Location Of Fracture Initiation Research Articles

Real-time monitoring of rock fracture by true triaxial test using fiber-optic strain monitoring in adjacent wells

The real-time monitoring of fracture propagation during hydraulic fracturing is crucial for obtaining a deeper understanding of fracture morphology and optimizing hydraulic fracture designs. Accurate measurements of key fracture parameters, such as the fracture height and width, are particularly important to ensure efficient oilfield development and precise fracture diagnosis. This study utilized the optical frequency domain reflectometer (OFDR) technique in physical simulation experiments to monitor fractures during indoor true triaxial hydraulic fracturing experiments. The results indicate that the distributed fiber optic strain monitoring technology can efficiently capture the initiation and expansion of fractures. In horizontal well monitoring, the fiber strain waterfall plot can be used to interpret the fracture width, initiation location, and expansion speed. The fiber response can be divided into three stages: strain contraction convergence, strain band formation, and postshutdown strain rate reversal. When the fracture does not contact the fiber, a dual peak strain phenomenon occurs in the fiber and gradually converges as the fracture approaches. During vertical well monitoring in adjacent wells, within the effective monitoring range of the fiber, the axial strain produced by the fiber can represent the fracture height with an accuracy of 95.6% relative to the actual fracture height. This study provides a new perspective on real-time fracture monitoring. The response patterns of fiber-induced strain due to fractures can help us better understand and assess the dynamic fracture behavior, offering significant value for the optimization of oilfield development and fracture diagnostic techniques.

Investigation on the fracture mechanics characteristics and crack initiation of deep dense shale

The mechanical behavior of shale fractures significantly affects the structure and stability of hydraulic fracturing networks in shale gas reservoirs. Currently, there is insufficient research on the mechanical characteristics of fractures, crack initiation, and the prediction of fracture trajectories in deep dense shale. This study involved conducting direct tension, shear, and three-point bending fracture mechanical experiments on shale samples, complemented by numerical calculations and mechanistic analyses. The results show a significant influence of the bedding plane angle (γ) on the initiation properties and locations of shale fractures. With γ ranging from 0° to 90°, shale fractures transition from alignment with the bedding plane to penetration through it. Fracture initiation locations show a significant degree of randomness when fractures propagate along the bedding plane. The horizontal tensile stress changes with increasing γ, initially increasing and then decreasing as it moves away from the center. The evolution of the horizontal tensile stress field primarily causes alterations in shale fracture paths, reaching its peak value at γ = 45°. Shale initiation fractures can be classified into four types: tensile fractures through the bedding plane, tensile fractures along the bedding plane, shear fractures across the bedding plane, and shear fractures along the bedding plane. Established a criterion called the “maximum stress strength ratio” to determine fracture initiation, which can predict the initiation properties, locations, and paths of shale fractures. When γ is small, fractures mainly propagate along the bedding plane, with a higher likelihood of initiation occurring at the center of the specimen. Conversely, when γ is large, fractures mainly propagate through the bedding plane. The spacing of the bedding planes minimally affects the predicted fracture paths of shale. This study is of significant theoretical importance for reservoir stimulation and stability assessment in deep dense shale gas reservoirs.

Fabric controls on fracture surface roughness of an architected rock material

Fluid flow through fractures is intimately linked to the fracture surfaces that define the void geometry through which fluids flow. Thus, an understanding of what controls fracture surface roughness is essential to the development of models for predicting fluid transport through fractured rock. The difficulty in predicting surface roughness arises from the complexity of rock which is inherently heterogeneous and nonuniform in composition, fabric, and structural components, even when samples are acquired from the same rock mass. Here, a benchmarked-simulation approach motivated from geo-architected 3D printed synthetic gypsum rocks is used to provide insight into the competing contributions from fabric and layering on fracture roughness formation. Simulation results from a discrete element model (Particle Flow Code, Itasca Consulting Group, Inc.) clearly indicate that the relative orientation between mineral layers and in-layer mineral fabric, and the variability in mineral bonding strengths determine whether anisotropic corrugated surfaces or isotropic surfaces are formed. Weak mineral layers oriented perpendicular to the applied load resulted in strong roughness anisotropy. Peak failure loads were found to vary up to 30% depending on the strength of the mineral fabric at the location of fracture initiation, which provides insight into the observed high variability in strength values of natural rock. The uniqueness of induced fracture roughness and peak failure load is intimately linked to layering, mineral fabric, and their distribution in the rock. These findings have important implications for any architected material fabricated through serial printing of layers with local compositional heterogeneity.

