of rock fracturing. The spatial distribution of earthquake’s hypocentres reveals fractals ranging from regional to worldwide distribution. If we extrapolate from laboratory measurements, it is possible to predict the occurrence of large earthquakes by the decrease in the fractal dimension.

Conditional simulations of fluid flow in three-dimensional networks of discrete fractures

Abstract: How to predict flow through a network of discrete fractures in a three-dimensional domain is investigated. Fractures are modeled as circular discs of arbitrary size, orientation, transmissivity, and location. A fracture network is characterized by the statistical distributions of these quantities. Fracture traces observed on a wall form the basis for estimates of mean fracture radius, fracture orientation parameters, and fracture density. Fracture trace lengths are estimated with the scanline method and from areal sampling on circular regions. The traces observed on the wall can also be used to condition the network. This trace conditioning is achieved by forcing the network generator to always reproduce the observed traces. Conditioning might be a means of decreasing the variability of the fracture networks. A numerical simulation model has been developed which is capable of generating a fracture network of desired statistical properties and solving for the steady state flow. On each fracture disc the flow is discretized with the boundary element method. A series of hypothetical examples are analyzed. These examples consist of sets of Monte-Carlo simulations of flow through a series of networks generated from the same statistical distributions. The examples lead to the following conclusions. Large fractures and high fracture density implies good connectivity in the networks. A high fracture density implies a small variance in the flow through the network. Trace conditioning decreases estimation variance only when the fracture network consists of large fractures. Fracture statistics can be estimated reasonably well from fracture traces observed on a wall.

Experimental study of the mechanics of fracture in WC-Co alloys

Abstract: Tungsten-carbide cobalt alloys in the range of technically relevant compositions an microstructures were fractured under controlled conditions. Stable and unstable crack propagation are shown to be equivalent with respect to crack geometry and energy release rates. Based on microscopic observations and on measurements using SEM micrographs, the nature of the process zone, the maximum extension of plastic deformation, the types of crack paths, and the area fractions of these crack paths are determined. The results, which are in partial disagreement with earlier work, provide the basis for the qualitative understanding and the quantitative description of fracture processes in composites combining a ductile phase embedded in a brittle matrix.

Sources of electromagnetic radiation from fracture of rock samples in the laboratory

Abstract: Rock, when fractured in the laboratory, emits electrical signals with a broad maximum in the power density spectrum in the band from 900 Hz to 5 kHz. Quartz-free basalt rocks radiate both light and low-frequency electrical signals as intensely as quartz-bearing rocks, suggesting that the piezoelectric effect of quartz is at most a minor contributor to the total power radiated. Newly created rock surfaces acquire a local net charge distribution when exoelectrons are expelled and removed from the vicinity of the new surfaces by collisional and other processes. The rotational, vibrational, and linear motion of rock fragments with charged surfaces is the major contributor to the observed power density spectrum of the low-frequency electrical signals. Neither optical spectral lines nor high-frequency electromagnetic radiation, characteristic of electric discharges, occur at fracture. Exoelectron bombardment of the ambient fluid surrounding the sample, not electric discharge, is the excitation source for the light emitted at fracture.

Quantitative assessment of rock texture and correlation with drillability and strength properties

Abstract: A dimensionless quantitative measure of rock texture, describing grain: —shape, orientation, degree of grain interlocking and relative proportions of grains and matrix (packing density) has been developed. Data required for the model are obtained by image analysis of thin sections and concerns percentage areas of grains and matrix, length, breadth, perimeter, orientation and area of each grain in the viewing window. The results of rock strength, diamond and percussion drillability tests in eleven sandstones, marbles and igneous rocks are reported, and correlated with the developed texture coefficient. The texture coefficient returns highly statistically significant correlations with rock strength and drillability data. Sandstones have low texture coefficients and high drillability whereas igneous rocks have high texture coefficients and low drillability. With particular reference to percussive drillability it is suggested that extensional crack propagation in the sandstones in an energy efficient process since fracture paths propagate through the weak phyllosilicate matrix. Extensional crack propagation in the igneous rocks is an energy intensive process since a significant proportion of the available drilling energy is consumed in the formation of intra-granular fracture paths. Observational and correlated data are supportive of the suggestion that the texture coefficient is a measure of the resistance of the microstructure of a rock to crack propagation, whether it be inter-or intra-granular. The texture coefficient can be used as a predictive tool for the assessment of drillability and rock strength properties. The technique offers a useful approach in understanding fracture initiation and growth as controlled by the texture of intact rock samples.

