Fracture Stage Research Articles

Non-extensive statistical analysis of acoustic emissions series recorded during the uniaxial compression of brittle rocks

Non-extensive statistical physics (NESP) provides the theoretical tool to investigate complex systems that appear in different length-scales and violate the classical Boltzmann–Gibbs statistics. In the present study, we apply the concept of NESP to analyze the Acoustic Emissions (AE) series recorded during uniaxial compression of brittle rocks, such as marble and sandstone specimens. In both types of rock samples, the cumulative distribution of inter-event times (interval times between successive AE hits) are well described by the q-exponential function. The variations of the q-index of Tsallis entropy and of the βq parameter with the applied stress follow distinct behavior in each type of rock, indicating in both cases the entrance of the system to the critical stage of fracture. The analysis of the experimental data suggests that NESP is suitable for describing AE activity in rock samples under compression, in a similar way to Earth’s seismic activity.

Pore Pressure Disturbance Induced by Multistage Hydraulic Fracturing in Shale Gas: Modelling and Field Application

Currently, there is no proper method to predict the pore pressure disturbance caused by multistage fracturing in shale gas, which has challenged drilling engineering in practice, especially for the infilling well drilling within/near the fractured zones. A numerical modelling method of pore pressure redistribution around the multistage fractured horizontal wellbore was put forward based on the theory of fluid transportation in porous media. The fracture network of each stage was represented by an elliptical zone with high permeability. Five stages of fracturing were modelled simultaneously to consider the interactions among fractures. The effects of formation permeability, fracturing fluid viscosity, and pressure within the fractures on the pore pressure disturbance were numerically investigated. Modelling results indicated that the pore pressure disturbance zone expands as the permeability and/or the differential pressure increases, while it decreases when the viscosity of the fracturing fluid increases. The pore pressure disturbance level becomes weaker from the fracture tip to the far field along the main-fracture propagation direction. The pore pressure disturbance contours obviously have larger slopes with the variation of permeability than those of the differential pressure. The distances between the pore pressure disturbance contours are smaller at lower permeability and higher viscosity. The modelling results of the updated pore pressure distribution are of great importance for safe drilling. A case study of three wells within one platform showed that the modelling method could provide a reliable estimation of the pore pressure disturbance area caused by multistage fracturing.

Fracability evaluation of lacustrine shale by integrating brittleness and fracture toughness

Rock brittleness and fracture toughness are important parameters for evaluating rock fracability. The stress-strain curves indicate that the lacustrine shale is strongly brittle. Brittle failure occurs rapidly when the stress of the lacustrine shale reaches its peak value. In addition, the lacustrine shale has different plastic strains before and after peak stress; this can relax the stress concentration of the crack tips. Therefore, the plastic strain that occurs before and after the peak stress can cause decreasing brittleness, which can be used to distinguish the brittleness and fracability in the formation of the lacustrine shale clearly. Moreover, this further enlarges the difference in the brittleness index. Based on the influence of plastic strain on brittleness, we have developed a new brittleness evaluation method that uses the ratio of linear elastic strain energy to the total strain energy before complete rock failure, which can indicate the difference of the lacustrine shale clearly. Fracture toughness is another important parameter that impacts the fracture extension and influences fracability. Based on the impact of brittleness and fracture toughness on the fracability, we have developed a new fracability evaluation method. The brittleness index increases with increases in the quartz content, and it decreases with increases in the albite feldspar and calcite contents. The fracture toughness decreases with increases in the quartz and clay contents, and it increases with increases in the siderite content. In addition, we established an empirical formula that can evaluate the brittleness index and the fracture toughness using mineral contents obtained from elemental logging. Using the new fracability evaluation method to optimize the fracturing stage, the preliminary field test indicates that the new approach was effective in the lacustrine shale formation.

