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油气田地应力可分为均匀和非均匀两类，相应的井眼和水泥环形状分别为圆形和椭圆形。文章应用有限元方法对不同地应力条件下水泥环形状对套管应力的影响进行了研究。得出结论认为：在均匀地应力条件下，圆形水泥环对套管受力是有利的，应在钻井过程中尽量保持井壁稳定;在非均匀地应力条件下，如果固井质量良好，那么椭圆形水泥环可使套管最大应力及受力不均匀程度有所下降，对套管保护有利，应当在设计与钻井作业中予以利用。另外，数值计算结果表明，在非均匀地应力条件下，套管偏心对套管应力载荷的影响并不大，但尽量保持套管居中仍是必要的。

INFLUENCE OF CEMENT SHEATH SHAPE ON CASING STRESS UNDER DIFFERENT GROUND STRESS CONDITIONS

The ground stresses in oil and gas fields may be divided into two categories,i.e.the homogeneous and inhomogeneous ground stresses,and the relevant shapes of the borehole and cement sheath are circular and elliptic respectively.The influence of cement sheath shape on casing stress under different ground stress conditions was studied by use of the finite element method.It is concluded that,under homogeneous ground stress condition,the circular cement sheath is beneficial to the casing load,so the borehole wall stability should be kept as far as possible in the process of drilling;and ,under inhomogeneous ground stress condition,the elliptic cement sheath is helpful to reduce the maximum casing stress and the inhomogeneous degree of casing load if the cementing qulity is good,which is favourable to protecting casing and should be applied to well design and drilling operation.In addition,through numerical calculation it was indicated that,under inhomogeneous ground stress,the influence of casing eccentricity on casing load was unnoticeable,but it is still necessary to keep the casing string at the center of the hole as far as possible.