Fracture Initiation Mechanism Research Articles

Challenges in hydraulic fracturing from perforated boreholes in unconventional reservoirs

In recent years, with the evolution of unconventional reservoirs, hydraulic fracturing has been applied to tight sandstone and shale formations to improve the hydrocarbon production. The application of hydraulic fracturing in cased boreholes is always associated with many difficulties because the fracture has to be initiated from the perforations. There have been many cases of improper fracture initiation in tight formations, which have then resulted in premature screen out, and have not improved reservoir production. In this study, initiation of hydraulic fracturing from a perforated tunnel was studied numerically using a finite element method. The numerical model was generated to represent a laboratory experimental test, which has been carried out on tight concrete cubic samples. A perforated wellbore in a linearly elastic tight formation was modelled using Abaqus software through three-dimensional numerical analysis. Two different perforation orientations were considered to analyse the fracture initiation pressure (FIP) and the location and initial direction of the crack. Different far field stresses were considered to study the effect of in-situ stresses and perforation directions on the fracture initiation mechanism. The results were then compared to laboratory and analytical outcomes, and good agreement was observed. The results provide a better understanding on how the stress regime, stress anisotropy, and perforation orientation could affect the pressure and geometry of fracture initiation in tight formations. Based on the outcomes of this study, better strategies can be decided for perforating a cased wellbore in a tight formation so that lower FIP is experienced and a better near wellbore fracture is created.

Mechanism for Network Fracturing in Natural Fractured Reservoir

By the methods of in-house experiment and theory analysis, the fracture initiation, stretch and interconnected mechanism of natural fracture is studied in the fracturing process of main fracture and branch fracture extending. The results have shown that, the hydraulic fracture in fractured formation is composed of a few main fractures in large size and many s secondary fractures in small size. The main fracture has the extending trend with the maximum crustal stress direction. The direction of sub fracture is network along the maximum crustal stress. The higher the degree of natural fractures, the difference between maximum and minimum crustal stress smaller. And the natural fracture is easier to form larger fracture. Fracture orientation and fracture width are determined by different combinations of natural fractures and the relative orientation with the maximum crustal stress.

Fracture Behavior of Intact Rock Using Acoustic Emission: Experimental Observation and Realistic Modeling

It is well known that acoustic emission (AE) is a powerful nondestructive testing tool for examining the behavior of materials deforming under stress. One can use it to monitor fracture or damage in a rock mass by listening to AE events during failure under compressive loads. In this paper, an experimental study on the AE source location in square cylinder granite specimens under uniaxial compression is reported. In order to determine the three-dimensional location of AE events, eight AE sensors were mounted on the specimen. The AE source location was determined via the acquisition of eight channel AE sensors after filtering, processing, reporting, and visualizing seismic data. On the basis of the laboratory experiment results, the granite sample was numerically simulated via the Burgers model using the discrete element program PFC2D (Particle Flow Code in Two Dimensions) to further study the mechanism of fracture initiation and propagation in intact rock. In PFC2D, materials may be modeled as either bonded (cemented) or unbonded (granular) assemblies of particles. It can describe nonlinear behavior and localization with accuracy that cannot be matched by typical finite element programs. The consistency of stress-strain curves obtained with PFC2D and with the test results shows that PFC2D is a practical tool for reproducing AE events in rock, and use of the Burgers model is feasible in the field of rock failure and provides an analysis of the microcracking activity inside the rock volume to predict rock fracture patterns under uniaxial loading conditions.

A methodology for pseudo-genetic stochastic modeling of discrete fracture networks

Stochastic simulation of fracture systems is an interesting approach to build a set of dense and complex networks. However, discrete fracture models made of planar fractures generally fail to reproduce the complexity of natural networks, both in terms of geometry and connectivity. In this study a pseudo-genetic method is developed to generate stochastic fracture models that are consistent with patterns observed on outcrops and fracture growth principles. The main idea is to simulate evolving fracture networks through geometric proxies by iteratively growing 3D fractures. The algorithm defines heuristic rules in order to mimic the mechanics of fracture initiation, propagation, interaction and termination. The growth process enhances the production of linking structure and impacts the connectivity of fracture networks. A sensitivity study is performed on synthetic examples. The method produces unbiased fracture dip and strike statistics and qualitatively reproduces the fracture density map. The fracture length distribution law is underestimated because of the early stop in fracture growth after intersection.