A microseismic-based fracture properties characterization and visualization model for the selection of infill wells in shale reservoirs

Microseismic monitoring is widely applied to detect microseisms touched off by shear slippage on bedding planes or natural fractures in the vicinity of the hydraulic fracture stages being stimulated, characterize full fracture geometry and orientation, fracture complexity and also measure the stimulated reservoir volume (SRV). Microseismic events are recorded by geophones at the surface, near-surface or in monitoring boreholes and their hypocenters are mapped via P and S wave arrival picking and hodogram analysis or seismic imaging technology. Fracturing treatment parameters and events attributes, such as seismic moment, rock rigidity, fluid efficiency, injected fluid volumes, displacement along the slip plane and hypocenter focal mechanism, can be used to measure the fracture geometry and orientation for 3D discrete fracture networks (DFNs) modeling. In the DFNs, every modeled fracture plane shaped by uniformly-spaced point arrays is centered on a microseismic event. All these pointsets are classified to different cells in a geocellular model. Then geocellular cubes with uniform dimension are created in the SRV and event-based fractures are mapped onto each cube. Based on the domain classification methodology, the total intersected fracture polygon area in each SRV cube was calculated for fracture properties (such as fracture intensity, fracture porosity) characterization. Fracture permeability scalar was proposed to capture the unscaled permeability enhancement in 3D geocellular framework. 3D visualization plots would be used to determine the high conductivity zone in the productive SRV, which further helps selecting the optimal position for the infill wells and further enhances the oil and gas recovery.

CAFE based multi-scale modelling of ductile-to-brittle transition of steel with a temperature dependent effective surface energy

It is still a challenge to numerically achieve the interactive competition between ductile damage and brittle fracture in ductile-to-brittle transition (DBT) region. In addition, since two types of fracture occur at two independent material length scales, it is difficult to process them with the same mesh size by using finite element method. In this study, a framework of modelling DBT of a thermal mechanical controlled-rolling (TMCR) steel is explored by using the cellular automata finite element (CAFE) method. The statistical feature of material's microstructure is incorporated in the modelling. It is found that DBT curve cannot be reproduced with only one temperature dependent flow property, for which another temperature dependent variable must be considered. A temperature dependent effective surface energy based on typical cleavage fracture stage is proposed and obtained through a continuum approach in present work. The DBT of TMCR steel is simulated by using CAFE method implemented with a temperature dependent effective surface energy. It is found that numerical simulation is able to produce a full transition curve, especially with scattered absorbed energies in the transition region represented. It is also observed that simulation results can reproduce a comparable DBT curve contrasting to the experimental results.

Numerical study on heat extraction performance of a multilateral-well enhanced geothermal system considering complex hydraulic and natural fractures

The complex fracture network is necessary to provide flow and heat transfer channels for working fluid of an enhanced geothermal system (EGS). It is significant to compare heat extraction performances of different complex fracture networks and to provide suggestions for fracturing operation of an EGS. Based on a 3D thermal-hydraulic-mechanical coupled model, this paper studies heat extraction performances of 11 different complex fracture networks with natural and hydraulic fractures for a multilateral-well EGS. Effects of natural and non-planar fractures on multilateral-well EGS performance are investigated. Influences of primary fracture stage quantity on multilateral-well EGS performance are studied. Contributions of primary and secondary fractures to heat extraction of multilateral-well EGS are compared. Results indicate that natural fractures should be considered when accurately estimating multilateral-well EGS performance. The model with planar fracture networks overestimates production temperature and thermal power of multilateral-well EGS. It is suggested that 3 stages of primary fractures are beneficial for multilateral-well EGS performance. Fracturing operation of multilateral-well EGS should focus on generating long fractures to connect natural fractures far from lateral wells rather than inducing numerous fractures around lateral wells. Simulation results provide significant suggestions for the fracturing operation of multilateral-well EGS.

Post-yielding tension stiffening of reinforced concrete members using an image analysis method with a consideration of steel ratios

When designing reinforced concrete (RC) members, the rebar is assumed to resist all tensile forces, but the resistance of the concrete in the tension area is neglected. However, concrete can also resist tensile forces and increase the tensile stiffness of RC members, which is called the tension stiffening effect (TSE). Therefore, this study assessed the TSE, particularly after yielding of the steel bars and the effects of the steel ratio on the TSE. For this purpose, RC member specimens with steel ratios of 2.87%, 0.99%, and 0.59% were fabricated for uniaxial tensile tests. A vision-based non-contact measurement system was used to measure the behavior of the specimens. The cracks on the specimen at the stabilized cracking stage and the fracture stage were measured with the image analysis method. The results show that the number of cracks increases as the steel ratio increases. The reductions of the limit state and fracture strains were dependent on the ratio of the rebar. As the steel ratio decreased, the strain after yielding of the RC members significantly decreased. Therefore, the overall ductility of the RC member is reduced with decreasing steel ratio. The yielding plateau and ultimate load of the RC members obtained from the proposed equations showed very good agreement with those of the experiments. Finally, the image analysis method was possible to allow flexibility in expand the measurement points and targets to determine the strains and crack widths of the specimens.

