Abstract: The borehole trajectory of extended-reach shale gas wells is complex,and the casing damage problem caused by cement sheath defects often occurs during the fracturing development process.In view of the characteristics of high pump pressure and large displacement in the fracturing development of shale gas wells,using the finite element analysis method,the paper optimizes the model mesh division,and establishes a casing-cement sheath-stratigraphic combination model for the calculation of casing mechanical integrity in complex borehole trajectories of shale gas wells,and finally explores the effects of angle,thickness,and length losses and composite defects of cement sheath on the mechanical integrity of casing in local flexure zones of the slanting and horizontal sections.The results show that the circumferential stress of the casing decreases with the increase of the elastic modulus of cement sheath and is less affected by the Poisson's ratio.The angle loss of cement sheath causes stress concentration in the casing,which is reduced when the missing angle exceeds 90°.With the increasing loss of cement sheath thickness,the stress on the casing increases.The effects of cement sheath thickness,casing size and pumping pressure on casing stress are similar under different missing angles in both inclined and horizontal sections,and the trend is that the casing stress increases and then decreases as the missing angle increases.The missing critical part of cement sheath will produce a sudden change in stress,and this location is also the most prone to casing loss.The maximum value of casing stress increases and then decreases with the increase of angle loss under the same thickness loss,and the maximum value of casing stress increases with the increase of thickness loss under the same angle loss.With the increase of angle loss,the equivalent force of the casing along the pipe length direction corresponding to the missing part of cement sheath first increases and then decreases.The thicker the cement sheath is,the smaller the casing equivalent force will be.Under the same thickness,the thinner the remaining thickness of cement sheath is,the greater the casing equivalent force will be,and the stress level reaches the maximum when the whole cement sheath is missing.

Key words: shale gas well, cement sheath, build-up section, horizontal section, casing mechanical integrity, finite element

摘要： 大位移页岩气井的井眼轨迹复杂,且压裂开发过程中经常出现由于水泥环缺陷引发的套变损伤问题。针对页岩气井压裂过程中高泵压大排量的特点,采用有限元分析方法,通过优化模型网格划分,建立了适用于页岩气井复杂井眼轨迹下套管力学完整性计算的套管-水泥环-地层组合体模型,讨论了水泥环缺失角度、缺失厚度、缺失长度及复合缺陷对造斜段及水平段局部屈曲部分套管力学完整性的影响。结果表明,套管的周向应力随水泥环弹性模量的增大而减小,受泊松比影响较小。水泥环角度缺失会使套管出现应力集中现象,当缺失角度超过90°,应力集中现象得到减缓。随着水泥环缺失厚度的增加,套管所受应力增大。无论是造斜段还是水平段,不同缺失角度下,水泥环厚度、套管尺寸及泵压对套管应力的影响类似,趋势均为随着缺失角度的增加,套管应力先增大后减小。水泥环缺失的临界部位会产生应力突变,该位置也是最容易发生套损的部位。同一缺失厚度下,套管应力最大值随着缺失角度的增加而先增加后减小;同一缺失角度下,套管应力最大值随着缺失厚度的增加而增加。随着缺失角度的增加,水泥环缺失部位对应套管沿管长方向的等效应力先增加后减小。水泥环越厚,套管等效应力越小。在相同厚度下,水泥环剩余厚度越薄,套管等效应力越大,全部缺失时应力水平最大。

关键词: 页岩气井, 水泥环, 造斜段, 水平段, 套管力学完整性, 有限元