注汽井套管的三轴预应力设计

TRIAXIAL PRESTRESS DESIGN OF CASING IN STEAM INJECTION WELLS

以暴露于热蒸汽中的目的层套管为研究对象,根据弹性力学理论,首先给出了套管在热应力和非均匀地应力作用下的三轴应力表达式,然后利用对套管施加预应力来降低套管的轴向热应力,以使得套管的有效应力控制在相应温度下套管的最小屈服极限内为原则,得到了热采井套管三轴预应力设计方法,并对辽河油田常用的N80钢级套管进行了预应力设计分析,给出了该套管在不同注汽温度和井深条件下所应使用的径厚比及相应的预应力值。研究表明,在实际注汽过程中适当增加注汽压力可以提高套管的安全性,传统的单轴预应力设计及只考虑热应力的三轴预应力设计结果偏于保守,建议以后注汽井在进行预应力设计时应考虑热应力和实际非均匀地应力的共同作用。

Based on elasticity theory, the triaxial stress formulas of casing exposed to thermal steam under thermal stress and non-uniform in-situ stress were derived. According to the principle that casing＇s Mises stress must be under the minimal yield strength, a new method was established to design the prestress of casing. Through designing the prestress of N80 casing which is often used in Liaohe Oilfield, the ratio of diameter to wall thickness and the corresponding prestress of N80 casing at different steam injection temperature and well depths are proposed. It is shown that the safety of casing can be enhanced by increasing steam injection pressure properly. The conventional uniaxial prestress design and the design of triaxial prestress that only consider thermal stress are conservative. It is suggested that the design of triaxial prestress considering thermal stress and non-uniform in-situ stress be used in steam injection wells.