Conventional Fracture Research Articles

Aggregate size effects on fracture behavior of concrete SCB specimens

Fracture in concrete and rock-like materials is the result of applying external loads, initiation, and propagation of the cracks, and finally, creating the failure surface in the material. The most common type of fractures in solid materials are usually formed under combined tensile/shear loading conditions. The stress intensity factors KI and KII, corresponding to opening and sliding modes (I and II), are the main fracture parameters, and their critical values (i.e., fracture toughness) show the material's resistance against the initiation and propagation of cracks. In this paper, the fracture toughness of concrete specimens is determined by considering different aggregate sizes and using the three-point bending test on semi-circular bend (SCB) specimen. The fracture toughness parameters have been determined by different fracture criteria and compared with the values obtained in the laboratory on concrete specimens. The effect of aggregate size on fracture parameters and crack propagation path has been evaluated. To predict thecrack initiation angle and fracture toughness values, themodified maximum tangential stress (MMTS) and extended maximum tangential strain (EMTSN) criteria along with conventional fracture criteriawere used. The results showed, that with the increase in aggregate size, the fracture toughness value under mode I, II, and mixed-mode (I-II) has increased. The laboratory results showed that specimens with aggregate sizes of 0–19 mm have the highest fracture load, and specimens with the aggregate sizes of 0–5 mm have the lowest fracture load

The fracture toughness of Schwarz Primitive triply periodic minimal surface lattice

Triply periodic minimal surface (TPMS) lattices have recently been shown to exhibit extraordinarily low densities and multi-functionalities. Nevertheless, the dependence of their fracture toughness KIc on relative density ρ¯, unit cell size l and the number of unit cells ahead of the crack tip suggest a lack of understanding of the governing parameters. This also extends to the adequacy of using the current conventional fracture testing protocols. Therefore, the objective of this work is threefold: (i) extract the minimum Schwarz Primitive unit cells for accurate toughness measurement, (ii) estimate KIc of polylactic acid Schwarz Primitive TPMS applying linear elastic fracture mechanics (LEFM) conditions, (iii) investigate the effect of relative density ρ¯ and unit cell size l on the fracture toughness. With 18 cells along the specimen width, KIc is shown to increase linearly with ρ¯ and with the square root of l. Outcomes from this work is aimed toward understanding the damage tolerance of TPMS lattices, and hence the reliability of such new emerging lightweight materials.

Comparative study on hydraulic fracturing using different discrete fracture network modeling: Insight from homogeneous to heterogeneity reservoirs

Conventional discrete fracture network (DFN) modeling is used to simulate hydraulic fracture propagation in fractured low permeability reservoirs. However, natural fractures have a certain degree of roughness, and reservoir rocks have significant heterogeneity characteristics. Therefore, it is necessary to compare the differences in hydraulic fracturing simulation of homogeneous and heterogeneous reservoirs caused by conventional and rough DFN modeling methods. Herein, new-generation algorithms for different DFNs are proposed. Combined the combined finite-discrete element method and different DFNs, four different simulation models are established, covering hydraulic fracturing models from relatively homogeneous to heterogeneous reservoirs. Then, the differences from different DFN modeling methods are compared and discussed. The results show that the relatively homogeneous and heterogeneous models established by conventional DFN may underestimate the bending propagation characteristics and fracturing time and overestimate the proportion of tensile failure. Meanwhile, when the nonuniform distribution of the mechanical parameters of rock blocks is considered, the bending propagation becomes more marked; thus, the fracturing time and the proportion of shear failure are further increased. The above results imply that to accurately predict the hydraulic fracture propagation in a real reservoir, it is vital to consider the roughness of real natural fractures and the nonuniform distribution of mechanical parameters in reservoir hydraulic fracturing modeling.