The essential work of plane stress ductile fracture of linear polyethylenes

Abstract: The specific essential works of plane stress ductile tearing of several high- and ultrahigh-molecular-weight polyethylenes were obtained from deeply edge-notched tension specimens, with either single or double notches, by extrapolating the straight line relationship between the total specific fracture work and ligament length to zero ligament. Provided the fracture morphologies of the torn ligament are not widely different, the specific essential work (we) is a material property dependent on thickness but independent of specimen geometry. The specific essential fracture work also can be identified with Jc the critical value of the J-integral along a contour immediately bordering the fracture process zone at the crack tip. There is good agreement between the experimental we values and theoretical Jc estimates for these polyethylene materials.

Locating microearthquakes induced by hydraulic fracturing in crystalline rock

Abstract: Microearthquakes induced by hydraulic fracturing in crystalline rock at a depth of 3.5 km were located with a precision of better than 30 m to obtain information about the geometry and dimensions of the fracture system produced. The induced microseismicity was monitored by a network of five vorehole seismic stations; a total of about 800 induced events were reliably located from arrival times. Event locations show a tabular distribution that strikes 350/sup 0/ and dips 65/sup 0/ east, subparallel to the injection well. The injection was intended to produce a fracture system that would hydraulically connect two subparallel wells. A lack of fluid communication between them is consistent with a lack of induced microearthquakes near the target wellbore. The 150 m thickness of the zone of seismicity far exceeds the relative locational uncertainties. The injected fluid appears to have stimulated a zone of rock, rather than simply a single fracture. The distribution of microseismic events presumably envelopes the zone of fluid paths created by the fracture experiment.

The role of damage-softened material behavior in the fracture of composites and adhesives

Abstract: Failure mechanisms of materials under very high strain experienced at and ahead of the crack tip such as the formation, growth and interaction of microvoids in ductile materials, microcracks in brittle solids or crazes in polymers and adhesives are represented by one-dimensional, nonlinear stress-strain relations possessing different post-yield softening (strain-softening) behavior. These reflect different ways by which the material loses capacity to carry load up to fracture or total separation. A DCB type specimen is considered in this study. * The nonlinear material is confined to a thin strip between the two elastic beams loaded by a wedge. The problem is first modelled as a beam on a nonlinear foundation. The pertinent equation is solved numerically as a two-point boundary value problem for both the stationary and the quasi-statically propagating crack. A finite element model is then used to model the problem in more detail to assess the adequacy of the beam model for the use of experimental data to determine in-situ properties of the thin interlayer.

Random stress and earthquake statistics: spatial dependence

Abstract: SUMMARY We calculate the 3-D stochastic rotations of focal mechanisms (disorientations) caused by stresses arising from the presence of many small, random, point defects that may surround the tip of an earthquake fault. These random stresses can be shown to be distributed according to a Cauchy distribution; as a consequence the next episode of fracture of a fault cannot be planar. The disorientation of focal mechanisms of these new rupture episodes is closely approximated by a rotational Cauchy distribution. As observed previously, the geometry of fault systems for natural earthquakes is consistent with these observations, since it too can be modelled by the Cauchy distribution. These observations indicate how the 3-D rupture process in rocks and other materials can be modelled. We also calculate distributions of disorientations caused by a uniformly random 3-D rotation of sources. These distributions are governed by symmetry properties of earthquake focal mechanisms. To take into account the source symmetry, we compare the Cauchy distribution which is due to influence of random stresses to the distributions caused by random rotations.

Fracture characterization by means of attenuation and generation of tube waves in fractured crystalline rock at Mirror Lake, New Hampshire

Abstract: Results are presented from experiments carried out in conjunction with the U. S. Geological Survey at the Hubbard Brook Experimental Forest near Mirror Lake, New Hampshire. The study focuses on our ability to obtain orientation and transmissivity estimates of naturally occurring fractures. The collected data set includes a four-offset hydrophone vertical seismic profile, full waveform acoustic logs at 5, 15, and 34 kHz, borehole televiewer, temperature, resistivity, and self-potential logs, and borehole-to-borehole pump test data. Borehole televiewer and other geophysical logs indicate that permeable fractures intersect the Mirror Lake boreholes at numerous depths, but less than half of these fractures appear to have significant permeability beyond the annulus of drilling disturbance on the basis of acoustic waveform log analysis. The vertical seismic profiling (VSP) data indicate a single major permeable fracture near a depth of 44 m, corresponding to one of the most permeable fractures identified in the acoustic waveform log analysis. VSP data also indicate a somewhat less permeable fracture at 220 m and possible fractures at depths of 103 and 135 m; all correspond to major permeable fractures in the acoustic waveform data set. Pump test data confirm the presence of a hydraulic connection between the Mirror Lake boreholes through a shallow dipping zone of permeability at 44 m in depth. Effective fracture apertures calculated from modeled transmissivities correspond to those estimated for the largest fractures indicated on acoustic waveform logs but are over an order of magnitude larger than effective apertures calculated from tube waves in the VSP data set. This discrepancy is attributed to the effect of fracture stiffness. A new model is presented to account for the mechanical strength of asperities in resisting fracture closure during the passage of seismic waves during the generation of VSPs.

