Fracture Stage Research Articles

Numerical simulation on the initiation and propagation of synchronous perforating fractures in horizontal well clusters

Horizontal well perforation is one of the main factors affecting formation fractures around the wellbore and subsequent hydraulic fractures extending along the predetermined path. In this paper, the RFPA2D-Flow software is used to conduct a numerical simulation study on the initiation and propagation of parallel perforation fractures in the perforation cluster. Then influence of different perforation density and different stress on the initiation and propagation of perforation fractures are studied. The results show that there is a competitive relationship between initiation and propagation of perforated fractures. Three parameters, the initial initiation rate and the length of the hydraulic fracture, and the generated fracture number, are put forward as the indexes to measure the competitive relationship. Perforated fractures will eventually form a fracture propagation mode dominated by several main fractures and supplemented by micro fractures. Then it shows that an obvious spatial form associated with asymmetric main fractures and micro fractures derived around them. Parallel perforation fracture initiation presents four modes of competitive initiation. With the increase of perforation density, the fracture initiation rate of parallel perforation in the initial stage of fracturing decreases gradually, the number and the complexity of fractures also increase gradually; with the continuous increase of the stress difference, the initiation rate at the initial stage of hydraulic fracturing and the number of the main formed fractures and derived micro fractures all show a decreasing trend.

Optimization design of horizontal well fracture stage placement in shale gas reservoirs based on an efficient variable-fidelity surrogate model and intelligent algorithm

Hydraulic fracturing technique increases shale gas well productivity significantly. Horizontal well fracture optimization has been studied by many researchers worldwide in the past decade. However, most of these researches relied on computationally expensive numerical simulation models for objectives evaluation during the optimization process. This affects the optimization efficiency significantly. To address this issue, surrogate model methods which adapt a simple approximate model are employed to lessen the computational burden. In this study therefore, a novel intelligent variable-fidelity radial basis function (VFRBF) surrogate-assisted model for multi-objective fracture stage placement optimization, namely VFRBF-FSO, is proposed to reduce the computational burden of the numerical simulation-based production optimization. In the VFBRF-FSO method, low-fidelity (LF) and high-fidelity (HF) samples were adopted simultaneously to establish the variable-fidelity (VF) surrogate model. To the best of our knowledge, this is the first time that variable-fidelity model is used for shale gas horizontal well fracture stage placement optimization. The uniqueness of this proposed method is that a scaling factor and an augment matrix are used to integrate the LF and HF samples to increase the accuracy of the surrogate model. Moreover, two cases with different wells and well types were studied to illustrate the effectiveness and accuracy of the VFRBF-FSO method. The optimization results showed that the VFRBF-FSO method performed comparably with the HF model-based method in terms of convergence and diversity. However, the VFRBF-FSO reduced the simulation runs on the two cases with different wells and fracture types to about five times that of the HF model.

Efficiency of High-Resolution MRI at Different Stages of Subchondral Insufficiency Fracture of the Knee

The history of subchondral insufficiency fracture is closely related to a pre-existing diagnosis of spontaneous osteonecrosis of the knee (SONK). Previously, it was thought that subchondral linear or lunate pathological changes on magnetic resonance imaging (MRI) scans in elderly patients with a history of osteoporosis are the result of spontaneous osteonecrosis, but in the 2000s, studies by T. Yamamoto showed that a small proportion of patients with osteonecrosis of the femoral head initially have a fracture of insufficiency that gets complicated by secondary osteonecrosis. Subsequent studies of SONK also showed that the subchondral insufficiency fracture is precisely the initial process, osteonecrosis is secondary and it is a complication of the fracture. The aim of our study was to evaluate the effectiveness of high-resolution MRI at different stages of this disease.

An experimental study on examining the outcome of including surfactant to cement spacer design on mud removal efficiency and bonding of cement with formation

