温-压耦合作用下水泥环参数对套管应力影响

The Influence of Cement Sheath Property on Casing Stress Considering Thermo-pressure Coupling Effect

套管应力的准确计算是保证井筒完整性的重要基础;但现有理论直接将地应力加载到模型中,计算所得的位移场包含了地层在原始构造应力下所产生的位移;而此位移并非是钻开井眼后所产生的,这样就会对套管应力的计算产生影响。在原有模型基础之上,消除了原始构造应力所产生的位移,建立井筒组合体温-压耦合模型。根据弹性理论,由界面应力、位移连续性条件,推导了组合体在温度和压力共同作用下的应力解析解。研究了水泥环弹性模量、泊松比和厚度对套管应力的影响规律。结果表明,对于低弹性模量的地层,现有模型的套管应力计算值远远高;对于高弹性模量地层会低。对于不同水泥环泊松比,现有模型计算值低。水泥环厚度较小时,现有模型计算值偏高;较大时则偏低。由此可见现有模型存在一定的差异,尤其是对于低弹性模量地层,需要对现有模型进行修正,才能为水泥环参数设计提供正确的理论指导。

The accurate calculation of casing stress is an important basis for ensuring the wellbore integrity.However,the geo-stress is directly loaded at the boundary into the existing theory model. The displacement field contains the displacement of the formation under the original tectonic stress,which is not generated after drilling.This will be some impacts on the accurate calculation of casing stress. Based on the existing model,the displacement caused by the original tectonic stress was eliminated. A new thermo-pressure coupling model of the wellbore combination was established. According to the elastic theory,the stress analytic solution of the combination was deduced from the interface stress and displacement continuity under the interaction of temperature and pressure. The influences of cement sheath elastic modulus,Poisson＇s ratio and thickness on the casing stress were analyzed. The results show that the casing stress calculated by the existing model is much higher than that by the new model,when the elastic modulus of stratum is small. For the high elastic modulus of stratum,it is lower than that by the new model. For different cement Poisson＇s ratio,the casing stress calculated by the existing model is lower than that of the new model. The smaller the thickness is,the larger the casing stress calculated by the existing model will be. The stress will be lower for the larger cement thickness. It can be concluded that there are some differences between the existing model and the new model. Especially for the low elastic modulus stratum,the existing model is needed to be amended so as to provide the correct theoretical guidance for the design of the cement sheath.