Elastoplastic Analysis of Cement Sheath of Injection & Production Wellbore Under Varied Casing Pressure

摘要

Sealing failure of cement sheath is one of the major factors threatening the integrity of underground gas storage (UGS) injection & production well due to continuously varied casing pressure and temperature. Focusing on the casing-cement sheath-surrounding rocks assembly, this paper derives the analytical solutions to elastoplastic stress and displacement of cement sheath subjected to inner casing pressure and outer radial far-field stress based on basic principles of elastoplastic theories with due consideration of casing pressure loading and unloading. In particular, the analytical model of micro-annulus due to plastic deformation in the process of casing pressure unloading is established. A practical case study is conducted and the results show that radial contact stress on the bonding surfaces of cement sheath increases nonlinearly and plastic deformation develops when casing pressure is loaded. When unloading casing pressure, if low far-field in-situ stress exists, the radial contact stress may transform from compression to tension leading to the separation of bonding surfaces. The mechanical parameters and far-field in-situ stress affect the sealing failure of cement sheath significantly. The reasonability of ultra-high toughness cement in UGS injection & production well has been verified. The results are of guiding significance to operating parameters optimization and micro-annulus prevention of UGS wellbore.