A microstructure-sensitive analytical solution for short fatigue crack growth rate in metallic materials

Short fatigue crack growth in engineering alloys is among the most prominent challenges in mechanics of materials. Owing to its microstructural sensitivity, advanced and computationally expensive numerical methods are required to solve for crack growth rate. A novel mechanistic analytical model is presented, which adopts a stored energy density fracture criterion. Full-field implementation of the model in polycrystalline materials is achieved using a crystallographic crack-path prediction method based on a local stress intensity factor term. The model is applied to a range of Zircaloy-4 microstructures and demonstrates strong agreement with experimental rates and crack paths. Growth rate fluctuations across individual grains and substantial texture sensitivity are captured using the model. More broadly, this work demonstrates the benefits of mechanistic analytical modelling over conventional fracture mechanics and recent numerical approaches for accurate material performance predictions and design. Additionally, it offers a significant computer processing time reduction compared with state-of-the-art numerical methods.

Determination of immune factor levels in serum and local hematoma samples of osteoporotic fracture patients and clinical study of the effect of active vitamin D3 treatment on immune factor levels

ObjectiveThe aim of this study was to investigate changes in systemic and local immune factors, namely, interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α, in patients with and without osteoporotic fractures and to explore the effects of active vitamin D3 treatment on immune function and fracture prognosis in patients with osteoporotic fractures.MethodThe mRNA expression levels of IL-1β, IL-6 and TNF-α were measured before the operation. After the operation, the patients in the control group were treated with conventional fracture treatment and calcium supplementation, and the patients in the treatment group were treated with calcium plus active vitamin D3 in addition to conventional fracture treatment. The serum of each patient was collected on the seventh day after the operation.ResultsThe expression levels of the three immune factors (IL-1β, IL-6 and TNF-α) in the fracture end hematoma samples were significantly positively correlated with those in the serum samples (P < 0.05). The mean values of the serums of IL-1β, IL-6 and TNF-α in the osteoporosis group were significantly higher than those in the non-osteoporosis group (P < 0.05). The average number of hematomas in the osteoporosis group was significantly higher than that in the non-osteoporosis group (P < 0.05). The results for the active vitamin D3 treatment group were significantly lower than those for the control group (P < 0.05). The mean wrist function score of the active vitamin D3 treatment group was significantly better than that of the control group (P < 0.05). The average fracture healing time of the treatment group was significantly shorter than that of the control group (P < 0.05).ConclusionThe relative expression of IL-1β, IL-6, and TNF-α in the fracture end hematoma samples was positively correlated with the corresponding levels of these immune factors in the serum samples. The levels of IL-1β, IL-6 and TNF-α in the serum and fracture end hematoma samples of the osteoporotic fracture patients were higher than those of the non-osteoporotic fracture patients. Active vitamin D3 treatment promoted fracture healing by affecting the levels of these immune factors.

Stage-by-Stage Hydraulic Fracture and Reservoir Characterization through Integration of Post-Fracture Pressure Decay Analysis and the Flowback Diagnostic Fracture Injection Test Method

Summary The post-fracture pressure decay (PFPD) technique is a low-cost method allowing for stage-by-stage hydraulic fracture characterization. The analysis of the PFPD data is complex, with data affected by both hydraulic fracture and reservoir properties. Available analysis methods in the literature are oversimplified; reservoir or fracture properties are often assumed to be constant along the horizontal well, and therefore changes in the trend of pressure decay data are attributed to hydraulic fracture or reservoir properties only. Moreover, methods analogous to those applied to the analysis of conventional diagnostic fracture injection tests (DFITs) are often used and ignore critical mechanisms involved in main-stage hydraulic fracture stimulation. A conceptual numerical simulation study was first conducted herein to understand the key mechanisms involved in main-stage hydraulic fracturing. An analytical model was then developed to account for the dynamic behavior of the hydraulic fracture, leakoff, proppant distribution, multiple fractures, and propped- and unpropped-closure events. The analytical model is cast in the form of a new straightline analysis (SLA) method that provides stage-by-stage estimates of the ratio of unpropped fracture surface area to total fracture surface area. The SLA method was validated against numerical simulation results. Moreover, to account for the variation of reservoir properties along the horizontal well, the PFPD model is integrated with DFIT-flowback (DFIT-FBA) tests, performed at some points along the lateral, to obtain a reliable stage-by-stage hydraulic fracture and reservoir characterization approach. The practical application of the proposed integrated approach was demonstrated using PFPD and DFIT-FBA data from a horizontal well completed in 22 stages in the Montney Formation. The numerical simulation study demonstrated that the use of proppant and injection into multiple clusters (creating multiple fractures) results in multiple closure events. The closure process may start early after the pump-in period at a pressure significantly higher than the minimum in-situ stress. Using DFIT-based analytical models, which ignore the presence of proppant, causes significant errors in hydraulic fracture and reservoir property estimation. The PFPD field data examined herein exhibited a similar pressure trend to the numerical simulation cases. The ratio of unpropped fracture surface area to total fracture surface area was determined stage by stage using the PFPD SLA method, constrained by DFIT-FBA data. Engineers can use this information to optimize the hydraulic fracture stimulation design in real time, optimize the well spacing, and forecast the production. The cost and time advantages of this diagnostic method make this approach very attractive.