Experimental Studies and Models for Ductile Fracture in ASTM A992 Steels at High Triaxiality

AbstractThis study presents the results of experimental investigation and models for ductile fracture in the ASTM A992 structural steels subjected to high stress triaxiality. To this end, uniaxial tension tests on the notched tensile specimens are conducted, and subsequent scanning electron microscopy (SEM) studies are performed on the fractured surfaces. From this SEM analysis, the fracture initiation mechanism in ASTM A992 steels is found to be the ductile fracture; however, the fracture propagation mode can be either ductile or cleavage fracture. Nonlinear finite-element analysis of the notched specimens is carried out to obtain the stress triaxiality distributions, and the location of the peak stress triaxiality is found to be strongly correlated to the location of the ductile fracture initiation zones. Finally, two uncoupled ductile fracture prediction models, the void growth model (VGM) and the stress modified critical strain model (SMCS), are calibrated, and the model parameters are provided for A9...

A New Model for Analyzing Hydraulic Fracture Initiation in Perforation Tunnels

Hydraulic fracturing has been used for many years in order to stimulate the oil and gas wells for improving reservoir production. The challenge of fracture initiation pressure and near-wellbore fracture propagation is a complex problem especially in deviated and cased perforated holes. Understanding the stress distribution around the perforations is the key for analyzing the fracture initiation mechanism and thereby realizing the near-wellbore fracture geometry. In this article, a generic model is developed for estimating the initiation pressure as well as the orientation geometry of the fracture in the perforations around the borehole. The results of this study show that at any specific stress regime around the wellbore, there is an optimum perforation orientation that leads to lower fracture initiation pressure.

Wave diffraction by a line of finite crack in a saturated two-phase medium

An application of the Biot’s theory to the diffraction problem of plane harmonic dilatational waves (P-waves) of the first kind and the second kind by a line crack or geometric discontinuity of finite width embedded in a saturated two-phase medium is presented in this paper. The crack surfaces are assumed impermeable, and the integral transform method is utilized to reduce the mixed boundary-value problem to a single Fredholm integral equation. The magnitudes of the intensity of the stress fields near the crack tips measured by Mode I dynamic stress-intensity factor (dimensionless) are computed and displayed graphically against dimensionless circular frequency (ω) for several dimensionless material property values, namely, viscosity-to-permeability and mass density ratios. In the case of the normally incident P-waves of the first kind, the results in terms of stress-intensity factor are also compared with the corresponding values of dry elastic material. All the stress-intensity factor curves are shown to exhibit a similar character in that they rise to the peaks at certain frequency values and then decay with increasing frequencies. At certain frequency ranges and material property values, amplification in the dynamic stress-intensity factor can be substantially larger than those encountered in dry elastic materials. The stress-intensity factor is found to be more affected by the changes in the ratio of viscosity-to-permeability at lower mass density ratio. With fluid mass density 10% of the bulk mass density, the viscosity-to-permeability ratio of 0.01 gives the highest increase of about 32% in the magnitude of stress-intensity factor compared to the dry material counterpart value, while a decrease of about 9% is observed for the viscosity-to-permeability ratio of 100. It is also found that change in mass density ratio has significant effect upon the magnitude of stress-intensity factor at lower ratio of viscosity-to-permeability. As for the normally incident P-waves of the second kind, the presence of the pore fluid affects both the magnitude and character of the stress-intensity factor. Large variations in the magnitude of stress-intensity factor are observed as viscosity-to-permeability ratio changes from 1 to 100. At the ratio of viscosity-to-permeability of 1.0, the stress-intensity factor curves increase gradually with frequency and exhibit the peaks in curves for mass density ratio of 0.3 and higher. As the viscosity-to-permeability ratio is raised to 100, the stress-intensity factor curves increase monotonically with frequency at a much faster rate throughout the frequency range of interest (ω=0–2), and the change in mass density ratio is shown to have little effect on the stress-intensity factor, especially within the low frequency ranges. The results obtained in this study are useful in the mechanics of fracture initiation of saturated porous materials under the fluctuating mechanical and/or pore fluid loadings that are periodic with time.