Effective spacer design is a key factor in cement's ability to produce a strong bond between the cement-casing and the cement-formation. Mud displacement, mud cake removal, and cement contamination with drilling mud are the different applications of the spacer. Furthermore applications include changing the wettability of the formation and casing, particularly when drilling is carried out with oil-based mud and the production zone is expected to be oil-wet. Oil wet formations can limit cement bonding with the formation, resulting in pressure transmission between hydraulic fracture stages and lack of zonal isolation.This paper investigates how adding surfactants to cement spacer design affects cement bond performance under reservoir temperatures, where the surfactants will help to mitigate the impact of oil base mud on cement performance, reduce drilling mud compatibility issues, change formation – casing wettability from oil-wet to water-wet, and reduce interfacial tension (IFT) between the mud and the cement.The investigation was conducted using Wolfcamp core samples. To simulate the oil well, the core samples were chopped into small pieces and aged in oil-based mud (8.8 ppg and 12.5 ppg) at reservoir temperature for 10 days. The wettability of the chip was then determined using the captive bubble method. The chips were then submerged in a spacer mixture. The experiment was carried out using three different surfactants (Anionic, Nonionic, and Zwitterionic surfactant). The wettability of the chips was tested again after they were exposed to the aforementioned spacers to see how surfactants affected the wettability. The interfacial tension (IFT) between the spacer and the crude oil was also measured. To investigate the mixability issue caused by the presence of surfactants in the spacer design, oil-based mud and spacer compatibility tests were done. In addition, beaker and rotor cleaning experiments were used to assess the efficacy of various surfactants in removing mud.

Compression Fractures and Partial Phenotype Rescue With a Low Phosphorus Diet in the Chihuahua Zebrafish Osteogenesis Imperfecta Model.

Osteogenesis imperfecta (OI) is a group of heritable disorders affecting bone and other connective tissues. Dominant OI forms are mainly caused by mutations in collagen type I. Patients suffer from skeletal deformities, fractures of long bones and vertebral compression fractures from early childhood onward. Altered collagen structure and excess mineralisation are the main causes for the bone phenotype. The Chihuahua (Chi/+) zebrafish has become an important model for OI. Given that reduced dietary phosphorus (P) intake reduces the bone mineral content and promotes bone matrix formation in teleosts, including zebrafish, we tested whether a low dietary P (LP) intake mitigates the OI phenotype in the Chi/+ model. To answer this question, we characterised the Chi/+ vertebral column phenotype at a morphological, cellular and subcellular level. We present the first description of vertebral compression fractures in Chi/+ and assess the effects of LP diet on the Chi/+ phenotype (Chi/+LP). Compared to untreated Chi/+, two months of LP dietary treatment decreases vertebral deformities in the abdominal region and reduces shape variation of caudal vertebral bodies to a condition more similar to wild type (WT). At the histological level, the osteoid layer, covering the bone at the vertebral body endplates in WT zebrafish, is absent in Chi/+, but it is partially restored with the LP diet. Whole mount-stained specimens and histological sections show various stages of vertebral compression fractures in Chi/+ and Chi/+LP animals. Both Chi/+ and Chi/+LP show abundant osteoclast activity compared to WT. Finally, the ultrastructure analysis of WT, Chi/+ and Chi/+LP shows Chi/+ and Chi/+LP osteoblasts with enlarged endoplasmic reticulum cisternae and a high protein content, consistent with intracellular retention of mutated collagen. Nevertheless, the secreted collagen in Chi/+LP appears better organised concerning fibre periodicity compared to Chi/+. Our findings suggest that a reduced mineral content of Chi/+ bone could explain the lower frequency of vertebral column deformities and the restored shape of the vertebral bodies in Chi/+LP animals. This, together with the improved quality of the bone extracellular matrix, suggests that two months of reduced dietary P intake can alleviate the severe bone phenotype in Chi/+ zebrafish.

Effects of nanoparticle surfactants on droplet splitting in a T-junction microchannel

