Fracture Void Research Articles

THE STOCHASTIC FORMULATION OF MICROCRACKS EVOLUTION

The general problem in various fracture processes microcracks or voids evolution has been investigated by employing non-equilibrium statistical concept and method. The kinetic equation of microcrack evolution is derived from multivariated master equation describing the process of microcracks evolution. The way how to connect micro mechanism with macro process is indicated. And the inherent properties of distrubution function of microcracks is discussed.

Kinetics of Void Development in Fracturing A533B Tensile Bars

The phenomenology of fracture in A533B pressure vessel steel was established by examining fractured and partially fractured round bar tensile and Charpy specimens. In the upper shelf region (above 200°F, 366°K) fracture occurs by the nucleation, growth, and coalescence of voids. Quantitative data establishing the void kinetics were obtained by counting and measuring fracture surface dimples and voids below the fracture surface. A computational fracture model is being constructed based on the observed fracture phenomenology and experimental void kinetics data. The model, written as a subroutine for use in finite difference computer codes, is intended to generate fracture toughness data that can be used to calculate critical loads in reactor components.

2相合金における引張変形挙動と破壊の起源について

For the composites, the strength and the site of crack nucleation are determined by a combination of mechanical and physico-chemical factors. The former provides the required stress concentrations and local plastic deformation, while the latter is represented by the strength of inclusion, interface and matrix, respectively. So, this problem may be solved by investigating the strength and mechanical properties of each alloy element and by calculating the stress distribution in plastic deformation. But the micromechanical calculations during plastic deformation were solved by the finite element method, that has been applied to the fracture mechanics. On the assumptions of the three different crack initiation modes, two-phase model alloys which were the Al matrix with Si, the Al matrix with Cu and the Cu matrix with Fe, were prepared by the hot-press method and the tensile behaviour was observed. On the other hand, the elastic-plastic finite element simulation was used for the micromechanical stress and strain calculations of these composite models in tension.There was good agreement in the load and the site of initiation of fracture voids between the results calculated from the simulation and experimental data on Si/Al, Cu/Al and Fe/Cu composite alloys. And the available results were computed for the initiation of ductile fracture voids.

延性破壊におけるvoidの起源と成長に関するシミュレーション

延性破壊におけるvoidの起源と成長に関するシミュレーション

Evaluation of Fractured Reservoirs

Abstract Efforts were made to find improved means for locating fractures penetrated by a wellbore and for estimating fracture reservoir volume. The four approaches to the problem utilized acoustic logs, porosity estimates from different sources, transient pressure and fluid flow data, and resistivity logs. Acoustic amplitude attenuation and acoustic variable-intensity interference patterns have been used to locate fractures. Although amplitude logs have often proved useful for fracture detection, they are frequently inconclusive when used alone. This is due partially to variations in amplitude caused by factors other than fracturing. The variable-intensity interference patterns produced by borehole discontinuities in casing and in open-hole fractured sections were found to be quite useful in detecting fractures; but, like amplitude logs, they are not definitive by themselves. A technique has been developed that uses porosity estimate comparisons to evaluate fractured reservoirs. If this technique is used with acoustic variable-intensity interference patterns, it may be helpful for delineating fractured zones and for estimating fracture porosity. Transient pressure behavior observed for a fractured reservoir was found to be in agreement with that theoretically predicted for linear flow systems. Since this behavior is markedly different from that of a homogeneous reservoir, interpretation of transient pressures may provide a means of recognizing fractures. Other helpful techniques employing pressure and fluid flow data are comparison of calculated kh's for injection and withdrawal of fluids, comparison of calculated kh's for different rates of injection, and calculation of k/phi's from pressure and log data, where k is permeability, h is producing thickness, and phi is porosity. Some of these techniques also present possibilities for calculation of reservoir volumes. The fourth approach to fracture detection showed that, under certain conditions, an induction log can be used to detect a resistivity anomaly opposite a fractured zone. Although some of the techniques discussed show promise of being helpful, further study will be required before evaluations of fractured reservoirs become as satisfactory as evaluations of "normal" porosity reservoirs. Introduction Experience has shown that the presence of a fractured system can improve the productivity of hydrocarbon reservoirs. In some cases, fracture void space also supplies a significant portion of the total porosity. However, quantitative determination of the contribution of fracture systems to production from petroleum reservoirs has proven difficult. This is due in part to lack of consistently successful techniques for locating and usefully describing fracture systems penetrated by a wellbore. This paper presents the initial results of a research program on the use of borehole measurements for evaluation of fractured reservoirs. The objectives of this research are to find improved means forlocating fractures penetrated by a wellbore, andestimating in-situ reservoir volume contained within fracture systems in communication with the wellbore. This progress report will describe our attempts to date to use four types of data for fractured reservoir evaluation:acoustic logs,porosity estimates from different sources,transient pressure and fluid flow data, andresistivity logs. ACOUSTIC LOGS Amplitude Logs The acoustic amplitude log is one of the most widely used measurements in attempts to detect fractures. SPEJ P. 28ˆ

