Numerical analyses of coupled thermo-mechanical behaviors of cement sheath in geothermal wells

The stability of cement sheath under high temperature and high pressure is one of the most critical issues for the durability of geothermal well systems. In this study, a two-dimensional plane-strain finite element code was developed to investigate the coupled thermo-mechanical behaviors of the casing-cement-formation system. Different from previous linear elastic analyses, a thermoelasto-plastic constitutive model based on the thermodynamic theory was adopted for the cement sheath. It is shown that the finite element simulations using the proposed model provide a more accurate and realistic prediction of stress–strain responses of the cement sheath under high temperature. The results demonstrate that the radial stress concentration and the tensile strain concentration occur at both the cement–casing interface and the cement–formation interface, where the cement sheath is most likely to fail. High strength and low stiffness in the cement sheath and the formation are preferred for the integrity of the system. Both large thermal cycles and large differences between the internal fluid pressure and the external pressure should be avoided during operation. The new code is an alternative tool for guiding the geothermal well design. The finite element framework described herein is universal for other thermo-mechanical applications, such as energy foundations and energy tunnels.