<p indent="0mm">Droplet microfluidics can generate and manipulate discrete droplets through immiscible multiphase flows inside microchannels, which effectively controls the size and generation rate of droplets. Due to its remarkable advantages, droplet microfluidics bears a significant value in an extremely wide range of areas, such as bio(chemical) analysis, particle preparation or petroleum engineering. Droplet splitting is an important form of obtaining monodisperse and small droplets in microfluidic devices, which can obtain smaller droplets to meet the requirements of biochemical reaction and material encapsulation. Nanoparticle surfactants, formed at liquid−liquid interfaces by the interactions between functional groups on nanoparticles and polymers having complementary end-functionality, are recently proposed as an excellent interface stabilizer to cover liquid droplets for applications of substance encapsulation and delivery. Apparently, understanding droplet splitting behavior with consideration of nanoparticle surfactants is of great significance for obtaining smaller and better dispersive functional droplets. In this paper, by using the approach of microfluidic experiment and theoretical analysis, the effects of nanoparticle surfactants on droplet splitting behavior within T-junction microchannels were investigated. Comparisons were conducted among clean droplets and those stabilized by nanoparticles alone, surface-active polymers alone and nanoparticle surfactants, respectively. Besides, the effects of concentrations of both nanoparticles and amino-terminal polymers, which determined nanoparticle surfactants concentration on the fluid-fluid interface, on the droplet splitting behavior have been systematically investigated. We have found that the droplet splitting process can be divided into three stages: Entering stage, squeezing stage and post-splitting stage. The generation of nanoparticle surfactants can effectively reduce the time required for droplet splitting. Besides, the nanoparticle surfactants induced reduction of the total split time is mainly attributed to the decrease of the entering time, while no significant change is presented in the squeezing and post-splitting time. The dynamical behavior of droplets splitting under different reagent conditions can be divided into four categories: Blocking splitting, transition state, non-blocking splitting and non-splitting. The blocking splitting state refers to that the droplets are always in contact with the channel wall during the splitting process. Otherwise, there is always a gap between the droplet and the channel wall, which is called non-blocking splitting. Besides, the transition state is a special state of non-blocking splitting, which is in contact with the upper wall of the channel and has a gap with the bottom of the channel. More importantly, by analyzing the transient decreasing of neck width, we found that the thinning rate of the droplet was suddenly accelerated in the process of neck thinning, i.e., the rapid fracture stage. During neck thinning, the depression is fanned out until it reaches a critical width and becomes funnel-shaped. By geometric analysis of neck shape, a theoretical model of neck width thinning was obtained, which is applicable to different reagent conditions. Besides, the influence mechanism of nanoparticle surfactants on droplet splitting was obtained, i.e., affecting necking rate via decreasing the interfacial tension. Finally, via analyzing the droplet splitting state, a theoretical formulation was established for predicting the transition of blocking splitting and non-blocking splitting, which shows a good agreement with the experimental data. These findings provide a theoretical basis for the preparation of smaller functional droplets by droplet splitting.

Shale well factory model reviewed: Eagle Ford case study

This case study highlights that shale wells drilled with close well spacing in the same landing zone in the same reservoir rock, using the same fracture treatment plan and parameters, commonly show large variances in well productivity. Searching for the root cause(s) of this variance in performance, we conclude that the factory model -- which assumes wells can be engineered in reproducible and identical ways using the same treatment plans -- is not yet feasible in practice. A thorough analysis of production data from wells completed in the Eagle Ford Formation shows variations in cumulative oil production cannot be attributed to lateral length, the number of hydraulic fracture stages or fluid/proppant load due to the almost constant values used in keeping with the “Factory Model” drilling method. Based on the dataset available, other parameters for the discrepancies in production volumes are proposed. Some parameters such as (1) reservoir quality and (2) well production control have been determined to have very low impact on production rates variability, while others such as (3) production timing, and (4) water cut may have a more moderate effect. The last defined parameters of (5) fracture quality/performance, and (6) well spacing are determined to have the largest impact on the cumulative production and were concluded to have the highest probability of being the main drivers behind the variations in well productivity. Use of production data with the Complex Analysis Method (CAM) to produce spatial drainage plots also provides insight into recovery factors from these wells. CAM results reveal relatively small, drained rock volumes (DRV) around the modeled hydraulic fractures, which is typical for unconventional shale wells.

Evolution mechanism of optical fiber strain induced by multi-fracture growth during fracturing in horizontal wells

A forward model for optical fiber strain was established based on a planar 3D multi-fracture model. Then the forward method calculating distributed fiber strain induced by multi-fracture growth was proposed. Based on this method, fiber strain evolution during fracturing of the horizontal well was numerically simulated. Fiber strain evolution induced by fracture growth can be divided into three stages: strain increasing, shrinkage convergence, and straight-line convergence, whereas the evolution of fiber strain rate has four stages: strain rate increasing, shrinkage convergence, straight-line convergence, and strain rate reversal after pumping stops. Fiber strain does not flip after pumping stop, while the strain rate flips after pumping stop so that strain rate can reflect injection dynamics. The time when the fracture extends to the fiber and inter-well pressure channeling can be identified by the straight-line convergence band of distributed fiber strain or strain rate, and the non-uniform growth of multiple fractures can be evaluated by using the instants of fractures reaching the fiber monitoring well. When the horizontal section of the fiber monitoring well is within the height range of a hydraulic fracture, the instant of the fracture reaching the fiber can be identified; otherwise, the converging band is not apparent. In multi-stage fracturing, under the influence of stress shadow from previous fracturing stages, the tensile region of fiber strain may not appear, but the fiber strain rate can effectively show the fracture growth behavior in each stage. The evolution law of fiber strain rate in single-stage fracturing can be applied to multi-stage fracturing.

