变内压条件下膨胀水泥性能对井筒完整性的影响

以膨胀水泥作为研究对象，利用弹性力学理论，采用有限元方法研究了变内压条件下膨胀水泥性能对井筒完整性的影响。研究表明，合适的水泥膨胀率可以降低套管内的最大米塞斯（Mises）应力；水泥膨胀率越大，水泥环内最大Mises应力越大，最大周向应力越小；有套管内压时，膨胀水泥弹性模量越大，套管内最大Mises应力越小，水泥环内最大Mises应力越大；套管内压较小时，水泥弹性模量越大，水泥环内最大周向应力越小，套管内压较大时则与之相反；膨胀水泥泊松比对套管内最大Mises应力的影响较小，水泥泊松比越大，水泥环内最大Mises应力和最大周向应力越小。对于弹性地层和蠕变地层情况，膨胀水泥性能对井筒完整性的影响规律相似。变内压条件下，膨胀水泥性能对水泥环的挤压破坏和周向拉伸破坏影响较为显著，使用膨胀水泥时应根据实际情况优选膨胀水泥石的各项性能。 

Effects of Properties of Expansive Cement on Wellbore Integrity under Changing Casing Internal Pressure

The effects of the properties of expansive cement on wellbore integrity have been studied using finite element method based on elastic mechanics theory. The study shows that cement with appropriate rate of expansion will reduce the maximum Mises stress inside casing. The higher the rate of expansion of cement, the higher the maximum Mises stress in side cement sheath, and the lower the maximum circumferential stress. When pressure exists inside casing, the higher the elastic modulus of the expansive cement, the lower the maximum Mises stress inside casing, and the higher the maximum Mises stress inside cement sheath. At lower casing pressure, the higher the elastic modulus of cement, the lower the maximum circumferential stress inside cement sheath, contrary to the situation in which the casing pressure is higher. The effects of Poisson ratio on the maximum Mises stress inside casing are lower. The higher the Poisson ratio of cement, the lower the maximum Mises stress and the maximum circumferential stress inside cement sheath. Similar pattern of the effects of the properties of expansive cement on the integrity of wellbore is found for elastic formations and creep formations. Under the circumstance of changing casing pressure, the properties of expansive cement exert greater influences on the compressive failure and circumferential tensile failure of cement sheath. These conclusions suggest that, in selecting expansive cement, the properties of the expansive cement should be taken into account, based on the actual application requirements.