Analysis on the casing stress of offshore HTHP wells

Casing failure is one of the important factors affecting the development efficiency of offshore high temperature and high pressure (HTHP) wells.So far, the influence of HTHP on casing stress has not been studied deeply.In this paper, wellbore temperature field was calculated accurately.Then, the coupling thermal stress state of casing-cement sheath-formation system was calculated in consideration of the influence of non-uniform in-situ stress, high formation pressure and thermal stress.Finally, the effects of the performance parameters of cement sheath and casing on the casing failure were analyzed.It is indicated that with the increase of radial distance from the casing center, the Mises stress decreases gradually, the radial stress increases firstly and then decreases, and the circumferential stress presents a "skip" change.It is comprehensively determined that the risk of casing failure is the highest at the inner wall.If HPHT is taken into account, the Mises stress of casing and the risk of casing failure will increase.With the change of circumference angle, the maximum values of Mises stress and shear stress appear at the circumference angle of 45°, and the maximum circumferential stress appears at the circumference angle of 0°.With the increase of the elastic modulus and thermal expansion coefficient of casing and cement sheath, the Mises stress of casing increases gradually.The research results are of some guiding significance to the analysis on the casing failure of offshore HPHT wells.