庄X区块不同地应力差下裂缝延伸模拟

体积压裂是非常规油气开采的关键技术,然而体积压裂形成缝网的机理极为复杂,裂缝之间的相互关系仍是目前研究的重点。本文采用边界元理论建立的数值模拟法,对庄X区块不同地应力差下裂缝的延伸情况进行模拟分析。研究结果表明:人工压裂缝及天然微裂缝的延伸受裂缝周围地应力影响明显。地应力差越小,裂缝越容易开裂延伸;压裂缝之间间距越小,诱导应力对裂缝的延伸影响就越明显。文章为预测和人为控制裂缝走向,以及区块后续压裂参数设计提供了参考。     

水平井压裂起裂规律研究现状

介绍了国内外在水平井压裂裂缝起裂规律的研究进展,主要包括运用解析法和有限元法两种不同方法,研究了水平井井筒周围地应力分布对裂缝起裂的影响,破裂压力及起裂方位,指出了目前研究中的不足,并对未来的研究方向进行了分析和展望。     

裂缝性储层水力裂缝扩展规律数值模拟研究

天然裂缝与水力裂缝的相互作用形成复杂裂缝的关键,采用耦合孔隙水压力的内聚力单元,模拟了天然裂缝存在下水力裂缝的扩展情况,探讨了逼近角及天然裂缝起裂应力对水力裂缝扩展及天然裂缝开启范围的影响。为保证缝内压力的连续性,在预置天然裂缝与水力裂缝的相交处耦合孔隙压力,裂缝位置采用变密度进行网格划分以提高收敛性。数值分析结果表明:水力裂缝的诱导应力场使得天然裂缝提前开启,天然裂缝的破坏形式以剪切破坏为主导;水平主应力差一定时,逼近角越小天然裂缝越易开启且开启长度越长;天然裂缝起裂应力增加,水力裂缝最大缝宽增加天然裂缝开启范围缩小。     

体积压裂应力干扰现场应用的规律

通过室内岩心地应力测试,利用多缝应力干扰理论分析应力场变化特征,结合现场压力测试诊断和微地震裂缝监测结果,验证不同缝间距下的应力干扰对裂缝延伸的影响规律.结果表明:裂缝一旦转向延伸后,随缝间距的缩小,应力干扰导致裂缝转向的程度增加,提高了体积缝的复杂程度,但抑制了缝长方向的延伸,影响裂缝对砂体控制范围.因此,对于既定应力差的地层,尤其对于水平应力差异小的地层,须合理设计缝间距,在利用应力干扰提高体积缝复杂程度同时,要考虑裂缝方向正常延伸,保证裂缝长度上对砂体的整体控制.     

致密砂岩储层压裂裂缝扩展形态试验研究

为研究致密砂岩储层水力压裂裂缝扩展形态及空间展布规律,利用真三轴水力压裂模拟实验系统,分析了水平应力差、压裂液黏度以及排量对复杂缝网形成规律的影响。研究结果表明:低水平应力差(3 MPa)下,水力裂缝更容易发生转向,趋向于沿着相对薄弱的天然裂缝和层理面延伸,使得裂缝趋于复杂,有利于致密砂岩储层形成复杂的裂缝网络;在水力裂缝扩展中,低黏度压裂液能充分保证滤失,在裂缝中流动摩阻小,有利于裂缝尖端的破裂,而且形成复杂缝网;压裂液泵注排量越大,破裂压力越大,裂缝越容易多处起裂,使得压裂裂缝形态趋于复杂。研究结果为致密砂岩储层现场压裂施工提供参考。     

气井重复压裂诱导应力场数学模型

重复压裂新裂缝的重新定向分析是具有相当难度和应用价值的核心问题,也是制约重复压裂优化设计的关键。重复压裂产生的裂缝方向取决于地应力状态,垂直裂缝井重复压裂前储层中的应力分布,尤其是水平主应力方向控制着重复压裂裂缝的起裂方位和延伸方向。本文系统研究了垂直裂缝井重复压裂前应力场的分布特征及影响应力大小和方向变化的主要因素,在此基础上应用弹性力学、流固耦合、孔隙弹性理论等建立了重复压裂气井诱导应力场的数学模型,实现了垂直裂缝气井重复压裂前应力场的定量分析与模拟。     