Investigation of mechanism of cleavage fracture initiation in intercritically coarse grained heat affected zone of HSLA steel

The degradation of the fracture toughness of high strength low alloy steels is attributed to the formation of ‘local brittle zones’ in the welded joint. These local brittle zones are mainly located within the coarse grained heat affected zone (CGHAZ) and the intercritically reheated CGHAZ (ICCGHAZ). Cracking and debonding of M-A constituents from the surrounding matrix are generally accepted as initiation events of fracture in the ICCGHAZ. In the present work, the low temperature fracture toughness of X80 pipeline steel was examined. The main purposes were (i) to evaluate possible crack initiation sites of cleavage fracture and (ii) to identify the mechanism by which M-A constituents deteriorate the ICCGHAZ toughness. The results revealed that the microstructure of ICCGHAZ contained blocky M-A constituents along prior austenite grain boundaries. Finally, it was shown that fracture initiation occurred preferentially at M-A constituents by a debonding mechanism rather than cracking of the M-A constituents.

Prior storage conditions and loading rate affect the in vitro fracture response of spinal segments under impact loading

Traumatic injuries of the spine are mostly the consequence of rapid overload e.g. impact loading. In vitro investigations on this topic usually encompass biomechanical testing using frozen/thawed specimens and employ quasi-static loading conditions. It is generally accepted that a freezing/thawing cycle does not alter mechanical properties for slow loading rates. However, this has never been investigated for high impact velocities. In order to assess the effects of freezing/thawing and the influence of different impact velocities, we loaded 27 fresh and 15 frozen/thawed cadaveric rabbit spinal segments (intervertebral disc with one third of the adjacent vertebrae) with different impact energies and velocities using a custom-made, dropped-weight loading device. Endplate fractures were assessed by micro-CT scans. Specimen dimensions (disk, bone, and total height) and vertebrae bone density (BV/TV) were compared pre- and post-trauma. Energy absorption by spinal segments was quantified by measuring the initial ball rebound. We found that freezing/thawing increased endplate fracture frequency and decreased the energy absorption of the segments. Higher impact velocities increased the energy absorption, while higher impact energy increased both energy absorption and fracture frequency. Two conclusions are drawn: first, under impact loading, freezing alters permanently the biomechanical response, and second, for different impact velocities, different fracture initiation mechanisms apply. Therefore, quasi-static loading of frozen/thawed spinal segments is not a valid model for traumatic endplate injuries. However, caution should be exercised in extrapolating these findings to human vertebrae until tests on larger vertebrae are performed.

An experimental study of fracture initiation mechanisms during hydraulic fracturing

The mechanism of fracture initiation is the basic issue for hydraulic fracture technology. Because of the huge differences in fracture initiation mechanisms for different reservoirs, some successful fracturing techniques applied to porosity reservoirs are ineffectual for fractured reservoirs. Laboratory tests using a process simulation device were performed to confirm the characteristics of fracture initiation and propagation in different reservoirs. The influences of crustal stress field, confining pressure, and natural fractures on the fracture initiation and propagation are discussed. Experimental results demonstrate that stress concentration around the hole would significantly increase the fracture pressure of the rock. At the same time, natural fractures in the borehole wall would eliminate the stress concentration, which leads to a decrease in the fracture initiation pressure.

Evaluation of dynamic fracture toughness based reference temperature ( [formula omitted]) of modified 9Cr–1Mo steel in phosphorus embrittled and cold-worked condition

In the present study, the DBTT of a high phosphorus bearing modified 9Cr–1Mo steel in normalized and tempered (N&T) condition and also after 5% and 10% cold work, has been evaluated by extending the reference temperature ( T 0) based Master Curve approach to dynamic loading conditions. The reference temperature in dynamic loading condition ( T 0 dy ) thus determined is found to be significantly high for the N&T Mod.9Cr–1Mo steel as compared to plain 9Cr–1Mo steel. Cold-work effect was not found to be significant in T 0 dy evaluation of Mod.9Cr–1Mo steel. The high T 0 dy of the present Mod.9Cr–1Mo steel is attributed to a fracture initiation mechanism predominated by decohesion of prior austenitic grain boundaries at the fatigue pre-crack front. This is attributed to segregation of phosphorus along the prior austenitic grain boundaries causing embrittlement, supported by SEM and Secondary Ion Mass Spectrometry (SIMS) observations.

Application of hydrogen influenced cohesive laws in the prediction of hydrogen induced stress cracking in 25%Cr duplex stainless steel

Cohesive zone finite element modeling is applied in the simulation of hydrogen induced stress cracking in 25%Cr duplex stainless steel. Hydrogen influence is implemented in linear and polynomial cohesive laws. Suitability of the laws in prediction of hydrogen induced stress cracking is investigated by applying models of U and V-notched tensile specimens representing a 25%Cr duplex stainless steel component submerged in sea water under cathodic protection (CP). Fracture prediction is performed by a three step procedure; elastic plastic stress analysis, stress assisted hydrogen diffusion and cohesive stress analysis. Local cohesive stress fields as well as the time to fracture initiation are investigated as a function of the shape of the traction separation laws and the element size for three levels of tensile stress. Simulated results are also compared with results from laboratory tensile tests and discussed with respect to the suitability of describing fracture initiation and fracture mechanism of the steel. The results show that the polynomial law and a mesh size of 0.5 μm gives the most accurate description of the local cohesive stress field. The simulated time to fracture is closest to laboratory test results for stresses of 0.85–0.9 times the yield strength.

