蠕变地层水泥环特性对套管应力的影响

蠕变地层套管应力计算是采取套损预防措施的依据，目前采用非均匀地应力作为套管水泥环力学模型边界条件的套管应力计算方法与实际情况不符。提出了蠕变地层套管应力计算应将均匀地应力作为其边界条件的有限元力学模型，并分析了水泥环椭圆度和弹性模量对套管应力的影响:当水泥环弹性模量小于等于地层弹性模量时，套管最大应力出现在最小水平地应力方向，反之则出现在最大水平地应力方向；套管峰值应力随着水泥环椭圆度的增大而增加，套管由缩颈损坏转变为挤扁损坏；蠕变地层水泥环弹性模量与地层弹性模量和井眼椭圆度之间存在合理的匹配关系，改变水泥环弹性模量是降低蠕变地层套管应力的最有效方法。该规律的得出有助于加深套损机理的认识和套损预防措施的实施。（）

Effect of cement sheath properties on casing stress in creep formation

Casing stress calculations in creep formation are the basis to take casing failure prevention measures.The current casing stress calculation method,which uses non-uniform formation stress as the boundary condition of cement sheath mechanical model,is inconsistent with the actual situation.The finite element mechanics model is put forward,which uses uniform formation stress as the boundary condition of casing stress calculation in creep formation.And the effect of cement sheath ellipticity and elastic modulus on the casing stress is analyzed.When the cement sheath elastic modulus is less than or equal to the formation elastic modulus,the maximum casing stress is in the same direction with the minimum horizontal in-situ stress,and on the contrary,it is in the same direction with the maximum horizontal in-situ stress.The peak casing stress increases as the cement sheath ellipticity increases,so the casing breaking may change from necking to collapsing.The cement sheath elastic modulus has a reasonable matching relationship with formation elastic modulus and borehole ellipticity,and it is the most effective way to reduce casing stress by changing the cement sheath elastic modulus in creep formation.The law would help deepen the understanding of casing failure mechanism and the implementation of prevention measures.