Staged numerical simulations of supercritical CO2 fracturing of coal seams based on the extended finite element method

As a water-less fracturing mining technology, supercritical CO2 (SC-CO2) fracturing has attracted increasing attention in the mining industry. In order to simulate the whole process of SC-CO2 fracturing and fully understand the fracturing mechanism of SC-CO2, the thermodynamic analysis method was used to analyze the phase transitions of CO2 in SC-CO2 fracturing, and the whole process of SC-CO2 fracturing was divided into the SC-CO2 fracturing stage and the CO2 phase-transition induced fracturing stage. Based on the extended finite element method, the fluid-solid coupling model at the SC-CO2 fracturing stage, the TNT equivalence of CO2 phase transition induced fracturing, pressure-time history curves of blast loads, and numerical model of CO2 phase transition induced fracturing stage were comprehensively analyzed, and the staged numerical simulation method of SC-CO2 fracturing was proposed. The simulation results showed that cracks in the coal seam were initiated and propagated at the SC-CO2 fracture stage, and the cracks broadened, and new cracks were generated during the CO2 phase-transition induced fracturing stage. At the same time, the injection rate of SC-CO2 fluid had a positive impact on its fracturing effect. The larger the injection rate of SC-CO2 fluid, the larger the length and width of crack propagation. This study is of great significance for enriching SC-CO2 fracturing theory.

Monitoring Damage Evolution in a Titanium Matrix Composite Shaft Under Torsion Loading Using Acoustic Emission

The damage behaviors of a titanium matrix composite shaft under torsion loading were monitored using the acoustic emission technique. The composite shaft with SiC fibers at ± 45° orientations was prepared by the solid-state fabrication process. Both the torsional rigidity and torsional strength of the TMC shaft were improved by SiC fibers. The acoustic emission responses during the loading–unloading–reloading, under quasi-static and cyclic torsion tests were investigated. Multiple acoustic emission signals were grouped as mechanical noise, matrix deformation, interface debonding and fiber fracture using amplitude, waveform shape and frequency centroid parameters. A substantial reduction of signals generated by matrix deformation was found in the reloading test. During the quasi-static torsion test, interface debonding and progressive breaks of SiC fibers occurred. According to different acoustic emission behaviors, the failure process in the torsion fatigue test can be divided into three stages: the initial stage, the fiber fracture stage and the fast fracture stage.

Visual & quantitative identification of cracking in mortar subjected to loads using X-ray computed tomography method

Cracking is a major form of failure in cementitious materials. In this study, the cracking process in a mortar is non-destructively investigated using an in-situ X-ray computed tomography (XCT) method. A novel image-processing method that allows for 3D visualization of the cracking process and for the distinction between pores and cracks is proposed to identify cracks inside concrete structures. With the proposed method, the fracture process in a mortar can be visualized, and quantitative information regarding the amount of solid, pores, and cracks inside the mortar can be obtained. Based on the experimental results, three fracture stages were observed: a compression stage, an expansion stage, and a cracking stage, which is consistent with the concrete compression failure process fracture theory.

CONTROLS ON FRACTURE FLOW POTENTIAL IN A TIGHT CARBONATE RESERVOIR: SAYINDERE FORMATION (CAMPANIAN), WEST ADIYAMAN BASIN, SE TURKEY