Blowout-Capping-Fracturing-Relief Well: A Full Cycle Workflow

Summary Failed well-capping attempts following offshore-well blowouts undergoing worst case discharge (WCD) can lead to fluid-driven tensile failures (de facto hydraulic fracture initiations). Subsequent propagation of these fractures may lead to broaching of overburden-rock-formation layers and even the seafloor, providing pathways for reservoir hydrocarbons to escape into the seawater. After capping stack shut-in, the pressure buildup in the fluid column inside the wellbore exposes vulnerable locations to tensile (Mode I) failure. If this shut-in wellbore pressure exceeds the formation breakdown pressure (FBP) in any of the exposed-rock-formation layers, a fracture will initiate and will continue to propagate as long as sufficient energy is provided by the reservoir. Scenarios where the propagating fracture(s) broached the seafloor in the past led to severe environmental impacts, disturbing the local ecology. The quintessential example is Union Oil’s 1969 “A-21 Well” blowout in California’s Santa Barbara Channel, where subsequent well-capping attempts led to multiple broaching instances on the seafloor near the well with thousands of hydrocarbon gallons gushing into the seawater (observable from the sea surface as “oil boilups”). In this paper, numerical modeling is performed on a hypothetical case study using deepwater Gulf of Mexico parameters in order to evaluate the likelihood of a similar scenario by modeling a planar-fracture propagation longitudinally-to-the-wellbore, upon well capping. A workflow is developed that integrates post-blowout WCD flowrate and volume estimations, fracture initiation and propagation modeling following the capping stack shut-in, pertaining to a “cap-and-contain” strategy (including predictions in regard to seafloor broaching), along with relief-well intersection followed by kill-weight-mud injection. The casing-shoe depth is the presumed location of fracture initiation, assuming perfect integrity of all casedhole sections above it. Several sensitivity analyses are performed to investigate the impact of the casing-shoe depth, along with the stiffness of the overburden-rock-formation layer, and the post-blowout-discharge duration on the resultant fracture growth. Finally, the mud density and pump flowrate necessary to compensate the oil column to successfully kill the blown well are quantitatively assessed.

Experimental and Numerical Fracture Characterization of DP1180 Steel in Combined Simple Shear and Uniaxial Tension

Current tests for plane stress characterization of fracture in automotive sheet metals include simple shear, uniaxial, plane strain, and biaxial tension, but there is a significant gap between shear and uniaxial tension. Presently, it remains uncertain whether the fracture strain experiences a reduction between simple shear and uniaxial tension or undergoes an exponential increase as the triaxiality decreases. Fracture in combined simple shear and tension is complicated by premature edge cracking in tension along with a strong sensitivity of fracture strain to the measurement lengthscale. To address these issues, several existing simple shear geometries were modified and evaluated, with a focus on obtaining approximately linear strain paths corresponding to combined uniaxial tension and simple shear suitable for experimental fracture characterization using digital image correlation (DIC). An experimental and numerical investigation was conducted using two planar geometries that do not require through-thickness machining and can be easily tested on a universal test frame. Finite-element analysis was used to investigate the influence of the notch eccentricity on the stress state and predicted fracture location. The most promising geometry in each coupon type was then selected and tested for a dual-phase advanced high-strength steel, DP1180. The performance of the two planar geometries was evaluated based on the linearity of strain and stress state, along with the location of fracture initiation. The best geometry was then used to evaluate and recalibrate the modified Mohr-Coulomb (MMC) fracture locus with data in combined shear and tension. The initial MMC calibration using four fracture tests that suppressed necking provided an accurate estimate for the fracture strain in combined uniaxial tension and simple shear. The MMC model correctly predicted a valley in the fracture strain between these two loading conditions.

