Visualization and characterization of experimental hydraulic fractures interacting with karst fracture-cavity distributions

Karst fracture-cavity carbonate reservoirs,in which natural cavities are connected by natural fractures to form cavity clusters in many circumstances,have become significant fields of oil and gas exploration and exploitation.Proppant fracturing is considered as the best method for exploiting carbonate reservoirs;however,previous studies primarily focused on the effects of individual types of geological formations,such as natural fractures or cavities,on fracture propagation.In this study,true-triaxial physical simulation experiments were systematically performed under four types of stress difference conditions after the accurate prefabrication of four types of different fracture-cavity distributions in artificial samples.Subsequently,the interaction mechanism between the hydraulic fractures and fracture-cavity structures was systematically analyzed in combination with the stress distribution,cross-sectional morphology of the main propagation path,and three-dimensional visualization of the overall fracture network.It was found that the propagation of hydraulic fractures near the cavity was inhibited by the stress concentration surrounding the cavity.In contrast,a natural fracture with a smaller approach angle(0°and 30°)around the cavity can alleviate the stress concentration and significantly facilitate the connection with the cavity.In addition,the hydraulic fracture crossed the natural fracture at the 45°approach angle and bypassed the cavity under higher stress difference conditions.A new stimulation effectiveness evaluation index was established based on the stimulated reservoir area(SRA),tortuosity of the hydraulic fractures(T),and connectivity index(CI)of the cavities.These findings provide new insights into the fracturing design of carbonate reservoirs.

Multi-Objective Optimization of Aluminum Alloy Electric Bus Frame Connectors for Enhanced Durability

The widespread adoption of aluminumalloy electric buses,known for their energy efficiency and eco-friendliness,faces a challenge due to the aluminum frame’s susceptibility to deformation compared to steel.This issue is further exacerbated by the stringent requirements imposed by the flammability and explosiveness of batteries,necessitating robust frame protection.Our study aims to optimize the connectors of aluminum alloy bus frames,emphasizing durability,energy efficiency,and safety.This research delves into Multi-Objective Coordinated Optimization(MCO)techniques for lightweight design in aluminum alloy bus body connectors.Our goal is to enhance lightweighting,reinforce energy absorption,and improve deformation resistance in connector components.Three typical aluminum alloy connectors were selected and a design optimization platform was built for their MCO using a variety of software and methods.Firstly,through three-point bending experiments and finite element analysis on three types of connector components,we identified optimized design parameters based on deformation patterns.Then,employing Optimal Latin hypercube design(OLHD),parametric modeling,and neural network approximation,we developed high-precision approximate models for the design parameters of each connector component,targeting energy absorption,mass,and logarithmic strain.Lastly,utilizing the Archive-based Micro Genetic Algorithm(AMGA),Multi-Objective Particle Swarm Optimization(MOPSO),and Non-dominated SortingGenetic Algorithm(NSGA2),we explored optimized design solutions for these joint components.Subsequently,we simulated joint assembly buckling during bus rollover crash scenarios to verify and analyze the optimized solutions in three-point bending simulations.Each joint component showcased a remarkable 30%–40%mass reduction while boosting energy absorption.Our design optimization method exhibits high efficiency and costeffectiveness.Leveraging contemporary automation technology,the design optimization platform developed in this study is poised to facilitate intelligent optimization of lightweight metal components in future applications.

Application of Spectral Decomposition to Detection of Fracture-Cavity Carbonate Reservoir Beds in the Tahe OUfield,Tarim Basin,NW China