Identifying controls on the permeability of fluid‐conductive fractures is critical in tight reservoirs, but this is challenging in tectonically complex regions such as foothills belts where there may have been multiple stages of deformation and fracturing. Fracture permeability depends on fracture aperture and connectivity, both of which are affected by tectonism and cementation. Among the many factors that control the cementation history, oil charging may play an important role. Important challenges in studies of fractured reservoirs in tectonically complex regions include determining the timing (and intensity) of fracturing events relative to that of the oil charge, verifying the presence of matrix storage, and establishing the fracture cementation history.This paper reports on a comparative fracture study of four small‐scale oilfields in the west Adıyaman Basin, located within the foothills belt of the Tauride suture zone in SE Turkey. Here the tight reservoir carbonates of the Sayındere Formation (Campanian) were subjected to repeated phases of structural deformation. Major deformation phases took place in Campanian and Maastrichtian times, before oil charging into the reservoir began in the Eocene; and in the Late Eocene – Oligocene and Late Miocene, after the oil charge. Fractures that were generated before oil emplacement appear to have been cemented or partially cemented by calcite as indicated by cross‐cutting cemented fractures on borehole images. Partially‐cemented fractures in cores are oil‐stained with cement‐lined walls, suggesting cementation began before oil emplacement but was not completed. Image logs and cores also show the presence of clean, open fractures with no cement present on the walls. These open fractures cut across the cemented or partially‐cemented fractures, and are in general related to Late Miocene compressional folding. Open fracture density is correlated to Late Miocene fold curvature and asymmetry in the four oilfields studied. Of these fields, the Șambayat structure is the tightest and most asymmetric anticline and hence has the maximum open fracture density; this field also has the highest oil potential.Although the available data is not sufficient to evaluate the effects of oil charging on fracture cementation definitively, the observations are consistent with a model that oil charge into the fractured Sayındere Formation carbonates inhibited or slowed calcite cementation. Hence fracturing of a carbonate reservoir after oil emplacement may significantly enhance the fracture permeability, and may even render a tight reservoir prospective.

Light and Sound Emission from Impact-Damaged ZnS and ZnSe Ceramics

The results of mechanoluminescence (ML) and acoustic emission (AE) studies of the response of the ceramic materials ZnS and ZnSe to impact damage to the surface are presented. The experiment simulates the process of dust erosion of optical components made from these materials and installed on high-speed mobile carriers. A comparison of a time series of light and sound emissions showed that ML manifests two peaks approximately 50 – 200 and 150 – 200 μsec after impact, while AE occurs in a single peak near 200 μsec. This result is explained by plastic deformation of the material at the first stage of fracture and subsequent formation of microcracks.

Real-Time Control of Nonflatness of Components of Infrared-Range Flip-Chip Photodetectors

A possibility of using the autocollimation and interference methods for real-time nondestructive check of the wafer nonflatness of photodetector arrays that are sensitive in the infrared range and are fabricated by the flip-chip technology is considered. These methods allow monitoring the wafer nonflatness and the maximum deflections. These methods are applied in the present study to measure the wafer nonflatness of fragments of silicon slices and arrays of photosensitive elements on GaAs substrates, as well as the wafer nonflatness of photodetectors at different stages of fracture and in the course of thermal cycling.

The effect of completion strategy on fracture propagation from multiple cluster perforations in fossil hydrogen energy development

The hydraulic fracturing is extensively used to stimulate the production of fossil hydrogen energy. Perforation parameters have a great influence on the performance of hydraulically fractured horizontal wells in fossil hydrogen energy development. In this work, a practical model to study the effect of a completion scheme on simultaneous fracture propagation is proposed, in which the coupling fluid flow and stress interaction is considered, and a fracture propagation uniform index model is used to optimize the completion strategy. Moreover, this model is well validated by Wu's model and Zhang's model. The numerical study results indicate that a uniform completion scheme causes a non-uniform fracture development; an optimal completion method is proposed by reducing the cluster number, non-uniform spacing, and practical limited entry technique in a fracturing stage. Furthermore, the combined use of unequal cluster spacing and a limited entry strategy can significantly reduce the suppression effects between multiple clusters, improve the fluid flow into each cluster, and enhance the uniform propagation to achieve the maximum production. Our present work illustrates a better understanding of the effect of the completion strategy on the multiple fracture propagation, and paves a path for a more optimal completion design for fossil hydrogen energy development.

Experimental study on acoustic emission characteristic of fatigue crack growth of self-compacting concrete

In order to study the fatigue crack growth (FCG) of self-compacting concrete, in this paper, the 100 mm × 100 mm × 400 mm specimens with a 30-mm initial notch were used for three-point bending test under three different stress levels (0.75, 0.8, and 0.85) and three different loading frequencies (0.5, 1, and 2 Hz), and the acoustic emission (AE) technique was used for monitoring the FCG. According to the results, the changes of the crack mouth opening displacement, the stiffness, and the effective crack length all present three obvious stages with the increase of fatigue cycles. According to Paris law, under the same stress level, the increase of loading frequency results in the increase of fatigue life and the decrease of FCG rate, but the curves of da/dN are approximately parallel under different loading frequencies. Furthermore, the research results of characteristic parameters of AE show that cumulative hits, counts and energy show three stages, which are similar to the change of stiffness, and there is a failure point in the curves of cumulative counts and cumulative energy. The cumulative laws of hits and counts can be used to well describe the FCG, and the b value can be used to analyze the three stages of fatigue fracture. Therefore, the AE technology can be used to monitor the FCG in concrete engineering.