Capturing fracture initiation and crack propagation of car windshields

To facilitate the development of accurate material models for glass, there is a need for reliable experimental procedures that capture the stochastic nature of the material. The experimental setup must be easy to model numerically and output relevant data for validation of important material characteristics. We conducted an experimental study on three types of windshields exposed to quasi-static loading. A robust and repeatable testing procedure was designed with the aim of revealing the stochastic behavior of windshields. 3D-Digital Image Correlation (3D-DIC) was used to monitor the windshield displacement field and we were able to capture fracture initiation locations and crack propagation data by using high-speed cameras combined with a pose estimation-based post-processing tool implemented in Python. We performed finite element (FE) simulations in LS-DYNA to estimate major principal stress values at first fracture. The results from the experimental study provided new insight and valuable data for the development of numerical models of the problem at hand.

Morphological characterization and reconstruction of fractured heat-treated glass

Fracture morphology has insightful information related to the residual effect of fractured structural glass, which is vital in assessing the post-fracture performance of glass members. This study experimentally characterized the fracture morphology of heat-treated glass and developed a novel method of morphology reconstruction, which aims to facilitate the numerical analysis of fractured structural glass. With the development of a computer-vision-based method for transparent objects, the morphology information from fragmentation tests was extracted and systematically investigated for monolithic heat-treated glass with various thicknesses, surface compressive stresses and fracture initiation locations, which are considered as the key influencing factors of heat-treated glass fracture. The geometrical features of fragments and their spatial distribution were quantitatively analysed, identifying their correlations with glass properties. The result indicates that the distribution of fragment centroids shows greater dispersion as the tempering level increases, and the fragments tend to be smaller and more rounded. The strain energy release at fracture was also assessed by fracture patterns, showing it presents high sensitivity to the glass thickness and surface compressive stress. Subsequently, a novel approach was proposed for the stochastic reconstruction of fracture morphology, combining feature points distribution and Voronoi tessellation concept. The control parameters are determined by data from the fragmentation tests and the influence of fracture load could be properly considered. The proposed method shows satisfactory outcomes and good agreement with the experimental records, which has further potential in developing refined numerical models by considering more realistic fracture morphology of glass members.

Initiation Pressure and Location of Fracture Initiation in Elliptical Wellbores

Initiation Pressure and Location of Fracture Initiation in Elliptical Wellbores

A numerical study on the tensile splitting of concrete with digital image processing

Numerical simulations are performed to investigate the mechanical behavior of concrete in uniaxial compression test and tensile splitting test. Especially, the digital image processing (DIP) techniques are utilized to differentiate the interface types on the fracture surface of the specimen that is already split, which involves transforming the color image of the fracture surface into a binary image through graying, binarization, and opening operation processing. The boundary data obtained from the binary image is further processed by mirrored comparison and contour simplification, and is finally imported into the numerical model to reconstruct the different interface regions on the splitting surface. Different material properties are assigned to the corresponding interface regions through intensive programming with Python, whereby the influence of heterogeneity on the splitting process is analyzed through the established numerical model. Through comparison with homogeneous model, it is found that the heterogeneous model is capable of reproducing a more genuine fracturing process, with respect to the fracture initiation locations and splitting sequence of different interfaces. The proposed modeling schemes are valuable for potential researches exploring the failure process of the materials such as concretes and rocks whose mechanical behavior is influenced by their inherent heterogeneity.

