Coupled Thermo-Mechanical Analysis of Bismuth Plug Deployment in Wellbore

Plug and abandonment (P&A) operations are a critical step in ensuring long-term well integrity and mitigating environmental risks. Bismuth-based alloys have emerged as promising alternatives to cement plug barriers due to their impermeability, corrosion resistance, and expansion upon solidification. However, their deployment requires in-situ melting at elevated temperatures, typically generated by thermite heaters, which may impose severe thermo-mechanical loads on surrounding wellbore components. Despite the increasing interest in bismuth plugs, limited research has systematically quantified the effects of these thermal loads on wellbore integrity. In this study, a fully coupled thermo-mechanical finite element model was developed to investigate the influence of elevated temperatures on the casing, cement sheath, and surrounding formation under shallow-depth conditions. Temperature-dependent elasto-plastic and damage-plasticity constitutive models were employed for the casing, cement and rock formation, respectively. A systematic sensitivity analysis was conducted by varying thermal and mechanical boundary conditions, including in-situ temperature, heating and holding times, maximum applied temperature, and in-situ stress. The results reveal that while the casing remained in the elastic regime and the formation experienced no failure, the cement sheath was highly susceptible to both tensile and compressive damage under rapid heating and elevated maximum temperatures. Lower heating rates and higher in-situ stresses mitigate failure. Results show that mechanical degradation from high temperatures was not the main cause of damage initiation. Instead, cement failure was mainly driven by casing expansion and the cement lower thermal conductivity compared to the casing, which together produced localized tensile stresses. The maximum temperature and heating rate were the factors affecting cement integrity, while holding time had no noticeable effect. These findings underscore the importance of controlling thermal input during bismuth plug placement to prevent cement failure and highlight the need for optimized deployment strategies. The study provides an insight for understanding thermo-mechanical interactions in shallow-depth P&A operations and offers guidance for improving the safety and reliability of bismuth-based sealing technologies.