Evaluation of Parent Well Depletion Effects on Fracture Geometry Based on Low-Frequency Distributed Acoustic Sensing in Hydraulic Fracture Test Site-2

Summary Low-frequency distributed acoustic sensing (LF-DAS) has recently received much attention for its ability to monitor fracture propagation at offset wells. Hydraulic Fracture Test Site-2 (HFTS-2), a Department of Energy–sponsored field-based research experiment, has acquired LF-DAS data sets during the stimulation of many horizontal wells in the Wolfcamp Formation of the Permian Basin. Over 100 stimulated stages with different completion designs in four horizontal wells were monitored by two horizontal offset wells and one vertical pilot hole with permanent fibers. The parent well depletion affected all four horizontal wells in almost half of their lateral section. Several studies have been performed on the acquired comprehensive data set. In this study, we apply our novel Green’s function-based inversion algorithm to calculate the fracture width of each stage and investigate the impact of parent well depletion on fracture geometry. The Green’s function-based inversion algorithm has been successfully applied to a few stages of field case studies. The Green’s function matrix was built based on linear elasticity theory (3D displacement discontinuity method) to relate the fracture openings with strain responses measured along the length of the fiber. This novel algorithm only relies on measured strain to solve fracture geometry at the monitoring well. Therefore, it is independent of the physics of the fracture propagation process and can be used to validate hydraulic fracture modeling results. Using our inversion algorithm, we can efficiently and quantitatively interpret LF-DAS data to provide information on fracture geometry and completion efficiency. We analyze more than 100 stages of the LF-DAS measurements obtained at the fiber wells B3H and B4H during B1H, B2H, and B4H stimulations. We apply our inversion algorithm for four data sets covering the stimulation of the abovementioned three wells. The fracture growth at stages in the parent well’s depleted zone is biased more toward upper formation and in the easterly direction. The fracture width at the stages in the parent well’s depletion zone is reduced compared to fracture widths at stages in the nondepleted zone irrespective of the monitoring well location relative to the treatment wells. This difference in fracture widths will affect the proppant distribution to a great extent, thereby affecting the effectiveness of the stimulation. We also illustrate the application of the inversion algorithm for stages that have both conventional fracture hits with “heart-shaped” signals as well as fracture reopening signals. Our inversion algorithm gives a reasonable estimate of the fracture width that aligns with the qualitative analysis of microseismic data sets and statistic summary of fracture hit numbers of HFTS-2. We believe that this quantitative study provides us with insights into the fracture geometry due to parent well depletion effects.

Retrospective Analysis On Changing Norms in Zygomatic Maxillary Complex Fractures.