Study on Applicability of Phase Field Method in Quasi-Static Fracture Behavior Simulation of PBX Components

Crack in polymer bonded explosive (PBX) components will influence the safety and reliability of weapon system. In order to accurately simulate the quasi-static fracture behavior of PBX components, the quasi-static arc Brazilian experiment of PBX explosive was carried out first. Then, the phase field method (PFM) and the traditional extended finite element method (XFEM) are used for the comparative simulation of the arc Brazil test process, in which both the maximum principal stress and the maximum principal strain initiation criterion are used in XFEM simulation. The calculation results show that the relative error of failure load calculated by PFM is small (less than 1%), while the error of XFEM calculation is large (the error of strain criterion and stress criterion are -61% and 42% respectively). Meanwhile, the fracture initiation locations calculated by PFM are located at 1/4 and 3/4 away from the end face of the Brazilian sample, which is in good agreement with the experimental results. In addition, PFM avoids the problems of crack mutation and discontinuity in XFEM. The results show that the PFM can accurately calculate the quasi-static fracture behavior of PBX components.

Evaluation of ductile fracture criteria in combination with a homogenous polynomial yield function for edge splitting damage of DP steels

Abstract This study investigates the formability characteristics of dual phase steels (DP600 and DP800) under flange stretching conditions through hole expansion tests. The hole-splitting initiation was numerically predicted using ductile damage functions coupled with an orthotropic plasticity model. Therefore, a polynomial yield criterion coupled with three damage criteria, namely generalized plastic work, void growth model, and a shear ductile fracture model, is implemented into Marc software by the user defined material subroutine. Thus, the fracture stroke, hole expansion ratio, and fracture initiation location were estimated for both steels. The polynomial yield criterion could capture the anisotropic features of the dual phase steels. Furthermore, the stress triaxiality-based criteria were reasonably accurate in stretching limit predictions of both steels’ grades. Nevertheless, plastic work predicted the fracture strokes and hole expansion ratios noticeably lower than the experimental outcomes for both steels. In addition, all the numerical results captured the exact fracture initiation location for DP600, while a slight discrepancy was observed for DP800. All ductile fracture models pointed out the identical crack location, which shows the cruciality of the yield criterion for locating the fracture initiation in hole expansion test. Consequently, both void growth model and shear ductile fracture model showed accurate performances conforming to the stress triaxiality was found to be more dominant than the Lode parameter.

Micromechanics-based simulation of quasi-static and dynamic crushing of double-chamber 6000-series aluminium profiles

Numerical simulation is becoming an increasingly important tool in the development of crash components in the automotive industry. With accurate numerical predictions of deformation and fracture, developers may be able to optimize and utilize the energy absorption capacity of the material to its full potential. This study aims to predict the crash behaviour of such components from previous experiments by Qvale et al. (2021) using explicit finite element simulations with a fine discretization of solid elements and the Gurson–Tvergaard–Needleman porous plasticity model. How well the experimental behaviour was predicted by the simulations was evaluated through comparison to computed tomography (CT) scans throughout the entire volume of the profiles. Despite the pragmatic approach of calibrating some parameters of the constitutive model to only uniaxial tension tests, and obtaining the remaining parameters from the literature, important features, such as local details of the folding pattern and the locations of fracture initiation, were represented remarkably well, while there is still room for improvement in the dynamic simulations, as the changes from quasi-static to dynamic axial crushing were slightly underestimated.

Multifracture initiation from a series of noncircular convex wellbores

In this study, a numerical model is proposed based on the dual toughness and strength criterion for fracture initiation to determine the fracture initiation location and initiation pressure. In the model, hydraulic fractures emanate from a noncircular convex wellbore, which is considered as a jointed cross between an ellipse and a circle. Numerical studies were performed to explore the behaviors associated with fracture initiation under different stress combinations. The results indicate that the initiation pressure is sensitive to the geometric parameters of the wellbore. The initiation location is mainly controlled by the wellbore shape under a near-equal stress state characterized by the ratio of deviation (S) to mean (P) stresses (S/P < 0.10); if S/P > 0.50, the initiation location is dominated by the stress state. For an intermediate stress state (0.10 ≤ S/P ≤ 0.50), the initiation location is affected by both the stress state and wellbore shape; multiple fractures are easier to initiate in the presence of a noncircular wellbore and tend to develop into an X shape. In addition, X-shaped fractures prefer externally expanded wellbores. The subsequent propagations of the X-shaped fractures were also modeled, and these fractures exhibited a strong near-well tortuosity.

