Cement sheath integrity analysis of underground gas storage well based on elastoplastic theory

For injection & production well built in underground gas storage (UGS), sealing failure of cement sheath is one of the major factors threatening wellbore integrity due to continuous varied casing pressure and temperature. Cement sheath may deform plastically in the process of gas injection and lose airtightness if unloading casing pressure. In view of this, an analytical model of the stress, displacement and interaction of casing-cement sheath-surrounding rocks subjected to inner casing pressure and outer radial far-field in-situ stress and debonding of cementing surfaces were derived with due consideration of varied casing pressure, temperature and pore pressure due to cyclic injection and production. In this model, casing, cement sheath and surrounding rocks were seen as multi-layer cylinders and were analyzed based on thermoelastic, elastoplastic and poroelastic theory, respectively. A demonstration calculation is performed to reveal the stress and displacement of cement sheath under conditions of cyclic loading and unloading. The model was compared and validated by finite element simulations. Factors affecting the interaction of cement-casing and cement-rocks and the formation of micro-annulus were analyzed. As a result, when loading casing pressure, the radial contact stress on the bonding surfaces increases nonlinearly and cement sheath may deform plastically. In the process of unloading casing pressure, if low far-field in-situ stress exists, the radial contact stress tends to transform from compression to tension leading to the separation of bonding surfaces. The variation of temperature, pore pressure and far-field in-situ stress are key factors affecting the stress & deformation of cement sheath and micro-annulus. For cement with large elasticity modulus and small Poisson's ratio, plastic deformation and micro-annulus are prone to forming. Thus, the reasonability of the application of ultra-high toughness cement in UGS well has been verified. The results are of guiding significance to the operating optimization, micro-annulus prevention and wellbore integrity improvement of UGS wells.