水平缝在螺旋射孔条件下的起裂规律研究

通过物理实验和数值模拟相结合的方式,研究了水平缝螺旋射孔条件下的裂缝起裂、扩展规律。根据水平缝地层的地应力大小和实验条件的相似准则确定物理实验方案,利用大尺寸真三轴实验装置对水平缝的起裂、延伸进行物理模拟;依据损伤力学原理,利用有限元法对部分实验的裂缝起裂、扩展进行数值模拟。结果表明:60°相位角、高密度射孔可以小范围降低水平缝的破裂压力;压裂过程中,井筒附近可能产生多点起裂,但是随着压裂过程的进行,只有最上面射孔位置或者最下面射孔位置的1条裂缝能继续向前延伸形成主裂缝,其他裂缝止裂。数值模拟结果与实验结果相一致,证明实验结果合理。     

分段压裂水平井压力动态影响因素分析

建立了分段压裂水平井不稳定渗流数学模型,利用有限元方法求得井底压力,绘制出压力动态曲线,对压力动态的影响因素进行了分析。研究结果表明:该模型的压力动态曲线分为井筒储集阶段、双线性流、裂缝附近径向流、地层线性流、拟径向流和拟稳态流等6个阶段。裂缝导流能力越强,双线性流持续的时间越短,裂缝附近径向流出现的时间越早;压裂裂缝半长越长,压裂裂缝附近的径向流出现的越早且持续时间越长;随着裂缝间距的增大,裂缝附近径向流越明显,且持续时间越长。该研究成果可为分段压裂水平井的压力动态分析和试井解释提供依据。     

支撑剂嵌入对致密砂岩储层裂缝导流能力的影响

致密砂岩储层经过水力压裂施工后,支撑剂通常会嵌入裂缝壁面导致有效缝宽减小,进而导致裂缝导流能力的降低,影响压裂施工的效果。以鄂尔多斯盆地某致密砂岩油藏储层段岩样为研究对象,评价了不同类型支撑剂、支撑剂粒径以及铺砂浓度条件下支撑剂嵌入对裂缝缝宽和导流能力的影响,结果表明：在相同的试验条件下,石英砂在致密砂岩板上的嵌入对缝宽和裂缝导流能力的影响要大于陶粒;支撑剂粒径和铺砂浓度越小,有效缝宽越小,支撑剂嵌入导致裂缝导流能力的下降幅度越大。在致密砂岩储层压裂施工过程中,应根据现场实际情况优化压裂施工设计,选择合适的压裂施工参数,尽可能地降低支撑剂嵌入对裂缝导流能力的影响。     

低渗气藏水平井分段压裂完井产能评价研究

水平井分段压裂完井是低渗气藏水平井长期高效开发的重要手段。目前国内学者在研究裂缝参数对压裂水平井产能影响时没有考虑不同气藏控制面积、不同气藏基质渗透率的情况,对压裂水平井裂缝间干扰作用也缺乏深入研究。为此采用数值模拟方法,建立数值模拟机理模型,在考虑不同气藏控制面积、基质渗透率的情况下,主要研究裂缝条数、裂缝长度、裂缝间距对低渗气藏水平井产能的影响,并提出了裂缝半长/裂缝间距的评价指标,同时参考模拟出的各条裂缝累积产气量及压力等值线图,对压裂水平井裂缝干扰情况进行分析。研究结果表明,在压裂水平井生产初期,各控制面积下累积产气量相同,但随生产时间的延续,气藏控制面积越大,累积产气量越大;基质渗透率越低,采出程度越小,需要压开更多的裂缝,裂缝半长/裂缝间距越大;裂缝条数越多,裂缝间产生的相互干扰也越严重,使每条裂缝的产量减小;裂缝间距越小,压力下降越快,裂缝间干扰作用越强。这为低渗气藏分段压裂水平井裂缝参数优化设计提供了理论依据。     