The zygomaticomaxillary complex fracture is unique and difficult to treat fracture, mostly because of its pentapod anatomic form which may necessitate a patient-specific treatment approach. This retrospective study aims at evaluating the changing trends in ZMC fractures. A total of 245 cases were included in this retrospective study, treated either surgically or conservatively, for ZMC fractures in the time period of 3 years (2017-2019). All the patients were assessed and compared based on these parameters- gender, age, aetiology, anatomic site of the fracture and type of treatment given and associated maxillofacial fractures. Fracture aetiology was segregated into: motorised road traffic accidents, road traffic accidents under the influence of alcohol, interpersonal violence, domestic violence, sports injury and self falls. 94.3% of the 245 study participants were men, while 5.7% were women. The most common age group was 21 to 40 years (60.8%). In our study, the most common cause of Zygomatic fracture was road traffic accidents with or without the influence of alcohol (41.6%). ORIF - 3 point fixation (32.7%) was the most commonly used treatment, followed by 4 point fixation (27.3%). The Maxillary buttress region was the most frequent site of fracture (93.5%), followed by the other sites. Due to the shifting patterns of injuries, most surgeons no longer see conventional fracture lines. Instead, patients have unusual and hybrid fracture lines, which necessitate more fixations due to the injury's complexity. The tendency is now shifting toward tailoring treatment choices for individual patients.

New Pump-In Flowback Model Verification with In-Situ Strain Measurements and Numerical Simulation

This study presents an analytical model for estimating minimum horizontal stress in hydraulic fracturing stimulations. The conventional Diagnostic Fracture Injection Test (DFIT) is not practical in ultra-tight formations, leading to the need for pump-in/flowback tests. However, ambiguities in the results of these tests have limited their application. The proposed model is based on the linear diffusivity equation and material balance, which is analytically solved and verified using a commercially available numerical simulator. The model generates a linear graph in which the pressure drop and its derivative are plotted versus the developed solution time function. The closure pressure is determined when the slope of the derivative deviates from linearity. The model was applied to several cycles of field flowback tests and found to eliminate the ambiguity associated with identifying the fracture closure. Furthermore, the minimum In-situ stresses estimated using this approach are verified via downhole strain measurement and synthetic data from a fully 3D commercial fracturing simulator. The proposed technique outperformed other conventional methods in analyzing challenging injection/shut-in tests, showing improved results and reducing uncertainty in estimated fracture parameters. This model is expected to scale down the need for multiple field trials and provide a reliable estimation of minimum stress.

Cyclic adaptive cohesive zone model to simulate ductile crack propagation in steel structures due to ultra‐low cycle fatigue

AbstractMicromechanics‐based continuum damage criteria have previously been developed to simulate the initiation of ductile fracture in structural steels under conditions with large‐scale plasticity where conventional fracture mechanics indices are invalid. Such models have been combined with methods to simulate ductile crack growth for monotonic loading. In this study, a micromechanics‐based adaptive cohesive zone model for simulating ductile crack propagation under monotonic loading is extended to handle cyclic loading. The proposed model adaptively modifies the cohesive traction–separation relationship for crack opening and closure, as the loading reverses between tension and compression. The approach is implemented into the finite element analysis platform WARP3D, and results of simulations that use the model are compared with data from coupon‐scale tests. The results demonstrate that the proposed model can accurately simulate the effect of crack propagation on specimen response, as well as other key aspects of observed behavior, including crack face closure and crack tunneling.

Rescue of High Glucose Impairment of Cultured Human Osteoblasts Using Cinacalcet and Parathyroid Hormone

Patients with type 2 diabetes mellitus (T2DM) experience a higher risk of fractures despite paradoxically exhibiting normal to high bone mineral density (BMD). This has drawn into question the applicability to T2DM of conventional fracture reduction treatments that aim to retain BMD. In a primary human osteoblast culture system, high glucose levels (25 mM) impaired cell proliferation and matrix mineralization compared to physiological glucose levels (5 mM). Treatment with parathyroid hormone (PTH, 10 nM), a bone anabolic agent, and cinacalcet (CN, 1 µM), a calcimimetic able to target the Ca2+-sensing receptor (CaSR), were tested for their effects on proliferation and differentiation. Strikingly, CN+PTH co-treatment was shown to promote cell growth and matrix mineralization under both physiological and high glucose conditions. CN+PTH reduced apoptosis by 0.9-fold/0.4-fold as measured by Caspase-3 activity assay, increased alkaline phosphatase (ALP) expression by 1.5-fold/twofold, increased the ratio of nuclear factor κ-B ligand (RANKL) to osteoprotegerin (OPG) by 2.1-fold/1.6-fold, and increased CaSR expression by 1.7-fold/4.6-fold (physiological glucose/high glucose). Collectively, these findings indicate a potential for CN+PTH combination therapy as a method to ameliorate the negative impact of chronic high blood glucose on bone remodeling.

