软岩层引起的套管外载计算方法研究

STUDY ON COMPUTATION METHOD FOR CASING LOAD INDUCED BY SOFT ROCKS

在我国部分油田中,软岩层流变致使套管挤毁,造成巨大经济损失。以黏弹性流变模型理论为基础,建立软岩层、水泥环和套管流变耦合的力学模型。从材料本构关系、几何变形协调、静力平衡三方面进行分析,推导出均匀地应力下水泥环和套管流变外载的表达式。编制相关计算程序,研究软岩层流变力学特性、井眼的初始蠕变和水泥环对套管外载的影响规律。研究结果表明,软岩层流变力学特性、井眼的初始蠕变、水泥环力学参数对套管外载的影响较大。井眼的初始蠕变可释放部分流变载荷,改善套管受力;水泥环使套管受力恶化,在井眼环空不注水泥能延长套管使用寿命。所得结论与一些油田的现场实践相吻合,研究成果已成功应用于江汉油田岩盐层固井和套管设计。

Casing failures are often caused by the rheological load generated by the soft rocks in China,which give rise to large economical loss.Based on the theory of the viscoelastic rheological model,the coupled mechanical model of the soft rocks,the concrete sheath and the casing is developed.Hence,the equations to compute the rheological load of casing and concrete sheath under the uniform in-situ stress are derived according to the analysis of constitutive law,geometric deformation compatibility,and static equilibrium.The software is developed;and the correlation between the casing load and the rheological parameters of the soft rocks,initial creep of the wellbore,mechanical properties of the concrete sheath is obtained.The results indicate that the rheological mechanical properties of the soft rocks,initial creep of the wellbore,mechanical properties of the concrete sheath have large effects on the casing load;and initial creep of the wellbore can reduce the rheological load and decrease the casing stress;however,no concrete sheath in the annulus can undergo the casing service life because the concrete sheath can increase the casing stress.The achieved results are coincident with the field data,which are already used for cementing and casing design in the Jianghan Oilfield,China.