TRIAXIAL PRESTRESS DESIGN OF CASING IN STEAM INJECTION WELLS

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.