Constraint characterization and fracture initiation analysis for semi-elliptical surface cracks in reactor pressure vessel

The constraint and fracture initiation for semi-elliptical surface cracks in an actual large scale reactor pressure vessel (RPV) have been investigated by finite element analysis. The results show that the overall constraint level of the cracks increases when the crack depth a/t increases and crack aspect ratio a/c decreases. For most cracks with a/t = 0.25–0.8 and a/c = 0.2–2, the constraint level is higher than the standard plane strain specimen, and the conventional fracture assessment may possibly yield non-conservatism. The aspect ratio a/c has obvious effects on the distributions of the new unified constraint parameters Ad* and the elastic-plastic stress intensity factor KJ and the fracture initiation position along the crack fronts. For low aspect ratio cracks with a/c < 1.0, fracture initiation probably appears in the deepest region of the crack. For high aspect ratio cracks with a/c ≥ 1.0, fracture initiation is likely to occur in the near-surface region. To improve the accuracy of fracture assessment for cracks in the RPV, the constraint effect requires to be incorporated. The engineering estimation methods of the parameters Ad* and KJ at the crack initiation positions have been provided and verified.

USE OF 3D-PRINTED FRACTURE MODELS VERSUS SAWBONES FOR SURGICAL RESIDENT TRAINING: A FEASIBILITY STUDY

Conventional fracture courses utilise prefabricated sawbones that are not realistic or patient specific. The aim of this study is to determine the feasibility of creating 3D fracture models and utilising them in fracture courses to teach surgical technique.We selected an AO type 2R3C2 fracture that underwent open reduction internal fixation. De-identified CT scan images were converted to a stereolithography (STL) format. This was then processed using Computer Aided Design (CAD) to create a virtual 3D model. The model was 3D printed using a combination of standard thermoplastic polymer (STP) and a porous filler to create a realistic cortical and cancellous bone. A case-based sawbone workshop was organised for residents, unaccredited registrars, and orthopaedic trainees comparing the fracture model with a prefabricated T-split distal radius fracture. Pre-operative images aided discussion of fixation, and post-operative x-rays allowed comparison between the participants fixation. Participants were provided with identical reduction tools. We created a questionnaire for participants to rate their satisfaction and experience using a Likert scale.The 3D printed fracture model aided understanding and appreciation of the fracture pattern and key fragments amongst residents and unaccredited trainees. Real case-based models provided a superior learning experience and environment to aid teaching. The generic sawbone provided easier drilling and inserting of screws. Preliminary results show that the cost of 3D printing can be comparable to generic sawbones.It is feasible to create a fracture model with a real bone feel. Further research and development is required to determine the optimum material to use for a more realistic feel.The use of 3D printed fracture models is feasible and provides an alternative to generic sawbone fracture models in providing surgical training to residents.

The Initiation of Frictional Motion—The Nucleation Dynamics of Frictional Ruptures

AbstractFrictional interfaces lose stability via earthquake‐like ruptures, which are close analogues of shear cracks that are well‐described by fracture mechanics. Interface ruptures, however, need to be first formed—or nucleated. Rupture nucleation therefore determines the onset of friction, replacing the concept of a characteristic “static friction coefficient”. Utilizing rupture arrest at an imposed barrier, we experimentally determine nucleation locations, times and stresses at the origin of each subsequent rupture event. This enables us study the nucleation process via real‐time measurements of real contact area and local strain. Nucleation events initiate as 2D patches that expand at nearly constant velocities, vnuc, that are orders of magnitude lower than the dynamic rupture velocities described by conventional fracture mechanics. We find that: (a) Nucleation has location‐dependent stress thresholds, (b) vnuc is roughly proportional to the local stress level, (c) the nucleation process continues until the patch size reaches Ltran ∼ LG, the Griffith length for the onset of dynamic fracture (d) scaling time by τ = Ltran/vnuc, nucleation patches exhibit self‐similar dynamics (e) dynamic ruptures' cohesive zones are not fully established until significantly beyond Ltran. Many details of nucleation are governed by the local contact area topography, which is roughly invariant under successive rupture events in mature interfaces. Topography‐dependent details of the nucleation process include: precise nucleation site location, patch geometry, critical stress thresholds and the proportionality constant of vnuc with stress. We believe that these results shed considerable light on both how frictional motion is triggered and earthquake initiation.

