压力和温度共同作用下的水泥环应力分析

高温高压井中井筒压力和温度变化较大,会给水泥环的力学性能带来严重影响.基于弹性力学多层厚壁圆筒理论,建立了耦合压力、温度的水泥环力学理论模型,研究了水泥环中径向应力和切向应力的分布规律.研究表明:持续套管压力值为10~50 MPa且井筒温度变化值为-50~50 ℃时,水泥环总体上沿径向受压,沿切向受拉,二者最大值均位于水泥环内壁;井筒温度变化值一定时,水泥环中径向压应力和切向拉应力随持续套管压力值的增大而增大;持续套管压力值一定时,水泥环中径向压应力和切向拉应力随井筒温度的升高而增大,随井筒温度降低而减小;井筒温度升高会加剧水泥环的切向受拉失效风险,井筒温度降低却可能导致水泥环径向密封失效.因此,高温高压井应考虑井筒温度变化对水泥环应力的影响,建议采用低弹性模量水泥浆固井,并提高水泥环与套管和地层的胶结强度,以提高水泥环的整体力学性能. 

Stress Analysis of the Cement Sheath under both Pressure and Temperature

Significant changes of the wellbore pressure and temperature in HPHT wells may bring great challenges to the mechanical integrity of the cement sheath.On the basis of the multi-layer thick wall cylinder theory of elastic mechanics,this paper proposed a mechanical theoretical model of cement sheath under both pressure and temperature,and investigated the distribution of radial and tangential stresses within cement sheath.Results of studies demonstrate that when the sustained casing pressure(SCP)is 10-50MPa and wellbore temperature change(WTC)is -50-50℃,the cement sheath is generally radially compressed and tangentially tensioned,and the maximum compressed and tangential stresses are both at the inner wall of the cement sheath.When the WTC keeps constant,both stresses increase with the increase of the pressure inside the casing.When the SCP is constant,both stresses increase with the increase of the WTC and decrease with the decrease of the WTC.A wellbore temperature increase will aggravate the possibility of tangential tensile failure in cement sheath,and a wellbore temperature decrease will lead to its failure due to radial tension.Consequently,wellbore temperature changes should be considered for their effect on the stress of the cement sheath,and the cement with lower elastic modulus and higher bond stength with casing and formation should be used to enhance the comprehensive mechanical performance of cement sheath in HPHT wells.