Ordovician fracture-cavity carbonate reservoir beds are the major type of producing formations in the Tahe oilfield, Tarim Basin. The seismic responses of these beds clearly changes depending on the different distance of the fracture-cavity reservoir bed from the top of the section. The seismic reflection becomes weak or is absent when the fracture-cavity reservoir beds are less than 20 ms below the top Ordovician. The effect on top Ordovician reflection became weaker with deeper burial of fracture-cavity reservoir beds but the developed deep fracture-cavity reservoir beds caused stronger reflection in the interior of the Ordovician. This interior reflection can be divided into strong long-axis, irregular and bead string reflections, and was present 80 ms below the top Ordovician. Aimed at understanding reflection characteristics, the spectral decomposition technique, which uses frequency to ＂tune-in＂ bed thickness, was used to predict Ordovician fracture-cavity carbonate formations in the Tahe oilfield. Through finely adjusting the processing parameters of spectral decomposition, it was found that the slice at 30 Hz of the tuned data cube can best represent reservoir bed development. Two large N-S-trending strong reflection belts in the mid-western part of the study area along wells TK440- TK427-TK417B and in the eastern part along wells TK404-TK409 were observed distinctly on the 30 Hz slice and 4-D time-frequency data cube carving. A small N-S trending reflection belt in the southern part along wells T403-TK446B was also clearly identified. The predicted reservoir bed development area coincides with the fracture-cavities connection area confirmed by drilling pressure testing results. Deep karst cavities occur basically in three reservoir bed-development belts identified by the Ordovician interior strong reflection. Spectral decomposition proved to be a useful technique in identifying fracture-cavity reservoir beds.

Numerical analysis of the hydraulic fracture communication modes in fracture-cavity reservoirs

The fracture-cavity carbonate reservoirs in the Tahe Oilfield in China are mainly exploited by fracturing.We need the hydraulic fractures to communicate with caves to create a flow channel.However,due to the existence of the fracture-cavity systems,the hydraulic fracture propagation morphology is complicated,while the propagation characteristics are not clear.To analyze the hydraulic fracture propagation in fracture-cavity carbonate formations,based on the discontinuous discrete fracture model,we developed a solid-seepage-freeflow coupled fracturing model for fracture-cavity formations,which can simulate the complex interaction behavior of fractures and caves.Based on the simulation results,we found the interaction rule between hydraulic fractures and fracture-cavity systems:the stress concentration around caves is the main factor that determines the fracture propagation path.Deflection due to stress concentration is usually not conducive to communication,while natural fractures distributed around caves could break the rejection action.Increasing the hydraulic energy in the hydraulic fracture can make fracture propagate directly and reduce the influence of deflection.The steering fracture formed by temporary plugging is beneficial to the communication of fracture-cavity systems in the non-principal stress direction.According to the simulation results of different fracture-cavity characteristics,we raised four optimization communication modes for fracture-cavity carbonate formation to provide references for fracturing optimization design and parameter optimization.

A Numerical Study on the Propagation Mechanisms of Hydraulic Fractures in Fracture-Cavity Carbonate Reservoirs

Field data suggests that carbonate reservoirs contain abundant natural fractures and cavities.The propagation mechanisms of hydraulic fractures in fracture-cavity reservoirs are different from conventional reservoirs on account of the stress concentration surrounding cavities.In this paper,we develop a fully coupled numerical model using the extended finite element method(XFEM)to investigate the behaviors and propagation mechanisms of hydraulic fractures in fracture-cavity reservoirs.Simulation results show that a higher lateral stress coefficient can enhance the influence of the natural cavity,causing a more curved fracture path.However,lower confining stress or smaller in-situ stress difference can reduce this influence,and thus contributes to the penetration of the hydraulic fracture towards the cavity.Higher fluid viscosity and high fluid pumping rate are both able to attenuate the effect of the cavity.The frictional natural fracture connected to the cavity can significantly change the stress distribution around the cavity,thus dramatically deviates the hydraulic fracture from its original propagation direction.It is also found that the natural cavity existing between two adjacent fracturing stages will significantly influence the stress distribution between fractures and is more likely to result in irregular propagation paths compared to the case without a cavity.