The influence of fracture development on quality and distribution of volcanic reservoirs: a case study from the carboniferous volcanic reservoirs in the Xiquan area, eastern Junggar Basin

The distribution pattern of fractures in the Carboniferous volcanic rocks controls the oil and gas distribution in the Xiquan area located in the eastern Junggar Basin. This paper describes the types, characteristics, distribution of fractures in the volcanic rocks, and the evolutionary history of fractures, as well as the influence of fractures on the reservoir distribution in this area, based on analyzing cores, casting thin sections, imaging logging data, the measurement of the physical properties and integrated with the production testing. The lithology of these Carboniferous volcanic rocks is primarily andesite and volcanic breccia. Basalt, dacite, tuff, breccia lava, tuffaceous sandstone, and tuffite were also present. The fracture types are mainly shrinkage fractures, weathered fractures, dissolution fractures, and structural fractures, the majority of which are structural fractures and weathered fractures. The distribution of fractures has obvious zonation characteristics in the vertical direction. Fractures are mainly distributed at four intervals under the top surface of the Carboniferous unconformity: 0–50 m, 65–115 m, 125–155 m, and 170–210 m. There are mostly weathered fractures and dissolution fractures in the 0–50-m interval. The physical properties and oil production of andesite are the highest, and weathering fractures and dissolution fractures play a major role in controlling the secondary pores of volcanic lava and the formation of favorable reservoirs in this interval. The structural fractures were developed in the intervals of 65–115 m, 125–155 m, and 170–210 m. The physical properties and oil production of volcanic breccia are the highest, and therefore, structural fractures mainly control the formation of favorable reservoir in volcanic breccia in these three intervals. The evolution of fractures in the Carboniferous volcanic rocks experienced four phases: the condensation and shrinkage fracture stage, which developed the primary pores in the middle-late Carboniferous period; the weathered fracture and dissolution fracture stage, which greatly increased the porosity in the late Carboniferous period to the early-middle Permian period; the structural fracture and weathered fracture stage, which greatly increased the porosity and permeability of the reservoir from the end of the Triassic to the early Cretaceous period; and the filled fracture and dissolution fracture stage, which can also greatly increase the porosity of the volcanic reservoir and occurred wholly within the late Cretaceous period until the present.

Mechanism of casing failure during hydraulic fracturing: Lessons learned from a tight-oil reservoir in China

Abstract Casing failures have been systematically encountered during multi-stage hydraulic fracturing of horizontal wells in some unconventional reservoirs of China. Such failures usually manifest themselves as excessive localized radial deformation of the casing after some fracturing stages, which prevent the bridge plugs of subsequent stages from being installed to the design depths, or even result in abandonment of all remaining fracturing stages. In this paper, aiming at clarifying the casing failure mechanism during hydraulic fracturing, we performed an integrated investigation of tight-oil wells in northwest China, by analyzing the field-gained data and modeling possible mechanisms responsible for the casing failure. The investigation results show that, radial deformation of the casing due to alteration of in-situ stresses, even jointly with poor cement sheath, is far smaller than that observed in the field which could reach the order of magnitude of 1–3 cm. In contrast, we find that shear slips of pre-existing fractures/faults crossing the casing can deform the casing to the observed magnitude, justifying a conclusion that casing failure is due to the shear of pre-existing fractures/faults. This modeling-derived conclusion is supported by the records of micro-seismic events. In addition, lead impression blocks have also run for determining the deformed shape of the casing, and the results match the modeling results for the casing sheared by activated fractures/faults. Effect of fracturing pressure, in-situ stresses, length and orientation of pre-existing fractures/faults on the slippage of the fractures/faults and thus the deformation of the casing are demonstrated by a series of sensitivity studies. Additionally, some insights are derived for mitigating the casing failure in future fracturing jobs in similar unconventional reservoirs.

