体积压裂套管剪切变形载荷分析及强度设计

页岩气开发过程中井筒穿越天然裂缝/断层,体积压裂时天然裂缝/断层容易被激活,导致套管剪切变形,套管内径缩小,井下工具无法下入。目前对套管剪切变形条件下的受力分析较少,现有研究主要采用套管Mises应力屈服准则判断套管失效,该准则对于套管大剪切变形问题已不适用。为探究天然裂缝/断层滑移导致的套管剪切变形问题,建立套管-水泥环-地层组合体有限元模型,对位移载荷作用下的套管力学行为进行研究。分析了各因素对套管受力及缩径量的影响,并提出套管缩径量与应力相结合的套管外载设计方法。研究结果表明:套管剪应力及Mises应力随地层滑移量增大而增大,在10 mm滑移量作用下,套管Mises应力已超过屈服强度,建议采用剪切载荷进行强度校核;套管缩径量主要集中在距离滑移面1 m范围内,套管局部区域出现膨胀现象;考虑套管变形量的载荷设计可以有效防止体积压裂过程中套管变形导致压裂工具无法下入的问题。

Research on Casing Shear Deformation Load and Strength Design during Volume Fracturing

In the process of shale gas development, the wellbore crosses natural fracture/fault, which are easily activated during volume fracturing, resulting in casing shear deformation, casing inner diameter reduction, and the failure of downhole tools to enter. At present, few stress analyses have been conducted on casing shear deformation. Existing researches mainly adopt the Mises stress yield criterion to judge casing failure,which is not suitable for large shear deformation of casing. To research the casing shear deformation caused by natural fracture/fault slip, a finite element model of casing-cement sheath-formation is established, and the mechanical behavior of casing under displacement load is studied. The various factors on casing stress and shrinkage are analyzed, and a casing load design method combining the reduction of diameter and stress is proposed. The results are as follows: (1) casing shear stress and Mises stress increase with the slip distance of formation, and the casing Mises stress has exceeded the yield strength under the 10mm slip distance. It is recommended to use shear load to check the strength; (2) casing shrinkage is mainly concentrated within 1m distance from the slip surface, and the casing expansion occurs in the local area; (3) load design considering the casing deformation can effectively avoid the failure of fracturing tools to run normally during volume fracturing.