Construction of carbonate reservoir knowledge base and its application in fracture-cavity reservoir geological modeling

To improve the efficiency and accuracy of carbonate reservoir research,a unified reservoir knowledge base linking geological knowledge management with reservoir research is proposed.The reservoir knowledge base serves high-quality analysis,evaluation,description and geological modeling of reservoirs.The knowledge framework is divided into three categories:technical service standard,technical research method and professional knowledge and cases related to geological objects.In order to build a knowledge base,first of all,it is necessary to form a knowledge classification system and knowledge description standards;secondly,to sort out theoretical understandings and various technical methods for different geologic objects and work out a technical service standard package according to the technical standard;thirdly,to collect typical outcrop and reservoir cases,constantly expand the content of the knowledge base through systematic extraction,sorting and saving,and construct professional knowledge about geological objects.Through the use of encyclopedia based collaborative editing architecture,knowledge construction and sharing can be realized.Geological objects and related attribute parameters can be automatically extracted by using natural language processing(NLP)technology,and outcrop data can be collected by using modern fine measurement technology,to enhance the efficiency of knowledge acquisition,extraction and sorting.In this paper,the geological modeling of fracture-cavity reservoir in the Tarim Basin is taken as an example to illustrate the construction of knowledge base of carbonate reservoir and its application in geological modeling of fracture-cavity carbonate reservoir.

Grading evaluation and prediction of fracture-cavity reservoirs in Cambrian Longwangmiao Formation of Moxi area,Sichuan Basin,SW China

By using core, thin section, well logging, seismic, well testing and other data, the reservoir grading evaluation parameters were selected, the classification criterion considering multiple factors for carbonate reservoirs in this area were established, and the main factors affecting the development of high quality reservoir were determined. By employing Formation MicroScanner Image(FMI) logging fracture-cavity recognition technology and reservoir seismic waveform classification technology, the spatial distribution of reservoirs of all grades were predicted. On the basis of identifying four types of reservoir space developed in the study area by mercury injection experiment, a classification criterion was established using four reservoir grading evaluation parameters, median throat radius, effective porosity and effective permeability of fracture-cavity development zone, relationship between fracture and dissolution pore development and assemblage, and the reservoirs in the study area were classified into grade I high quality reservoir of fracture and cavity type, grade II average reservoir of fracture and porosity type, grade Ⅲ poor reservoir of intergranular pore type. Based on the three main factors controlling the development of high quality reservoir, structural location, sedimentary facies and epigenesis, the distribution of the 3 grades reservoirs in each well area and formation were predicted using geophysical response and percolation characteristics. Follow-up drilling has confirmed that the classification evaluation standard and prediction methods established are effective.

Experimental investigation and analytically modeling of hysteretic behavior of rigid bus-flexible connectors of 220 kV electrical substations

This paper describes the quasi-static testing and analytical modelling of the hysteretic behavior of aluminum alloy rigid bus-flexible connectors of 220 kV electrical substations.The main objective of the study is to experimentally investigate the hysteretic behavior of six different types of rigid bus-flexible connectors 220 kV electrical substations when subjected to cyclic loading.Another objective is to theoretically study the flexibility and effectiveness of a previously proposed analytical model in fitting the experimental hysteresis loops of the tested rigid bus-flexible connectors.The experimental investigation indicates that the tested rigid bus-flexible connectors exhibit highly asymmetric hysteresis behavior along with tension stiffening effect.The theoretical study demonstrates that the generalized Bouc-Wen model has high flexibility and is effective in fitting the experimental hysteresis resisting force-displacement curves of the six tested rigid bus-flexible connectors.

套筒灌浆搭接及对接接头高应力反复拉压试验

为比较套筒灌浆搭接及对接接头间力学性能差异,进行了41个搭接接头和20个对接接头的单拉及高应力反复拉压试验。结果表明:单拉及高应力反复拉压时,两种接头均能实现最大力下总伸长率大于6%、延性系数大于4,强度基本满足规范要求;高应力反复拉压后单拉时,两种接头承载力均有所提高,但接头初始刚度和延性下降;防偏转措施可减少搭接接头残余变形,但其约束刚度有限,搭接接头残余变形略大于对接接头,防偏转搭接接头及对接接头残余变形基本满足规范要求;高应力反复拉压后单拉时,搭接接头套筒中部截面在加载前期纵向受拉、环向受压,加载后期纵向受压、环向受拉,对接接头套筒加载过程中均为纵向受拉、环向受压;高应力反复拉压结束后单拉时,防偏转、不防偏转搭接接头套筒中部截面近钢筋侧最大纵向拉应变分别为对接接头的0.10~0.39倍、0.13~0.18倍,最大环向压应变分别为对接接头的0.09~0.49倍、0.02~0.32倍,搭接接头对套筒材性要求较低;钢筋直径相同时搭接接头材料成本较对接接头降低约35%。