Nonlocal analysis of edge‐cracked nanobeams under Mode I and Mixed‐Mode (I + II) static loading

AbstractIn the present paper, the behavior of an edge‐cracked nanobeam under both Mode I and Mixed‐Mode (I + II) static bending loading is analyzed by using the stress‐driven nonlocal model (SDM), being the SDM applied for the first time in the case of Mixed‐Mode loading. More precisely, the cracked nanobeam is modeled by using a modification of the classical cracked‐beam theory, consisting in dividing the beam into two beam segments connected through a massless elastic rotational spring located at the cracked cross section. The bending stiffness of the cracked cross section is computed by exploiting: the Griffith energy criterion and the conventional linear elastic fracture mechanics. Cracks under both Mode I and Mixed‐Mode (I + II) loading are considered. Finally, an edge‐cracked cantilever nanobeam subjected to a transversal point load at the free end is analyzed. The static deflection is calculated and discussed for different values of relative crack depth, crack location, and crack orientation.

A New Technique for Estimating Stress from Fracture Injection Tests Using Continuous Wavelet Transform

The diagnostic fracture injection test (DFIT) is widely used to obtain the fracture closure pressure, reservoir permeability, and reservoir pressure. Conventional methods for analyzing DFIT are based on the assumption that a vertical well is drilled in ultra-low permeability reservoirs with potential multiple closures but fails to consider horizontal wells. There is still significant debate about the rigorousness and validity of these techniques due to the complexity of the hydraulic fracture opening and closure process and assumptions of conventional fracture detection methods. The paper introduces a new method for detecting fracture closure pressure using the continuous wavelet transform (CWT). The new method aims to decompose the pressure fall-off signal into multiple levels with different frequencies using the CWT. This “short wavy” function is stretched or compressed and placed at many positions along the signal to be analyzed. The wavelet then convoluted the signal yielding a wavelet coefficient value. The signal energy is observed during the fracture closure process (pressure fall-off) and the fracture closure event is identified when the signal energy stabilizes to a minimum level. A predefined simple commercial fracture simulation case with known fracture closure, flow regime modeling, and actual field cases was used to validate the new methodology.

A comprehensive study on the influence of T-stress term on crack initiation angle in orthotropic materials based on the modification of MTS, MSS, SER and SED criteria

Theoretical predictions using conventional fracture criteria indicate that the theoretical approaches are not compatible with the experimental data. The most important reasons for the inconsistency between theoretical criteria and experimental data are lack of attention to accurate prediction of the crack initiation angle in fracture phenomenon and the role of T-stress term in the theoretical analysis. In available theoretical criteria, the fracture behavior of orthotropic materials was studied roughly considering crack initiation angle in the fiber direction. In this way, the accurate prediction of this angle has a significant impact on the extraction of the fracture limit curve consistent with experimental data. In the presented work, a comprehensive study on the role of the T-stress term on the fracture assessment of orthotropic materials based on the maximum strain energy release rate, minimum strain energy density, maximum tangential stress, and maximum shear stress criteria is performed. These criteria can characterize the mechanism of crack initiation under mixed-mode I/II loading and consider the effect of the T-stress term on the crack initiation angle. The formulations of the presented criteria for orthotropic materials are developed using the orthotropic stress field. Based on these criteria, the crack initiation angle is extracted in each mode mixity and the effect of the T-stress term on crack initiation angle is studied. A new approach is used to derive the T-stress term. The results indicate that by crossing pure mode I and approaching pure mode II, the T-stress term has a remarkable influence on the crack initiation angle and fracture assessment of orthotropic materials. Also, based on these criteria, FLC’s are extracted for orthotropic materials and the influence of this stress on crack behavior is evaluated. To verify the proposed formulations of these criteria, the fracture behavior based on the presented criteria is compared with the available experimental data and the accuracy of proposed theoretical approaches is discussed. It reveals that the presented criteria reliably estimate crack initiation angle and the crack behavior more accurately than the conventional fracture criteria for orthotropic materials.

