Failure mechanism and control method of cement sheath sealing integrity under alternating thermal-stress coupling in geothermal wells

In geothermal wells, exposure to extreme conditions of alternating temperature-pressure coupling (T-P-C) may lead to repeated expansion and contraction of the casing and cement sheath. This can degrade the cement sheath sealing integrity, resulting in continuous annular pressure. Consequently, an elastic-plastic mechanical model of the casing-cement sheath-formation (C–S–F) system under T-P-C was established based on thermodynamic theory and elastic-plastic theory with consideration of additional heating strain and thermal load, and it is solved by global search in MATLAB. The elastic-plastic mechanical behavior and development of interfacial contact stress are investigated under four typical working conditions. The failure mechanisms include interfacial debonding and micro-annulus formation between the casing and cement sheath under strong alternating temperatures (SAT). Cycling of temperature and accumulated plastic strain caused by the cement sheath entering plasticity are the major causes of interfacial stress inversion, which leads to interfacial debonding during temperature unloading. The influences of the mechanical and thermodynamic parameters of the cement sheath on its sealing integrity are analyzed. Based on this, an integrity control method is proposed, which comprises a cement slurry system and considers cement mechanical and thermodynamic parameters and environmental conditions. The results provide an important reference for the optimal design of cement slurry systems and for preventing cement sheath integrity failures due to SAT.