基于SSTDR的光伏连接器故障检测

随着运行时间的增长,光伏连接器会出现氧化、老化、松动等现象,易导致接触不良、发热等问题,最终可能引起断路、电弧等故障,对光伏系统的高效、安全运行造成不良影响。由于光伏连接器故障会引起其等效阻抗的变化,该文采用扩频时域反射法(spread spectrum time domain reflectometry,SSTDR)来进行检测:通过向光伏连接器所在的光伏组件串注入正弦高频信号调制的伪随机序列序列测试信号,分析入射信号与反射信号的相关特性,再与健康状态下的特性进行比较,来实现光伏连接器故障在线诊断。对此进行仿真计算并在4块光伏板组成的光伏组串中进行实验,发现开路故障时包络面积最大可达到5×10^(5),而脱离故障时包络面积最小为0.8×10^(5),二者皆远大于健康时的包络面积0.07×10^(5),可有效诊断光伏连接器是否发生故障。

不同锈蚀程度下栓钉连接件的抗剪性能分析

为研究栓钉连接件锈蚀条件下的抗剪性能退化规律,建立ABAQUS有限元模型进行数值模拟.针对均匀圆环、受压区半环、受拉区半环及拉压区半环4种不同的锈蚀分布形态,考虑不同的锈蚀高度、锈蚀深度进行数值分析.结果表明,当锈蚀深度相同时,4种锈蚀分布形态的栓钉抗剪承载力随锈蚀高度的增加而降低,在锈蚀高度为10 mm时降到最小,且锈蚀深度越大,承载力降低程度也越大;当锈蚀高度相同时,随着锈蚀深度的增加,抗剪承载力和抗剪刚度基本上呈线性下降趋势;当锈蚀高度和深度相同时,均匀圆环锈蚀的抗剪承载力和刚度的降低程度最大,相比其他3种情况降低60%左右,受压区半环锈蚀的抗剪承载力和刚度的降低程度最小.研究成果可为钢混组合结构栓钉连接件耐久性设计提供参考.

国际空间站宇航光纤通信器件发展综述

光纤通信技术在国际空间站(ISS)上首次大规模应用,是光通信技术从地面进入太空应用的一个重要里程碑。回顾了ISS舱内光纤通信所用宇航光缆和光纤连接器的技术发展历程,阐述了ISS对光纤通信技术的特殊需求;介绍了美国航空航天局(NASA)开展的光纤器件空间适应性试验研究情况,NASA和欧洲太空总署(ESA)负责建造舱段的光纤通信系统,以及NASA针对ISS光纤通信器件缺陷问题的质量调查情况和整改过程;结合ISS光纤通信器件质量问题的案例分析,总结了宇航光缆和光纤连接器在制造、工艺、材料、环境、检测和管理方面存在的问题和经验教训。最后,对宇航光纤通信器件的研发方向进行了展望。

