径向孔眼对井壁稳定性影响的数值模拟

Simulation of influence of radial well on well stability

钻井、水力径向射流等作业施工会打破岩石的初始平衡状态,影响井壁、孔眼壁的稳定性。常规数学模型多为二维模型,且假设条件过多,无法真实反映地层的应力分布。借助ABAQUS有限元软件,考虑地应力、孔隙压力等诸多参数,可以真实有效地模拟观察地应力在井壁、井周的分布,克服常规数学模型的不足,对现场施工具有重要的指导意义。通过在ABAQUS中引入最大剪切应力(Tresca)准则、形变比能(Mises)准则及最大主应力(Max Principle)准则,分别评价研究对象的塑性及脆性破坏,分析应力在套管、水泥环及井壁的分布。结果表明:水泥环内外界面存在应力差,内界面为最危险界面;Mises和Tresca应力均于水平最小主应力方向达到极大值,因此水平最小主应力方向最易发生塑性破坏;Max Principle应力均于水平最大主应力方向达到极大值,因此水平最大主应力方向最易达到拉伸极限进而发生拉伸破坏;Mises和Tresca应力均随孔径、方位角、水平地应力差的增大而增大,Max Principle应力随孔径、方位角的增大而减小,随水平应力差的增大而增大。最后,运用灰色关联分析法分析了各影响因素对井壁稳定性的影响权重。

Stability of radial well is influenced by operation of drilling or radial jet due to the initial equilibrium state of reservoir changes. The assumed condition of normal mathematical model is too strict and intricacte to simulate the distribution of stress in reservoir. As the parameters of crustal stress, pore pressure and so on are taken into consideration in ABAQUS software, the distribution of stress around the well and radial perforation can be observed visually, which overcomes the shortcomings of normal mathematical model and is significant for field application. In this article, the tensile failure and plastic failure are evaluated by Tresca, Mises and Max Principle respectively, and also the distribution of stress on casing, cement sheath and rock are analyzed. The results show that the internal interface of cement sheath is the most dangerous interface because of the stress difference between internal and external interface. The maximums of Mises and Tresca are located in the direction of minimum horizontal principal stress, so the direction of minimum horizontal principal stress is at the risk of plastic failure. The maximium of Max Principle is located in the direction of maximium horizontal principal stress, so the direction of maximium horizontal principal stress is at the risk of tensile failure. Mises and Tresca increase as the diameter of radial well, azimuth and horizontal stress difference increase.Max Principle reduces as the diameter of radial well and azimuth increase, and it increases as horizontal stress difference increases.At last, the impact degree of the parameters on stability of radial well is researched by Grey Relation Analysis.