Fracture of elastomers under static mixed mode: the strain-energy-density factor

This work deals with the fracture of rubbers under a mixed mode loading (I + II) and it is an extension of our previous papers on that subject [Aït Hocine N, Naït Abdelaziz M, Imad A (2002) Int J Fract 117:1–23; Aït Hocine N, Naït Abdelaziz M (2004) In: Sih GC, Kermanidis B, Pantelakis G (eds) 6th international conference for mesomechanics. Patras (Greece), May 31–June 4, pp 381–385]. An experimental and a numerical analysis were carried out using a Styrene Butadiene Rubber (SBR) filled with 20 and 30% of carbon black. Sheets with an initial central crack (CCT specimens) inclined with a given angle compared to the loading direction were used. The J-integral and its critical values J c (fracture surface energy) were determined by combining experimental data and finite element results. These critical values, determined at the onset of crack growth, were found to be quite constant for each elastomer tested, which suggests that J c represents a reasonable fracture criterion of such materials. Then, the strain–stress field and the strain-energy-density factor S, earlier introduced by Sih [Sih GC (1974) Int J Fract 10(3):305–321; Sih GC (1991) Mechanics of fracture initiation and propagation. Kluwer Academic Publishers, Dordrecht, 428 pp] were numerically calculated around the crack tip. According to the experimental observations, the plan of crack propagation is perpendicular to the direction of the maximum principal stretch. Moreover, as suggested by Sih in the framework of linear elastic fracture mechanics (LEFM), the minimum values S min of the factor S are reached at the points corresponding to the crack propagation direction. These results suggest that the concept of the maximum principal stretch and the one of the strain-energy-density factor can be used as indicators of the crack propagation direction.

Distinct element modeling of hydraulically fractured Lac du Bonnet granite

The aim of this study is to better understand the mechanics of fracture development and propagation during hydraulic fracturing. This paper presents some development and applications of discrete particle modeling of this problem. A discontinuum modeling approach idealizes the material as separate particles bonded together at their contact points and utilizes the breakage of individual structural units or bonds to represent damage. The numerical models are correlated with existing hydrofracture laboratory experiments, which are presented in other publications. A simulation of a laboratory‐scale hydrofracture experiment and the acoustic emission (AE) data from the experiment is used to validate the synthetic AEs produced in the hydrofracture model. This technique has been used to examine the mechanics of fracture initiation and time and spatial distributions of AE. The modeling results demonstrate that the mechanism of hydraulically induced fracture in the Lac du Bonnet (LdB) granite core sample is predominantly tensile failure and that the shear cracks recorded in the hydrofracture experiment were due to slip on preexisting fractures. Numerical modeling of hydrofracture on homogeneous and heterogeneous synthetic samples seems to capture much of the behavior observed in the laboratory hydrofracture experiments.

Experimental and modeling study of fluid pressure‐driven fractures in Darley Dale sandstone

We report measurements of acoustic emissions (AE) generated during formation and growth of pressure driven fractures in cylindrical samples of Darley Dale sandstone that were co‐axially pre‐drilled in order to allow an internal pressure to be applied. A set of 3 to 6 fractures initiate at the wall of the internal bore at a fluid pressure around three times that of the confining pressure, but only 3 propagate to the outer wall of the sample. Time and spatial distributions of acoustic emissions show two distinct bursts of activity, associated with initiation and propagation, respectively. A Particle Flow Code (PFC) model has been used to reproduce the mechanics of fracture initiation and the time and spatial distributions of AE. In both the experiments and the model, we observe a distinct phase of accelerating AE activity preceding fracture formation.