Study on the long-term behaviour of glass fibre in the tensile stress field

Glass fibre is a random network structure composed of [SiO₄] tetrahedra. The structure contains a large number of defects, which act as crack initiation points. Under tensile stress, cracks undergo crack initiation, stable propagation, failure propagation, and fracture, and the stress that begins after unstable propagation is called the critical fracture stress. When the stress is less than the critical value, the crack is subject to the force of chemical bonds during the crack propagation process, and crack arrest occurs. When the stress is greater than the critical value, the glass fibre will undergo destructive fracture. In this paper, long-term tensile tests were carried out on glass fibre and a glass fibre composite under different constant tensile stress conditions. The fracture times of the glass fibre and glass fibre composite under different tensile stresses were obtained, the critical fracture stress of glass fibre was inferred, and the fracture mechanism was explained.     

A quantitative analysis of the indentation fracture of fused silica

It is unclear how the densification of fused silica influences the damage of its precision optics subjected to machining. This paper presents a quantitative analysis of the indentation fracture of fused silica involving densification with the embedded center of dilation (ECD) model. The Hertzian stress field and the ECD-induced stress field were superposed to provide the overall stress distribution in the loading stage. A new method was established to accurately determine the strength of the ECD-induced stress field with densification effects. With the aid of the ECD model, the starting locations, initiation stages and initiation sequence of crack morphologies were predicted by analyzing the stress fields. To quantitatively study the initiation of conical cracks in fused silica, the strain energy release rate was calculated by linear elastic fracture mechanics (LEFM). The predicted minimum threshold load leading to conical cracking was consistent with the measured values.     

金属基聚合物内衬复合压力容器界面剥离失效及预防

基于构建的裂纹剥离扩展失效过程的模拟方法，提出了预测临界载荷的方法，并通过界面的临界应变能释放率与损伤起始应力，构建了预防界面裂纹剥离扩展失效的等值临界真空压力约束预防控制线和设计准则。结果表明，临界压力载荷受控于界面初始预裂纹长度、复合界面的临界应变能释放率（GIC）和损伤起始应力（T0），与界面初始预裂纹长度呈负关联关系，而与临界应变能释放率与损伤起始应力呈正关联关系；当初始预裂纹长度由11.11 %增至15.55 %时，临界压力载荷（Pc）由87.6 kPa降至为57 kPa，降幅为34.9 %；内衬界面剥离韧性参数（T0，GIC）的坐标点位于等值临界真空压力约束控制线之上，可有效预防内衬界面剥离扩展失效。     

真空玻璃的应力分析及强度设计

基于真空玻璃的典型结构特征,分析了大气压差、温差及风载荷作用下真空玻璃的应力分布特征,给出了最大弯曲拉应力定量计算公式。基于结构抗力设计方法,分析了长期和短期应力协同作用下真空玻璃的承载性能设计。结果表明,在给定基片厚度情况下,随着支撑物间距的增大,真空玻璃最大弯曲拉应力呈近似线性增长,在支撑物间距不变情况下,真空玻璃最大弯曲拉应力随基片厚度增大呈指数式下降趋势。温差引起的最大弯曲拉应力与玻璃基片厚度及长宽尺寸无关,但与膨胀系数有关,温差值与其引发的真空玻璃最大弯曲拉应力呈线性关系。相同条件下,真空玻璃抗风压性能弱于与其等厚度的单片玻璃。在长期和短期应力协同作用下,宜分别计算不同应力作用时间下真空玻璃的效应设计值进行校核。     

Analysis of interfacial crack initiation mechanism of thermal barrier coatings in isothermal oxidation process based on interfacial stress state

In this paper, the interfacial stress state is used to analyze the interfacial crack initiation mechanism of the thermal barrier coatings (TBCs) during isothermal oxidation. The influence of thermal growth stress, initial residual stress, and creep behavior on the stress distribution is considered to have an accurate simulation result. A parameter that integrates the effects of interfacial normal and tangential stress is modified for evaluating interfacial crack initiation. It is found that, in the cooling stage, the interfacial cracks sprout at the top coat (TC)/thermally grown oxide (TGO) interface valley region and the TGO/bond coat (BC) interface peak region, which agrees with the experimental results. Furthermore, the influence of interfacial roughness on crack initiation is investigated. The result shows that different interfacial roughness affects the sprouting region of interfacial cracks and cracks within the TC layer.     