大直径盾构隧道新型纵缝接头抗弯性能试验研究

纵缝接头是盾构隧道受力性能的薄弱部位,管片接头的形式直接影响盾构隧道的力学性能。目前国内的盾构隧道纵缝接头使用的连接件多为螺栓,新型纵缝接头使用了一种新的连接件—滑入式连接件,此种接头在大直径盾构隧道中的受力性能有待研究。本文以新型纵缝接头为研究对象,针对两种不同型号的滑入式连接件,采用模型试验的方法探究了大直径盾构隧道新型纵缝接头的受力性能,通过理论分析计算了新型纵缝接头的极限承载力,并比较分析了传统螺栓纵缝接头和新型纵缝接头的受力性能。结果表明:新型接头衬砌管片的薄弱部位在连接件周围的混凝土区域;滑入式连接件的型号直接影响新型纵缝接头的受力性能,两种接头中连接件尺寸较大的接头转角刚度相较于连接件尺寸较小的接头转角刚度增加了7.2~169.5%,极限承载能力增加了69.9%;新型纵缝接头比螺栓接头有更高的转角刚度,受力性能更好,适用于大直径盾构隧道。

电连接器用聚氨酯胶密封件贮存可靠性统计模型的加速试验验证与评估

针对电连接器用聚氨酯胶密封件在长贮条件下的寿命评估问题,从机理层面分析了聚氨酯胶密封件内聚破坏和边界破坏导致性能下降的原因。依据从机理层面建立的电连接器用聚氨酯胶密封件贮存可靠性统计模型,利用聚氨酯胶密封件温湿度综合应力加速退化试验数据,综合应用粒子群优化算法和回归分析、AD检验及拟合优度等检验方法,从数据统计层面验证了电连接器用聚氨酯胶密封件失效物理方程和退化轨迹模型的合理性。利用扫描电镜和能谱分析验证了失效机理分析的正确性。最后,利用所建模型评估了电连接器用聚氨酯胶密封件在贮存环境下的可靠寿命。

钢-UHPC结合段PBL连接件和钢梁端面承压联合作用研究

钢-混结合段作为钢梁和混凝土连接为整体的核心部件,传力构件的受力性能对钢-混凝土结合段至关重要。为提高结合段的力学性能,进行了仅有钢梁端面承压、PBL连接件和两者联合作用3组推出试验。结果表明:3组试件最终破坏裂缝分布均为钢梁端面侧“八”字形分布;钢梁端面组(D组)试件破坏模式主要为UHPC的开裂破坏而失效,PBL连接件组(P组)和联合作用组(PD组)试件主要是贯穿钢筋的剪切破坏而失效;3组试件的荷载-滑移曲线均包含弹性、裂缝开展和屈服3个阶段,由于贯穿钢筋和UHPC榫具有抑制裂缝开展和提高试件延性的作用,P组和PD组试件裂缝开展阶段较D组更短,屈服阶段更长,延性更好。对荷载-滑移曲线进行拟合,提出了钢梁端面承压和PBL连接件承载力随滑移量变化的计算公式。引入滞后滑移差Δs_(j),当Δs_(j)=0~0.67 mm时,联合作用计算结果偏于安全。计算得到钢梁端面承压传力比例约为43%,PBL连接件传力比例约为57%,两者传力效果基本相当。利用UHPC作为外包混凝土时,建议采用承载能力更高的贯穿钢筋,以充分发挥UHPC的高强性能,从而提高结合段的承载能力。

新型自复位钢框架-复合墙连接节点研究

内填RC复合墙钢框架结构在框架与内嵌墙之间设置开缝并安装耗能连接件能减小结构损伤,改善结构力学性能。提出在内填RC复合墙和钢框架结构之间采用新型折板型耗能连接件,并采用ABAQUS对连接件的受力性能进行分析,研究了节点的受力过程和破坏模式。将耗能连接件应用于两侧开缝的密肋复合墙-钢框架结构中实现滑移连接。结果显示:折板连接件在竖向拉压荷载和水平侧向荷载作用下的变形损伤模式存在差异,折板角和折板弯折数目对连接件受力性能影响显著。折板连接件用于结构中时,结构加载前期滞回曲线表现出残余变形小的特点,具有自复位性能,因此折板连接件是一种具有自复位能力的连接形式。