Observation and modelling of fault‐zone fracture seismic anisotropy ‐ I. P, SV and SH travel times

Abstract: Summary. Three-component VSP borehole seismograms taken in the vicinity of an active normal fault in California show strong systematic shear-wave splitting that increases with proximity to the fault. Using Cervený's method of characteristics for ray tracing in anisotropic heterogeneous media and Hudson's formulation of elastic constants for media-bearing aligned fractures, we have fitted a suite of P, SV and SH hanging-wall and foot-wall travel times with a simple model of aligned fractures flanking the fault zone. The dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault. The increase in fracture density is non-uniform (power law or Gaussian) with respect to distance to the fault. Although the hanging-wall and the foot-wall rock are petrologically the same unit, the fracture halo is more intense and extensive in the hanging wall than in the foot wall. Upon approach to the fault plane, the fracture density or fracture-density gradient becomes too great for the seismic response to be computed by Hudson–Cervený procedures (the maximum fracture density that can be modelled is about 0.08). Within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low-velocity waveguide. We observe production of trapped waves within the confines of the intense fracture interval.

Calculation of vertical fracture containment in layered formations

Abstract: An analytic procedure for calculating vertical fracture extent in symmetrical trilayered formations was extended to multilayered, asymmetrical formations using a semianalytic technique. The fracture extends computed by this method were compared with those calculated with the finite-element method. It was found that even for modulus variations between layers as large as a factor of 5, the semianalytic procedure gave exactly the same results as the finite-element solution in a fraction of the computation time and with significantly less manual data manipulation. It is recommended that the analytic and numerical procedures be used in a complementary manner to calculate fracture-width profiles in layered formations.

Fracture characteristics of Al-4 pct Mg mechanically alloyed with SiC

Abstract: Subcritical crack growth and rapid fracture of the mechanically alloyed aluminum alloy IN-9052* reinforced with SiC particles have been investigated. Fatigue crack growth rates for the composite exceed those of the unreinforced alloy, except that the threshold stress intensity for growth is higher for the composite. Fracture toughness of the composite is about 9 MPa√m compared to a (reported) value of 29 MPa√m for the unreinforced alloy. The contributions to fracture toughness from work done within the plastic zone and in formation of the void sheet have been computed using analytical models. Fracture toughness is shown to result almost entirely from work done within the plastic zone of the growing crack. The matrix microstructure and the particulate characteristics are found to account for the elastic and fracture properties of this composite.

Simultaneous hydraulic fracturing

Abstract: A process and apparatus for simultaneous hydraulic fracturing of a hydrocarbonaceous fluid-bearing formation. Fractures are induced in said formation by hydraulically fracturing at least two wellbores simultaneously. While the formation remains pressurized curved fractures propagate from each wellbore forming fracture trajectories contrary to the far-field in-situ stresses. By applying simultaneous hydraulic pressure to both wellbores, at least one curved fracture trajectory will be caused to be transmitted from each wellbore and intersect a natural hydrocarbonaceous fracture contrary to the far-field in-situ stresses.

Laminated Composites Containing an Elliptical Opening. I. Approximate Stress Analyses and Fracture Models

Abstract: Approximate stress analyses are presented for orthotropic composite laminates contain ing an elliptical opening under uniaxial normal loads. Examining by the use of exact elasti city solution shows that these approximate solutions have excellent accuracy. Utilizing these solutions, the point stress fracture criterion and the average stress fracture criterion are generalized for the strength prediction of fiber-reinforced composite laminates contain ing an elliptical opening. Comparisons show that these models have good correlation with experimental data.

Deformation and fracture characterization of inelastic composite materials using potentials

Abstract: An approach using strain energy-like potentials to characterize deformation and fracture of inelastic, nonlinear composite materials is described. The inelasticity may be due to various causes, including microcracking, microslipping, and rate processes responsible for fading memory (viscoelasticity). The concept of work potentials is introduced first, and then arguments are given for their existence for inelastic materials. Emphasis in the paper is on elastic composite materials with changing or constant states of distributed damage. Experimental results on polymeric composites are subsequently presented to illustrate this approach to deformation and fracture characterization. Finally, extension to viscoelastic behavior is discussed.