Fracture behavior of slag &amp; fly ash-based geopolymer concrete in comparison with opc-based conventional concrete, including the effect of steel and hybrid fibers

The study presents an experimental investigation of the fracture behavior of hardened slag and fly ash-based alkali-activated normal and high-strength geopolymer concrete compared with conventional Ordinary Portland Cement (O.P.C.) based concrete with steel and hybrid fibers. The fracture parameters considered in the experimental investigation include fracture energy, stress intensity factor, energy release rate, and characteristic length. The study concludes that the observed differences in conventional and geopolymer concrete's fracture and mechanical performance agree with the microstructural differences between these concrete systems reported in past literature. The slag-based geopolymer concrete is marginally inferior to the O.P.C.-based concrete, with similar compressive strength in fracture performance. Also, hybrid fiber reinforcement improves the fracture performance of geopolymer concrete more than steel fiber alone. Contrary to geopolymer concrete, steel fiber reinforced conventional concrete is superior to hybrid fiber reinforced conventional concrete in terms of fracture behavior.

Study on the method of identifying fractures by ant tracking technique

Fracture is an effective reservoir space and important seepage channel for low permeability sandstone reservoirs, which is very important for reservoir exploration and development. Conventional fracture prediction methods have high cost, slow operation speed and low resolution, which limits the promotion of fracture identification methods. Therefore, this paper proposes a fracture prediction method based on ant tracking. This method is based on post-stack seismic data. Firstly, the seismic data is denoised and the fault continuity is enhanced by using structural smoothing and variance body attributes. On this basis, the active ant tracking is used to enhance the fault boundary of the data body. Then, the data body is subjected to multiple ant tracking calculations by optimizing parameters, and finally the suitable parameter values are determined to realize the prediction of natural fractures. The results show that the technology is used to predict natural fractures, and the prediction results have high resolution and reasonable fracture distribution.

Discrete fracture model with unsteady-state adsorption and water–gas flow for shale-gas reservoirs

A shale-gas reservoir possesses many complex characteristics, including coexisting free and adsorbed gases, a complex distribution of fractures, and the existence of movable water in these fractures. In this study, we improved the conventional discrete fracture model by considering viscous flow, diffusion flow, and slip flow to describe the transport of free gas in the matrix, the unsteady-state adsorption to describe the mass exchange between the free gas and adsorbed gas, and the gas–water two-phase flow to describe the fluid transport in fractures. We compared the proposed model with a previous discrete fracture model and found it to be credible. In addition, we discussed different influence factors on the results and found the following: (1) The net desorption rate was most dependent on fracture and development time, which had different influences on the net desorption rate; the existence of a fracture improved the net desorption rate, but this effect could be reduced along with the development time. (2) The adsorption and free-gas distribution were more related to surface diffusion and fracture, which showed different sensitivity to the influence of those two gases. The effect of surface diffusion on the distribution of adsorbed gas was more sensitive than that of free gas, whereas the influence of fractures was more sensitive to the distribution of free gas than adsorbed gas. Moreover, we used the proposed model to simulate a shale-gas reservoir developed by a horizontal well in the Ordos Basin in China. The study findings showed that if we used the model considering only single-gas transport in the fracture to simulate this reservoir, the average relative error could reach up to 135%, whereas if we used our model, the average relative error was only 1%, which indicated that our model was accurate for actual shale-gas reservoir.