Prestress analysis of casing strings in heavy oil thermal production wells

Based on the elastic theory,this paper derived the triaxial stress formulas of casing exposed to heating steam under thermal stress and non-uniform geologic stress,and established a new method to design the prestress of casing.The casing’s effective stress should be under the minimum yield.Through analyzing the prestress design of N80 casing which was often used in Liaohe Oilfield,this paper also gave the ratio of diameter to wall thickness and the corresponding prestress of N80 casing at different steam injection temperature and well depths.The study manifests that the safeness of casing can be enhanced by pulling up prestress properly and the conventional uniaxial prestress design has evident defects.As thermal stress,non-uniform geologic stress and effect of sand production on stress are considered in triaxial prestress design,it is suggested that triaxial prestress design is adopted in heavy oil thermal production wells.