Using Compressional and Shear Acoustic Amplitudes for The Location of Fractures

Abstract Field results have shown that fractured zones may be located by their attendant reduction of acoustic amplitude. Laboratory and theoretical investigations confirm this technique, but interpretation of amplitude logs is complicated by the many variable factors encountered in actual logging operations. The acoustic amplitude investigations covered by this paper were made by continuous measurements of the peak amplitudes of single, and well defined compressional and shear-wave arrivals. A simultaneously recorded measurement of interval transit time or total travel time, in each case, indicated whether or not there had been continuous amplitude measurement of the same wave arrival. Investigations have shown that the angle at which a fracture plane crosses a borehole affects the attenuation of acoustic signals. Theoretically, horizontal fractures (those perpendicular to the axis of the borehole) should cause little or no attenuation of the compressional wave; this is confirmed by field examples. Shear-velocity waves, on the other hand, are significantly attenuated by horizontal fractures. While oblique fractures cause a reduction of compressional-wave amplitude, shear-wave amplitude measurements in such cases may not be as definitive. Since the early compressional arrivals are not subject to interference complications, as are shear arrivals, both measurements should be made and used to complement each other. Introduction An important problem in formation evaluation is the location of fractured zones. Because of the relatively low ratio of fracture void to bulk volume, down-hole measurements of formation resistivity, acoustic velocity, or density have been unsuccessful in satisfactorily detecting fractures in most cases. A Possible solution to the problem was noted on early Sonic Logs run in hard formations. Cycle-skipping. indicating weak compressional arrivals, occurred opposite suspected fractured intervals. Subsequent developments have permitted recording acoustic amplitude as well as studying scope pictures of the wave train. Several papers have shown the usefulness of these methods to locate fractures. The process of fracture detection from signal amplitude variation is complex. Laboratory and field studies have shown that the effects of a number of variables must be considered in the interpretation of amplitude changes. It is the purpose of this paper to explore the use of the acoustic signal to detect fractures, taking into consideration the effect of these other variables. Propagation of The Acoustic Signal A simplified representation of the sonic waves formed by a source of sound in a liquid-filled borehole is given in Fig. 1. Capital letters designate the wave when it is traveling in the mud, subscribed letters when it travels in the formation. A theoretical description of these waves for the analogous problem of wave propagation in a liquid layer over a solid half space was given by Strick. The waves labeled P and St are the direct and Stoneley waves, respectively. The theory of their propagation in a borehole has been developed by Biot and they have been observed by Pickett. The other waves in the borehole labeled PpP and P, P, are the refracted compressional arrival and the shear velocity arrival. The R wave will be mentioned later. The Refracted Compressional Wave If compressional velocity in the formation Vp, is enough higher than mud velocity C the first arrival in the sonic waveform is a wave that has traveled from the transmitter to the formation as a compressional wave in the mud, has been refracted at the borehole wall, and has traveled along the wall at the compressional-wave velocity in the formation. In an infinite medium, each small particle affected by a compressional wave oscillates only in the direction of wave propagation as the compressions and rarefactions travel past it. When a borehole exists. particle motion is more complex. JPT P. 623ˆ