Bending fatigue strength of aerospace quality gear steels at ambient and elevated temperatures

Gear bending fatigue testing was conducted on six commercial aerospace quality steels, in the carburised, shot-peened and super finished condition, establishing S-N curves at ambient and elevated temperatures, providing evidence of higher than predicted strength established in accordance with ISO 6336:2006. At elevated temperatures, a systematic drop in mean strength was experienced for all candidate steels. Detailed fractography established the fracture initiation site, unique to each steel, illustrating a propensity for surface initiation in the finite life region and subsurface beyond the knee point, whilst certain grades displayed a distinct change in the location of fracture initiation at elevated temperatures.

Evaluation of cellular structures with triply periodic minimal surfaces fabricated by additive manufacturing

Cellular structures are formed by struts and edges, which makes them difficult to fabricate by conventional technologies. The emergence of 3D printing has enabled the production of such complex structures. In the present work, the Fused Filament Fabrication method was used to obtain cellular structures formed by the repetition of triply periodic minimal surfaces unit cells. Due to the complexity of the cells, several defects occurred until the procedure was fine-tuned. Failure analysis of samples was performed after bending and compression tests. Failure occurs at the same locations of the samples independently of their relative density, for the three densities tested. These fracture initiation locations correspond to the zones where the von Mises stress is highest.

Optimization of Indirect Fracturing Process Parameters Based on Mechanical Properties of Fractured and Low-Permeability Coal

The coal-bed methane (CBM) resources in soft and low-permeability coals are assumed to be as much as 15 × 1012 m3 in China. Indirect fracturing technology can be an effective method to successfully extract methane within soft coals. The key to the success of this technique is to optimize the parameters, such as water injection flow rate and fracture initiation location, so that the hydraulic fracturing parameters enable the fractures to pass through the interface between coal and rock and propagate sufficiently into the coal. This paper focuses on solving the above problems by focusing on discontinuities and plastic characteristics of soft coals. Voronoi polyhedron was used to simulate the discontinuities of coal, and the constitutive relations of ductile fracture-seepage and elastoplastic damage-seepage are, respectively, given to the discontinuities and coal matrix. A numerical model was established based on the above theory to simulate the effect of stress difference Δσ, coal-rock interface friction coefficient fc,r, water injection flow rate i w , and distance between the well and the interface Dop on indirect fracturing fractures. The results show that the HFs area in the coal is positively correlated with Δσ, fc,r, and i w , and it first increases and then decreases with the decrease of Dop. The above results were applied in the Zhaozhuang mine of Qinshui Basin by optimizing Dop = 1 m and iw = 8 m3/min, so that CBM production has been greatly increased. The results can provide theoretical support for the efficient development of CBM in fractured and low-permeability coal seam areas.

Synthesis, characterization, and modelling the behavior of in-situ ZrO2 nanoparticles dispersed epoxy nanocomposite

The cured epoxy has poor toughness owing to highly cross-linked nature. The present work reports inclusion of ZrO2 nanoparticles with epoxy via ultrasonic mixing process for improving the toughness. The developed nanocomposites are characterized using mechanical tests, fractographic analysis and dynamic mechanical thermal analysis (DMTA). The mode-1 fracture toughness (K1C), fracture energy (G1C), storage modulus (E') and glass transition temperature (Tg) for different filler concentration are evaluated. A theoretical explanation for improved ZrO2-epoxy nanocomposite behaviour is reported by analyzing the fracture parameters and morphology of fractured samples. A micromechanical finite element (FE) model is constituted for predicting macroscopic elastic properties of composites using numerical homogenization scheme. The model predicted location of fracture initiation under triaxial and uniaxial loading conditions. The SEM micrographs and modelling results collectively explained the fracture mechanism. The experimentation and modelling results are compared.