非金属密封垫片应力场的数值模拟

为研究聚四氟乙烯密封垫片的应力分布规律,根据有限元理论,建立了管法兰连接有限元模型。研究了垫片在预紧状态和工作状态下的Von Mises应力和接触应力分布,分析了管道内流体压力、螺栓预紧力和垫片材料弹性模量对其应力值和应力分布的影响。结果发现:垫片在工作状态下的Von Mises应力大于其在预紧状态;流体压力越大,垫片和Von Mises应力越大,但是其有效接触宽度越小;螺栓的预紧力越大,垫片的Von Mises应力和接触应力越大;垫片材料的弹性模量值对垫片接触应力值的影响较小。     

裂缝性储层压裂水平井压力动态分析

利用点源函数和叠加原理,建立并求解裂缝性储层压裂水平井数学模型,绘制并分析压力动态曲线。结果表明:裂缝性储层压裂水平井的流动形态可分为井筒储存、第一线性流、第一径向流、第二线性流、拟稳态窜流和第二径向流等6个主要阶段;裂缝半长一定下,裂缝间距越大,第一线性流持续时间越短,第一径向流持续时间越长;储能比决定窜流过渡段的凹槽宽度和凹陷深度;窜流系数决定窜流过渡段的凹槽位置。该研究成果可为裂缝性储层压裂水平井的动态分析及试井解释提供依据。     

Effects of high hydrostatic pressure on the mechanical behavior of a crystalline polymer–polyoxymethylene

The true stress-true strain behavior of polyoxymethylene, n(-CH₂O), as an example of a bulk semi-crystalline polymer, has been investigated for constant hydrostatic environmental pressures from 1 atmosphere to 8 kilobars with the principal objectives of elucidating the factors controlling flow and fracture. Experiments were conducted in uniaxial tension at room temperature and constant strain rate. The tensile observations were supplemented by measurements of bulk compressibility and stress relaxation behavior at pressure. In contrast with metals and inorganic compounds, the modulus, yield stress and fracture stress of POM increase strongly with pressure by a factor of approximately three at 8 kilobars. The modulus increase is shown from the stress relaxation measurements to be associated with a pressure-induced increase in the β-transition temperature which points to the potential usefulness of the concept of pressure-temperature super-position of mechanical behavior. The characteristics of the pressure dependence of the yield stress demonstrate that yield criteria based on continum mechanics considerations, including the Mohr or Coulomb-Navier criterion, are not valid for general deformation (non-plane strain) conditions in this polymer. The concept of a critical volume change determining the initiation of yielding is suggested to be applicable to semi-crystalline polymers. Comparison with analogous changes in yield stress with temperature points to an increasing contribution to the control of yielding by the initially disordered regions with increasing pressure or decreasing temperature. The fracture behavior observed at pressure eliminates the concepts of a critical stress as a fracture criterion for POM and of a simple reduction in normal stress at points of stress concentration as the principal effect of the applied pressure on fracture.     

Influence of crack initiation length on fracture behaviors of reinforced concrete

For the purpose of discussing the fracture features of high strength reinforced concrete, under the conditions of the fundamental assumptions to reinforced concrete, the calculating process of fracture parameters for three point bending beam of high strength reinforced concrete is given, and through a total of 16 specimens with the same sizes, with the different initial crack length 40 mm to 100 mm on three point bending beam of high strength reinforced concrete, it is performed to examine the calculation method. The results show that the fracture features of high strength reinforced concrete are different from normal concrete, both the initiation toughness and the unstable toughness of high strength reinforced concrete gradually increase with the crack-to-depth ratios, and the ratios of initial load to maximum load are different depending on the initial crack length, the bigger the crack-to-depth ratios, the lower the ratio, and the ductility is best to the high strength reinforced concrete.