Efficient Prediction of Proppant Placement along a Horizontal Fracturing Stage for Perforation Design Optimization

Summary Multistage plug and perforation (plug-n-perf) fracturing is commonly used for horizontal well completion in unconventional reservoirs. Uniform distribution of proppant across all clusters in each stage has proved to be challenging with low viscosity slickwater owing to its limited transport capability. Computational fluid dynamics (CFD) has been used to model proppant transport in wellbore to improve perforation and fracturing design for achieving uniform proppant placement. However, traditional CFD modeling of a full-scale stage is computationally expensive, which limits its applicability in the completion design optimization. A new approach was developed in this paper to efficiently predict proppant placement along a multicluster stage based on a machine learning (ML) model trained with extensive CFD modeling results. Its high computational efficiency permits quick sensitivity analyses to optimize perforation and fracturing designs. The new approach was validated against full-stage CFD modeling results as well as post-treatment field diagnostics. Sensitivity analyses show that proppant inertia effect is a key factor affecting proppant placement in heel clusters with higher slurry flow rates, allowing more proppant carried to the toe owing to its higher density in comparison with fluid. Proppant settling allows bottom perforations to accept more proppant than top perforations. This gravitational effect is not negligible near the heel at high flow rates and becomes more dominant near toe clusters where the flow rate is reduced. Near-uniform proppant placement is achievable via perforation design optimization by taking advantage of these two key mechanisms controlling proppant transport in horizontal wellbores. It is demonstrated that in-line perforating designs with all perforations having the same orientation in each cluster or the entire stage, especially with perforations at the bottom or on the side of the wellbore, improve the proppant placement uniformity. However, it is recommended that the optimum perforation design should be identified case by case depending on specific input parameters. The ML-based model developed in this study has overcome some of the limitations from existing models in the literature and is able to provide quick and yet reliable solutions to proppant placement prediction and design optimization.

Characteristics of Acoustic Emission Waveforms Induced by Hydraulic Fracturing of Coal under True Triaxial Stress in a Laboratory-Scale Experiment

Hydraulic fracturing (HF) is an effective technology to prevent and control coal dynamic disaster. The process of coal hydraulic fracturing (HF) induces a large number of microseismic/acoustic emission (MS/AE) waveforms. Understanding the characteristic of AE waveforms’ parameters is essential for evaluating the fracturing effect and optimizing the HF strategy in coal formation. In this study, laboratory hydraulic fracturing under true triaxial stress was performed on a cubic coal sample combined with AE monitoring. The injection pressure curve and temporal variation of AE waveforms’ parameters in different stages were analyzed in detail. The experimental results show that the characteristics of the AE waveforms’ parameters well reflect the HF growth behavior in coal. The majority of AE waveforms’ dominant frequency is distributed between 145 and 160 kHz during HF. The sharp decrease of the injection pressure curve and the sharp increase of the AE waveforms’ amplitude show that the fracture already runs through the coal sample during the initial fracture stage. The “trapezoidal” rise pattern of cumulative energy and most AE waveforms with low amplitude may indicate the stage of liquid storage space expansion. The largest proportion of AE waveforms’ energy and higher overall level of AE waveforms’ amplitude occur during the secondary fracture stage, which indicates the most severe degree of coal fracture and complex activity of internal fracture. The phenomenon shows the difference in fracture mechanism between the initial and secondary fracture stage. We propose a window-number index of AE waveforms for better response to hydraulic fracture, which can improve the accuracy of the HF process division.

