基于无网格法的井下岩层复合射孔爆燃气体劈裂过程分析

Analysis of fracture process of the rockness in the oil well, drivened by the detonation gas during the complex perforation is a complex coupling problem includeing multiphase and multiphysical fields.At present,many modeles based on the quasi-stationary theory can not describe the complex process of gas flow and crack propagation truely,because the instantaneous change of gas,the dynamic response of rockness and their coupling relationship were simplfied and neglected.In this study,we conside those dynamic influence and analysis whole fracture process by meshless method mainly.Firstly,a theoretic model of gas flow in preliminary crack is established according to the flow property of detonantion gas in oil well.A pressure distribution function from analytic results of gas flow along the crack faces is concluded.Then,a new model simulated the crack propogation of the rockness under detonation gas loading based on the meshless method is presented.In this model,enriched meshless methods for crack approximations based on the idea of partition unity method and Meshless Local-Petrove procedure are developed,using a jump function by means of the displacement discontinuity along the crack faces and the asymptotic solution enrichment near the crack tip.With this model only nodal data are used to describe the crack,no geometrical entity is introduced for the crack trajectory,and no partitional differential equations need to be solved to update the level sets.Thirdly, using the both models introduced forward,coupling process between denotion gas flow and crack growing of rockness in the oil well is solved based on the combination of the fliud-solid governing equations.The mixed-mode stress intensity factors are evaluated by the interaction integral method. And the crack opening dispalcement and growing path are calculated by referencing relationship between crack normal displacment and pressure along the crack in gas flow model.Lastly,sevelal principle factors are discussed by numerical test ,and found out systematic measures for fracture works in oil well. This study not only does good to realize fracture nature of rockness with crack deeply,improve the level of study ,but also can give better advises for oil engineering and parameter optimization of the complex perforation.

井下岩层复合射孔爆燃气体劈裂过程分析是一个多相、多物理场的耦合作用问题，传统有限元在求解时面临诸多困难。本项目在流体力学、断裂力学和岩体力学的理论基础上，引入单位分解扩展无网格法，重点研究爆燃气体在裂缝内的流动规律、爆燃气体对含裂纹岩体的劈裂作用以及裂纹扩展对爆燃气体压力重分布的耦合机理，开发由复合射孔爆燃气体流动分析、岩体动态响应的无网格分析，裂纹动态扩展的无网格分析三大模块组成的软件包，讨论爆燃气体驱动裂纹扩展过程中井下岩体的受力特点，分析复合射孔技术参数、不同储层条件对裂纹形态的影响。与许多准静态理论模型相比，本项研究着重考虑缝内高压气体的瞬态变化、含裂纹岩体的动态响应以及它们之间的耦合作用，能较真实地反映气体流动和裂纹扩展的复杂过程。本项研究是岩土力学的前沿交叉课题，不仅有利于深入认识岩体断裂机理，提高致裂作用理论研究水平，而且能更好地指导工程实践，推动复合射孔参数的优化设计。

随着国民经济的快速发展和石油资源日益枯竭的现状，加强对低渗透油、气层的勘探和开采具有重大的战略意义。本课题借助无网格数值方法，对爆燃气体作用下井下岩层的劈裂过程开展数值模拟和分析。主要工作和重要结论为：（1）从单位分解思想出发，发展了单位分解扩展无网格法。该方法是求解岩石断裂力学十分有效地数值工具，不但继承了无网格Galerkin法（EFGM）计算精度高的特点，而且可以准确描述含裂纹岩体的不连续位移场和裂尖奇异场；（2）针对含裂纹岩体的裂纹动态扩展问题，提出了水平集与无网格耦合法。采用相互正交的两个水平集函数，隐式描述裂纹并自然地在裂纹尖端构成局部坐标系。与传统的无网格法相比，水平集与无网格耦合法无需使用可视法、衍射法或透明法，克服了这些方法在裂尖处人为引入的不连续且能很好地再生裂纹尖端奇异应力场场，提高了弹性问题的收敛性；节点影响域不受裂纹线切割的影响，在计算中往往使用较小的影响域，保持了整体刚度矩阵的带状、稀疏性；另外，水平集简化了附加节点的选取和附加函数的建立，其更新过程无需求解偏微分方程，实现简单且易于编程。数值算例表明，该方法模拟的裂纹路径与试验结果吻合的非常好，是追踪裂纹扩展十分有效的方法；（3）针对岩石上、下裂纹面的接触摩擦问题，提出了单位分解扩展无网格线性互补模型。该模型将接触方程转化为互补方程，使接触问题转化为线性规划问题求解，从而抛开了引入人工参量和繁琐的开、闭迭代运算。数值计算结果表明该方法可以有效地控制裂纹面间的相互嵌入，同时在裂纹面上可以得到与实际情况相同的应力分布；（4）提出了可求解多裂纹问题的改进无网格法，研究了多条次裂纹的长度、倾角对主裂纹应力强度因子的屏蔽作用；（5）研究不同水化学溶液对钙质砂岩力学特性和能量机制的影响,利用WDT–1500多功能材料试验机对化学腐蚀后饱和状态与自然状态钙质砂岩进行单轴压缩试验。试验结果表明:水化学溶液对钙质砂岩的强度特性与变形特性影响显著,腐蚀后其强度特征与抗变形能力均呈现不同程度的劣化,并具有明显的时间阶段性,化学腐蚀后砂岩有具有明显的脆性向延性转化的特征。

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