采用T型钢连接的可拆换钢梁抗震性能试验与数值研究

提出了一种采用T型钢连接的可拆换钢梁,在地震作用下T型钢集中耗散地震能量,并在震后通过更换T型钢实现使用功能的恢复。通过合理设计T型钢连接、钢梁中间段和悬臂段的抗弯承载力关系,定义设计承载力削弱系数的取值范围,从而实现T型钢集中损伤耗能,钢梁中间段和悬臂段保持弹性的屈服机制。为了使T型钢连接件实现预期的损伤和最优的耗能效果,设计了4个腹板采用不同截面和材料的T型钢连接件,并对T型钢连接的可拆换钢梁试件进行了低周往复加载试验和有限元分析。结果表明:T型钢连接的可拆换钢梁可以实现预期的屈服机制,达到T型连接件集中耗能的目的;T型钢腹板的残余变形和螺栓应力可以满足拆换条件,拆换后结构功能不变,实现了较为理想的可更换机制。狗骨式腹板截面的T型钢连接件塑性变形更均匀,耗能区域更大,耗能效率提高约30%,拆换时间和残余变形与普通截面相差不大,建议腹板采用狗骨式截面;采用低屈服点钢材可以提高T型钢的耗能效率约40%,在设计承载力系数取值合理的情况下,T型钢腹板可以选择低屈服点钢材。最后,基于试验和有限元结果,考虑到T型钢连接件的可拆换性和钢梁材料利用率,建议设计承载力削弱系数取值为0.65~0.85。

基于温度循环加速退化试验的光纤连接器寿命评估

针对光纤连接器在温度交替变化下寿命评估难的问题,提出一种基于修正Coffin-Manson模型的寿命评估方法。首先,分析了光纤连接器失效模式、机理。接着,开展光纤连接器多个加速应力水平下的温度循环加速退化试验,基于插入损耗加速退化数据建立幂函数退化轨迹模型。然后,结合插入损耗的失效阈值,通过假设检验推导不同加速应力水平下光纤连接器的伪失效寿命。最后,引入修正Coffin-Manson加速模型,进而外推空间环境下光纤连接器寿命。本方法评估的某光纤连接器在实际应力水平下的寿命与实际寿命比较接近,验证了修正Coffin-Manson模型方法对光纤连接器寿命评估的有效性和可行性。

接触件结构参数对连接器插拔特性的影响分析

在航空航天、国防等不同系统中,电连接器都发挥着至关重要的作用。这里对某型号电连接器的插拔特性进行研究。通过仿真和正交试验,分析不同的结构参数和材料对该特性的影响。仿真结果表明:插孔簧片的开槽宽度对电连接器性能影响最小,一般不考虑;而簧片的长度、缩口量和簧片厚度均对连接器的接触性能有不同程度的影响;铍青铜触头材料的整体性能优于其他两种材料(锡青铜和黄铜);另外,用圆角槽底代替直角槽底后,三种材料的最大等效应力减小约23%,最大插入力和接触压力都略有增加(约2.17%,3.2%),槽底应力集中的现象得到较大改善。

输电铁塔装配式基础抗剪键连接的剪切性能

预制构件连接方法及其界面剪切性能影响整个输电铁塔装配式基础的受力和变形.针对螺栓连接易松动、变形大之类的问题,提出采用高强灌浆料灌注成形的十字形抗剪键作为输电铁塔装配式基础连接方案.采用室内剪切试验和数值模拟,研究该抗剪键连接的剪切性能,并与常用的螺栓连接进行对比分析.结果表明:十字形抗剪键剪切破坏模式为剪断型脆性破坏,破坏形态表现为以抗剪键为中心向外略有扩展成菱形.和螺栓连接相比,抗剪键连接的剪切强度略低,但剪切刚度显著提高.浇筑过程中灌浆料外溢对界面抗剪强度有提高作用.抗剪键的抗剪性能与灌浆料强度正相关,但提高灌浆料强度对抗剪性能提升并不明显.抗剪键深度对抗剪性能有影响,但在抗剪键深度超过20 mm后,抗剪键的抗剪性能不再提高.根据剪切试验和数值模拟结果,建议十字形抗剪键设计为长200 mm、宽20 mm、深20 mm.