Investigation of Flowback Behaviours in Hydraulically Fractured Shale Gas Well Based on Physical Driven Method

Due to the complex microscope pore structure of shale, large-scale hydraulic fracturing is required to achieve effective development, resulting in a very complicated fracturing fluid flowback characteristics. The flowback volume is time-dependent, whereas other relevant parameters, such as the permeability, porosity, and fracture half-length, are static. Thus, it is very difficult to build an end-to-end model to predict the time-dependent flowback curves using static parameters from a machine learning perspective. In order to simplify the time-dependent flowback curve into simple parameters and serve as the target parameter of big data analysis and flowback influencing factor analysis, this paper abstracted the flowback curve into two characteristic parameters, the daily flowback volume coefficient and the flowback decreasing coefficient, based on the analytical solution of the seepage equation of multistage fractured horizontal Wells. Taking the dynamic flowback data of 214 shale gas horizontal wells in Weiyuan shale gas block as a study case, the characteristic parameters of the flowback curves were obtained by exponential curve fittings. The analysis results showed that there is a positive correlation between the characteristic parameters which present the characteristics of right-skewed distribution. The calculation formula of the characteristic flowback coefficient representing the flowback potential was established. The correlations between characteristic flowback coefficient and geological and engineering parameters of 214 horizontal wells were studied by spearman correlation coefficient analysis method. The results showed that the characteristic flowback coefficient has a negative correlation with the thickness × drilling length of the high-quality reservoir, the fracturing stage interval, the number of fracturing stages, and the brittle minerals content. Through the method established in this paper, the shale gas flowback curve containing complex flow mechanism can be abstracted into simple characteristic parameters and characteristic coefficients, and the relationship between static data and dynamic data is established, which can help to establish a machine learning method for predicting the flowback curve of shale gas horizontal wells.

Developmental characteristics and formational stages of natural fractures in the Wufeng - Longmaxi Formation in the Sangzhi Block, Hunan Province, China: Insights from fracture cements and fluid inclusions studies

The developmental characteristics and formational stages of natural fractures are of great significance for shale oil and gas accumulation in complex structural areas. We determined the developmental characteristics and formational stages of fractures in the Wufeng - Longmaxi Formation in the Sangzhi Block, Hunan Province, China based on outcrops, cores, optical microscopy, cathodoluminescence, scanning electron microscopy, energy spectrum analysis, fluid inclusion homogenization temperature, micro laser Raman spectrum analysis, and acoustic emission analysis. Furthermore, we deduced the possible fracture formational depth, filling time, and stress magnitude of structural fractures in rocks, and established the development model of structural fractures of time series. The results show that the Wufeng - Longmaxi Formation fractures can be divided into tensile fractures, compression-torsion fractures, and strike-slip fractures in terms of the combination characteristics of fracture plane and the roughness of fracture plane. The fractures mainly occurred in the late Yanshanian - Himalayanian period after the violent uplift. In the process of burial-hydrocarbon generation, it experienced at least six stages of repeated process of splitting, healing, re-splitting, which reflected the periodic release of local stress caused by the continuous westward push of Xuefeng uplift. The microcracks of different periods have obvious differences in orthogonal polarization and cathodoluminescence reflection characteristics. The cathodoluminescence spectrum ranges from orange red to bright yellow, and some even do not glow, which is a comprehensive performance of mineral filling composition, crystal size, crystallization degree and alteration degree. The gas generated in the early stage are easy to enter the early opened fractures. With the continuous opening and healing, the shale gas has a continuous dynamic process of enrichment, dispersion, re-enrichment, and re-dispersion, which affects the distribution and preservation of shale gas.

Кинетика поврежденности предварительно циклированной малоуглеродистой стали с крупнозернистой и субмикрокристаллической структурой

The kinetics of accumulation of microcracks and fracture at various stages of tension of grade 20 steel specimens with coarse-grained and ultrafine-grained (UFG) structure obtained by the equal-channel angular pressing (ECAP) has been studied. To study the effect of mechanical degradation, a part of the samples was subjected to preliminary cycling to a relative lifetime of 50 %, followed by tensile tests, during which the parameters of acoustic emission, deformation fields obtained by digital image correlation were evaluated, and the intensity of the residual magnetic field was measured. The length and density of surface microcracks were measured by optical microscopy using computer image analysis. It is established that pre-cycling causes hardening of the material and a decrease in its plasticity. The fracture stages were revealed, and fatigue crack origins were found on the internal delaminations of specimens with a UFG structure after preliminary cyclic loading. It is shown that the formation of the UFG structure after the ECAP of specimens made of grade 20 steel leads to hardening, reduction of the fracture work, the area of the plastic zone, maximum major deformations and damage, as well as to an increase in the number of AE signals and the intesity of the residual magnetic field.

