水泥环缺陷对套管强度安全性的影响分析

Influences of Cement Sheath Defect on the Safety of Casing Strength

固井过程中造成的水泥环缺陷是不可避免的,并会对套管强度安全性产生影响.考虑水泥环在实际工程中的缺失形态,从缺失环向开度、缺失厚度以及缺失位置等角度综合考虑,利用有限元数值计算方法,建立套管-含缺陷水泥环-地层模型.考虑套管服役时内压和非均匀地应力的作用,对水泥环缺失部位套管应力进行模拟,重点讨论不同环向开度缺失的水泥环对套管应力的影响规律以及不同位置和厚度缺陷的水泥环对套管应力的影响规律.分析表明:套管在水泥环缺口处的应力分布情况根据环向开度大小分为两种情形,即环向开度小的应力集中在缺口中心位置,环向开度大的应力集中在缺口边缘位置;水泥环缺失环向开度为65°时套管应力值可达到最大;水泥环内壁缺失厚度变化对套管上的应力波动影响较小,外壁缺失厚度变化对套管上的应力波动影响较大.本算例中套管应力最大值在水泥环内壁缺失时比在外壁缺失时高13%,说明套管应力极值受水泥环内壁缺失时影响较大.上述结论可为固井过程中判断套管强度安全提供依据.

Cement sheath defects caused during well cementation process are inevitable and have an impact on the safety of casing strength.Considering the defect forms of cement sheath in practical engineering,from the perspectives of circumferential opening degrees,defect thickness and position,the finite element numerical calculation method is used to establish a model of casing-defective cement sheath-stratum.Considering the effect of internal pressure and non-uniform ground stress during service,the casing stress of the cement sheath defect part is simulated.The influence law of circumferential opening degrees of cement sheath defect on casing stress and the influence law of different positions and thickness of cement sheath defects on casing stress are discussed.The analysis shows that the stress distribution of the casing at the gap of the cement sheath is divided into two situations according to the sizes of the circumferential opening,that is,the stress with a small circumferential opening concentrated in the center of the gap while the stress with a large circumferential opening is concentrated at the edge of the gap.The casing stress can reach the maximum when the circumferential opening is 65°.The change of defect thickness of the cement sheath inner wall has a little effect on the casing stress,while defect thickness of the cement sheath outer wall has a great effect on the stress fluctuation on the casing.In our example,the maximum casing stress of the inner wall defect is 13%higher than the outer wall defect,indicating that the extreme value of the casing stress is affected more by the defect of the cement sheath inner wall.The above conclusions can provide a basis for judging the safety of casing strength during well cementation process.