Fatigue Crack Initiation Behavior of Welded AA5083 in a Seawater Environment

Aluminum Alloy 5083 is increasingly used in the ship building industry due to its excellent corrosion resistance and its favorable mechanical properties. A significant application of this alloy is in high speed ferries, where the influence of dynamic loads is of great importance as well. However, the influence of a corrosive environment and/or a welded condition on the fatigue properties of nautical aluminum alloys has been studied only superficially. In the present paper the influence of a seawater environment in combination with a welded state is described. Both fatigue limits and SN-curves are given, as well as a fracture surface analysis. From these results the fracture and crack initiation mechanisms are explained. Particularly the penetration of seawater into sub-surface pores, that are present in the welded material, results in a significant drop in fatigue life time when the welded alloy is tested in seawater.

Fracture initiation mechanisms of thin hard coatings during the impact test

The impact test can be successfully applied for the characterization of thin and thick coatings fatigue and creep properties, respectively. In the present paper experimental and analytical investigations concerning thin films fracture initiation during this test are introduced. The damage initiation of thin hard coatings deposited on smooth surfaces is mainly induced by fatigue mechanisms and characterized by the removal of film segments in the contact area between the ball indenter and the specimen in highly superficially loaded contact regions during the impact test. Herein the currently used coating stress determination methodologies, when applied, lead in most cases to sufficiently accurate results. Superficial PVD layers deposited on a basic coating do not affect the occurring stress field practically and thus no differences concerning the fracture initiation in the basic coating are encountered. When the surface roughness increases, localized stress concentrations throughout the contact area occur, resulting in film failures in randomly distributed spots, at predictable high impact loads. In cases of relatively soft PVD films or at elevated substrate roughness, the abrasive wear contribution to the coating damage initiation has to be considered. In such cases, as will be explained, the first film fracture appears in a region, close to the middle of the contact radius.

Computer simulation study of the atomistic mechanism of deformation and fracture initiation in thin fcc metal films

A molecular dynamics computer simulation was performed using a potential of embedded atom method (EAM) of copper so as to study the atomistic mechanism of plastic deformation in thin metal films. When a thin film of Cu crystal is elongated by more than 8%, small islands of surplus atoms start to nucleate between (111) planes. The formation of new (111) islands occurs by the movement of atoms along the 〈111〉 direction from two successive (111) planes. Multiplication of these (111) planes serves as the mechanism of elongation, releasing the accumulated elastic stress. Vacancies and small clusters, thereof, are left behind at the positions from which atoms have migrated and also at the part of newly nucleated (111) planes in which atoms were not filled completely. Stacking fault tetrahedra (sft) are nucleated directly in the deformed region as a result of movement of atoms along 〈111〉 on the tetrahedral (111) cap.

Failure analysis of gas turbine last stage bucket made of Udimet 500 superalloy

ABSTRACT This study presents the failure analysis of the 37.5 MW gas turbine third stage buckets made of Udimet 500 (U-500) superalloy. The buckets experienced repetitive integral tip shroud fractures assisted by a low temperature (type II) hot corrosion. A detailed analysis of all elements which had an influence on the failure initiation was carried out, namely: the commensurate loss of a load bearing cross-sectional area of the bucket tip shroud by the conversion of metal to the corrosion product (scale) and related increase in stress; the influence of the tip shroud microstructure, e.g. a presence of equiaxed and columnar grains, their distribution and orientation; evidence of the transgranular fracture initiation and intergranular creep mechanism fracture propagation. Finally, the most probable mechanism for bucket damage, conclusions and recommendations to prevent failures of the buckets are presented.

The effect of constraint-induced normal stress on the failure of notched TiAl components

Titanium aluminides are candidates for replacing nickel superalloys in some aircraft engine components. In uniaxial tension tests, these materials experience plastic strains at failure that place them in-between traditional definitions for ductile and brittle materials. This study considers the appropriate continuum mechanics failure criterion for these materials under multiaxial loading conditions (based on either the maximum equivalent plastic strain or the maximum normal stress). The material tested is a Ti-47.0Al-2.0Cr-1.9Nb alloy having a predominantly lamellar γ/α 2 microstructure. Cylindrical notched tensile specimens that experience elevated normal stresses in their interiors due to circumferential constraint during plastic deformation have been investigated. Results are presented that quantify reductions in failure loads due to elevated normal stress, compared to those predicted by finite element models using a maximum equivalent plastic strain criterion. To properly interpret the experimental results, the effects of notch strengthening must be included in the model predictions. Model and experimental results suggest that this TiAl alloy has some sensitivity to normal stress and that a combined failure criterion is needed to accurately predict failure under multiaxial loading conditions. A fracture initiation and failure mechanism requiring a combination of normal stress and plastic straining is suggested that is consistent with observed features at fracture initiation sites.