СРАВНИТЕЛЬНЫЙ АНАЛИЗ 1: НАЧАЛЬНАЯ СТАДИЯ ДИНАМИЧЕСКОГО ДЕФОРМИРОВАНИЯ И РАЗРУШЕНИЯ АЛЮМИНИЕВЫХ СПЛАВОВ 1561 И 1565

Transition of dynamically deformed solids into structure-unstable state is analyzed theoretically and experimentally from the physical kinetics approach. Propagation of shock wave in heterogeneous medium is considered to be in the form of superposition of two modes: (i) propagation of plane front with the mean velocity u(x,t) and (ii) random motions of local regions of medium because of chaotic stress fields. resulting from interaction of shock wave with the heterogeneous medium. This interaction results in particle velocity distribution, the quantitative characteristic of which is the particle velocity dispersion. Transition from one scale of dynamic deformation to another is found to be the so-called defect (decrease) of the particle velocity which characterizes the intensity of energy and momentum exchange between scale levels. Both characteristics are registered in real time in the process of shock-wave tests by using precise interferometric technique. Two materials, 1561 and 1565 aluminum alloys, were tested under uniaxial strain conditions within impact velocity range of 250–750 m/s. Owing to fine focusing of laser beam of interferometer up to 50–70 µm, the registered dynamic strength (spall strength) of material corresponds to single mesoscale structural element, which allows the incipient stage of dynamic fracture to be revealed in detail. The incipient strength is found to be maximum when velocity variation becomes equal to the velocity defect.

Failure analysis of porous segmented aluminа by digital image correlation method

The behavior of porous, segmented, 50% porosity aluminа under uniaxial compression loading has been studied using digital image correlation (DIC) method. The main stages of fracture were high-lighted as well as qualitative and quantitative characteristics given. In the process of compressing the sample, a series of images of the material surface was obtained. The use of the DIC technique made it possible to obtain a series of displacement vector fields, and the corresponding spatial distributions of deformation were transformed into pseudo-images. The use of the DIC algorithm made it possible to obtain the fields of deformation vectors on their basis.

Pre-folding fracturing in a foredeep environment: insights from the Carseolani Mountains (central Apennines, Italy)

AbstractBedding-perpendicular joints striking parallel (longitudinal) and perpendicular (transverse) to both the axis of the hosting anticline and the trend of the foredeep-belt system are widely recognized in fold-and-thrust belts. Their occurrence has been commonly attributed to folding-related processes, such as syn-folding outer-arc extension, although they can also be consistent with a pre-folding foredeep-related fracturing stage. Here we report the pre-folding fracture pattern affecting the Pietrasecca Anticline, in the central Apennines (Italy), resolved by a detailed field structural analysis. Field observations, scan-lines and interpretation of virtual outcrops were used to study the intensity, distribution and the orientations of fracture pattern along the anticline. The fracture pattern of the Pietrasecca Anticline consists of longitudinal and transverse joints, oriented approximately perpendicular to bedding, and of a pre-folding longitudinal pressure-solution cleavage set, which is oblique to bedding regardless of the bedding dip. Cross-cutting relationships show that joints predated the development of the pressure-solution cleavage. Furthermore, joint intensity does not relate to the structural position along the anticline. Taken together, these observations suggest that jointing occurred in a foredeep environment before the Pietrasecca Anticline growth. Our work further demonstrates that joints striking parallel and orthogonal to the main fold axis do not necessarily represent syn-folding deformation structures.

Stratigraphically Controlled Stress Variations at the Hydraulic Fracture Test Site-1 in the Midland Basin, TX

We investigated the relationship between stratigraphy, stress, and microseismicity at the Hydraulic Fracture Test Site-1. The site comprises two sets of horizontal wells in the Wolfcamp shale and a deviated well drilled after hydraulic fracturing. Regional stresses indicate normal/strike-slip faulting with E-W compression. Stress measurements in vertical and horizontal wells show that the minimum principal stress varies with depth. Strata with high clay and organic content show high values of the least compressive stress, consistent with the theory of viscous stress relaxation. By integrating data from core, logs, and the hydraulic fracturing stages, we constructed a stress profile for the Wolfcamp sequence, which predicts how much pressure is required for hydraulic fracture growth. We applied the results to fracture orientation data from image logs to determine the population of pre-existing faults that are expected to slip during stimulation. We also determined microseismic focal plane mechanisms and found slip on steeply dipping planes striking NW, consistent with the orientations of potentially active faults predicted by the stress model. This case study represents a general approach for integrating stress measurements and rock properties to predict hydraulic fracture growth and the characteristics of injection-induced microseismicity.

