Analysis on the elastoplastic of cement sheath in underground gas storage wellbore

In this paper, the casing-cement sheath-surrounding rock assembly was taken as the study object to reveal the stress and deformation of cement sheath in underground gas storage wellbore in the case of continuous injection and production. The analytical solutions to the elastoplastic stress and displacement of cement sheath subjected to internal and external pressure in the working conditions of injection and production were derived based on the basic principles of elastoplastic theory combined with the influential factors, e.g. temperature, pore pressure and far-field in-situ stress. The calculation examples show that in the process of gas injection, the cement sheath may deform plastically and the radial contact stress on the inner and outer wall increases nonlinearly with the increase of internal pressure. The variation of temperature and pore pressure during gas injection and production has significant effect on the stress of cement sheath. The elastic limit internal pressure of cement sheath increases with the increasing of pore pressure and the decreasing of temperature. As temperature and pore pressure increase, the radial contact stress of cement sheath on the casing and surrounding rock increases. The research results provide the technological support for optimizing underground gas storage operating parameters and preventing the formation of micro-annulus.