Study on temperature distribution prediction of horizontal wells during fracturing treatment

In recent years, fiber-optic distributed temperature sensors (DTS) have been widely used for the dynamic monitoring of horizontal well during fracturing treatments. It is proposed to address the frequent issues of unknown artificial fracture initiation location, unclear crack extension pattern, unknown fracturing fluid injection success, and difficult fracturing effect evaluation in the hydraulic fracturing of horizontal wells. The temperature prediction model is the basis for fracturing diagnosis based on DTS monitoring, however, the quantitative prediction of temperature distribution throughout the horizontal well fracturing process remains a challenge. In this paper, a set of temperature-distribution prediction model for multi-stage fracturing in horizontal wells is established, with various micro-heat effects taken into account, and the models are solved through coupling. The temperature distribution during multi-cluster fracturing, and the temperature distribution characteristics during staged fracturing of horizontal wells have been analyzed. Moreover, the influence of fracturing fluid displacement, formation filtration coefficient, fracture width, and height on temperature distribution has also been clarified. The findings of this study provide theoretical support for fracturing performance diagnosis, fracture initiation identification, and analysis of the whereabouts of fracturing fluids using DTS monitoring. It has a great significance on fracturing effect evaluation and fracturing design optimization for horizontal wells.

COUPLING A GEOMECHANICAL RESERVOIR AND FRACTURING SIMULATOR WITH A WELLBORE MODEL FOR HORIZONTAL INJECTION WELLS

Reservoir cooling during waterflooding or waste-water injection results in thermo-poro-elastic stresses that can significantly alter the reservoir stress tensor field. In addition, colloidal particles in the injected water can filter on the borehole and fracture surfaces resulting in matrix permeability reduction. Fractures are likely to initiate and propagate from injectors because of these thermal and filtration effects. These fractures are of great concern for both environmental reasons and their strong influence on reservoir sweep and oil recovery. In this paper, a fully coupled reservoir-fracture-wellbore model was developed. Fluid flow, solid mechanics, energy balance, fracture initiation and propagation, and particle filtration are modeled in the reservoir, fracture, and wellbore domains. The coupled non-linear systems of equations are solved implicitly using the Newton-Raphson method. Simulation results show that water quality and thermal effects control fluid leak-off and fracture growth. While it is difficult to predict the exact location of fracture initiation due to reservoir heterogeneity, we propose a reasonable method to handle fracture initiation and growth without a predefined fracture location. In open-hole completions, "thief" fractures may propagate deep into the reservoir. Thermal stress changes in the injection zone are shown to be significant because of the combined effects of forced convection, heat conduction, and poro-elasticity. Accurate predictions of thermal stress in different reservoir layers allow us to conduct numerical studies on fracture height growth and containment. We show that controlling the injection water temperature and the water quality are an effective way to ensure fracture containment.

Fracture prediction in transverse fillet welded joints of high strength structural steel

Due to the huge property difference between high strength structural (HSS) steels and mild steels after welding, mechanical behavior and failure mode of fillet welded joints of HSS steels should be studied further. Numerical simulations can provide increased insight into this topic. This paper concerns finite element simulations of component tests of two types transverse fillet welded joints of HSS steels: lap-welded transverse fillet welds and cruciform type transverse fillet welds. In order to investigate the influences of triaxial stress states on fracture ductility of weld metals, a series of tests on specimens with various initial geometries are carried out. By comparing three typical uncoupled fracture models (stress triaxiality-dependent ductile fracture model (VGM), Lode angle-dependent ductile fracture model (MSSM) and combination of stress triaxiality and Lode angle ductile fracture model (BWM)), BWM gives precious prediction of ductile fracture of weld metals. The behaviors in terms of load-displacement and failure modes of transverse fillet welds are all well captured by the numerical simulations with calibrated ductile fracture models. Specifically, the location of fracture initiation and propagation of cracks obtained from simulations are both identical with real responses.