Anisotropic damage model of wellbore cement sheath for underground salt cavern gas storage

Abstract

Underground salt cavern gas storages have high injection-production efficiency and fast gas injection-delivery switching speed, which has a significant advantage in peak shaving of gas supply. As the weak part of the gas storage wellbore, the cement sheath requires stability analysis. The main motivation for this paper is to propose a model that can characterize the anisotropic damage and use it for cement sheath failure analysis. First, an anisotropic damage evolution model is established based on the principle of thermodynamic irreversibility, and a numerical solution method is given for this model. Then, the model has been verified from three parts: the damage differences in tension and compression and the strengthening effect of confining pressure were verified by mechanical tests; the relationship between anisotropic damage and micro-cracks initiation was verified by test deformation results; a two-stage loading and unloading test was performed to verify the mechanism of anisotropic damage, and the prediction error of cement strength was less than 5%. Based on a user-defined function in the finite element software, the model was developed into a constitutive equation and embedded in the numerical calculation. Finally, the anisotropic damage model is applied to analyze the failure of the cement sheath and to predict the formation of the micro annulus and radial cracks in the wellbore of a gas storage salt cavern. Compared with isotropic damage, the prediction accuracy of the anisotropic damage model for the micro annulus and radial cracks is improved by 50%.