An analytical model to predict the impact of a bullet on ultra-high molecular weight polyethylene composite laminates

High-performance fiber reinforced polymers are widely used in military and aerospace applications and easily subjected to high-velocity bullet impacts. Despite the abundantly available experimental and numerical studies related to energy absorption mechanisms of laminates under impact loading, measuring levels of the absorbed energy from different failure modes of laminates is a challenging task. Acquiring this important information using simplified analytical models is vital to evaluate the bulletproof ability of laminates and effectively optimize its structures. The present work develops an analytical model to predict the ballistic limit of a laminate and the energy absorption mechanism is considered from matrix crush, laminate shear, laminate compressive, fiber stretch, fiber break and delamination. The penetration process of a bullet striking laminates is divided into four sequential stages, including crush stage, compression-shear stage, stretch-shear stage, and fiber fracture and delamination stage. The analytical model predicts the ballistic limit, energy absorbed by each failure mode, and time histories of velocity and acceleration. The results are validated with experimental data and computational results, and good correlations are found for ballistic limits and thickness of the laminate, diameters of the deformation cone and thickness, ballistic limits, and non-dimension masses as well as energy absorptions and thicknesses.

Coupling Model of Stress-Damage-Seepage and Its Application to Static Blasting Technology in Coal Mine.

Most coal mine field application processes are carried out using empirical formulas because of the insufficient understanding of the fracture development law of the static blasting technology. This lack of understanding results in poor coal seam gas extraction. In this study, a stress–damage coupling model was established to investigate the construction parameters of the static blasting technology using COMSOL simulation software. Then, a stress–damage–seepage coupling model was designed to study the evolution of the fracture field (seepage field) during the static blasting process using realistic failure process analysis simulation software. Finally, the influencing factors and fracturing effects were analyzed comprehensively. The research results show the following. (1) Comparing the simulation results with previous field tests reveals that the seepage law of the numerical simulation of the static blasting technology is consistent with the field test results, verifying the rationality of the stress–damage–seepage coupling model. (2) The development of coal seam fractures is affected by the expansion pressure, elastic modulus, and guide hole arrangement; the guide hole arrangement can play a role in guiding the development direction of fractures and enhancing the effect of fracturing. (3) The coal body mainly experienced the following five stages of fracturing: coal body compaction, microdamage formation, microfracture formation, large fracture formation, and fracture propagation. In addition, because of the rapid release of soundless cracking agents during the large fracture formation stage, the gas flow decreased in a short time. (4) The static blasting technology causes the coal seam permeability coefficient to increase. Compared with conventional extraction, the effective influence radius in the horizontal direction increases by 5.1 times, and the effective influence radius in the vertical direction increases by approximately 3 times. The static blasting technology can increase the number of coal seam fractures and significantly reduce the coal seam gas pressure, thereby enhancing coal seam permeability and realizing safe coal mining.

Numerical simulation of the non‐Newtonian fracturing fluid influences on the fracture propagation

AbstractHydraulic fracturing fracture propagation is the main factor affecting the fracturing effect. In view of the more diversified selection of fracturing fluid, the problem of fracture propagation in non‐Newtonian fluid fracturing is often encountered in fracturing. In this paper, the influencing factors of fracture propagation of non‐Newtonian fluid fracturing fluid are analyzed. Based on PKN model, a three‐dimensional fracture propagation model considering the rheology and filtration of fracturing fluid is established, the better fracture shape simulation results are given, and the effects of fluid properties, rock physical properties, and injection parameters on fracture propagation are analyzed in detail. The research shows that (1) in the process of fracture propagation, the fracture length and width gradually increase, and the corresponding pressure in the fracture also increases. The overall increase trend is that the growth is rapid in the initial stage of fracturing and gradually slows down in the later stage. (2) The greater the consistency coefficient and rheological index, the greater the fracture width and pressure in the fracture, and the easier it is to form wide fracture. The larger the filtration coefficient is, the weaker the fracture forming ability is, and the fracture width, length, and pressure in the fracture are reduced. (3) Rock physical properties have a certain influence on fracture propagation, Young's modulus has a great influence on fracture propagation, while Poisson's ratio has little influence on it. However, the larger the Young's modulus, the more difficult the fracture is to expand, and the smaller the corresponding fracture width. (4) The larger the pump injection rate, the larger the fracture size and the higher the pressure in the fracture. The model can provide a theoretical basis for fracturing design and fracture shape prediction.