Optimization of acid fracturing for a tight carbonate reservoir

Abstract In this study, acid fracturing treatments were simulated for a tight limestone reservoir within a shale formation using FRACPRO software. The purpose was to investigate the optimum acid fracturing design that leads to a higher fracture etched length and width, and higher fracture conductivity. Moreover, the impact of the rock-acid contact time and whether to consider a post-flush or fluids flowback, on the acid fracturing outcomes were also investigated. A simple geological model was constructed which consists of different lithological layers. Different acid fracturing design scenarios were considered starting from a single stage of acid injection to multi-stage treatment. In multi-stage acid treatment, alternate acid-slickwater injection was considered. Plain HCl acids with different concentrations and other acids that are already included in FRACPRO database were used. The results showed that the acid loss during post-flush is among the main problems of the acid fracturing in tight carbonate shale reservoir. For the single stage of acid injection, it is recommended to flow back the acid after well shut-in instead of considering a post-flush stage. The multi-stage alternate acid slickwater injection reduces or even eliminated the acid loss. However, it is recommended to inject a slickwater before well shut-in to reduce the rock-acid contact time, thus reducing the formation damage. The results also showed that the created fracture etched width decreases and the fracture etched length increases as the fracturing stages increase. In this study, because of the low carbonate layer permeability and compressive strength, a two-stage alternate 28% HCl and slickwater injection with a post-flush stage is recommended.

Fatigue damage evolution of micro friction stir blind riveting joint through acoustic emission analysis

This research explored the fatigue behavior for a newly developed multilayer dissimilar material joining technique, micro friction stir blind riveting (viz. µFSBR), through integrating acoustic emission (AE) data with cyclic loading data. Multilayer µFSBR joints consisting of one Cu layer and three Al layers connected through Al rivet were investigated, where the fatigue behavior was encapsulated through examining failure modes, damage mechanism and in-situ damage evolution with AE monitoring. It was revealed that the µFSBR process mechanics influenced the failure modes as the resultant effects of friction stirring, leading to the failure in the stirred region for Cu layer at high loads and failure in base metal for Cu layer at low loads. The damage mechanisms were identified through hysteresis loops, where cyclic creep or ratcheting strain occurred in the low cycle fatigue scenarios while fatigue damage was the governing damage mechanism in high cycle fatigue paradigms. In-situ damage characterization was performed by introducing a novel analysis scheme using AE hit datasets, which effectively identified three distinct stages, including (1) initial deformation, (2) cyclic hardening at low cycle or softening at high cycle fatigue, and (3) crack initiation and growth till fracture. The stage of crack initiation and growth till fracture was further explained by leveraging the cumulative AE hit count rate. Furthermore, the relationship between the life fraction-load-AE amplitude was developed to characterize the damage source mechanisms in the entire fatigue life at different load levels by integrating both load and AE datasets.

Numerical Investigation of Hydraulic Fracture Propagation Based on Cohesive Zone Model in Naturally Fractured Formations

Complex propagation patterns of hydraulic fractures often play important roles in naturally fractured formations due to complex mechanisms. Therefore, understanding propagation patterns and the geometry of fractures is essential for hydraulic fracturing design. In this work, a seepage–stress–damage coupled model based on the finite pore pressure cohesive zone (PPCZ) method was developed to investigate hydraulic fracture propagation behavior in a naturally fractured reservoir. Compared with the traditional finite element method, the coupled model with global insertion cohesive elements realizes arbitrary propagation of fluid-driven fractures. Numerical simulations of multiple-cluster hydraulic fracturing were carried out to investigate the sensitivities of a multitude of parameters. The results reveal that stress interference from multiple-clusters is responsible for serious suppression and diversion of the fracture network. A lower stress difference benefits the fracture network and helps open natural fractures. By comparing the mechanism of fluid injection, the maximal fracture network can be achieved with various injection rates and viscosities at different fracturing stages. Cluster parameters, including the number of clusters and their spacing, were optimal, satisfying the requirement of creating a large fracture network. These results offer new insights into the propagation pattern of fluid driven fractures and should act as a guide for multiple-cluster hydraulic fracturing, which can help increase the hydraulic fracture volume in naturally fractured reservoirs.