The effect of well temperature on the microstructure and the mechanical performance of bismuth-based plugs in well plugging and abandonment operations

Bismuth-tin (BiSn) alloys are considered promising candidates as potential alternatives to cement for well plugging and abandonment (P&A) applications to prevent any uncontrolled flow of deep reservoir fluids to the surface. As the temperature in the well varies with depth, the final microstructure of the BiSn alloy after its solidification might be influenced, and consequently the mechanical performance of the alloy will also be affected. The aim of this study is to evaluate the microstructural evolution of the BiSn alloy after solidification, at temperatures between 20 °C and 90 °C, to reflect in situ conditions from approximately 800m to 3 km depth. Microstructural characterization was carried out by using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The microstructural analysis revealed that the plug subjected to the lower temperature resulted in finer microstructure, whereas the grain size of the material was increased at higher temperatures. Additionally, the theoretical lever rule was employed to validate the microstructural findings. The observed microstructures were also correlated with tensile tests, which showed that the plug at 20 o C had a yield strength of 67,25 MPa, whereas the yield strength of the plug at 90 o C was only 41,64 MPa. In addition, hydraulic push-out tests further confirmed that lower testing temperatures correlate with higher pressure tolerance of the plug. The plug at 20 o C could withstand 113,5 bar, compared to the plugs at 60 o C and 90 o C which could withstand 90,4 and 50,9 bar, respectively. From all the experimental tests conducted, the outcome was that the lower temperature of 20 o C resulted in a material with a finer microstructure, which plays an important role in enhancing the mechanical properties of the bismuth-based plug. This study aims to advance the understanding of BiSn alloy performance in plug and abandonment (P&A) operations, thereby supporting the industry's efforts to qualify BiSn alloys as effective well-barrier materials. • Exploring a BiSn alloy as a potential substitute for cement in Plugging and Abandonment applications of oil and gas wells. • Investigating the effect of well temperature (20, 60 and 90 o C) on the behavior of BiSn alloy. Changes in well temperature lead to variations in the microstructure, consequently directly impacting the mechanical properties of the BiSn alloy; tested with hydraulic push-out tests and tensile tests. • The lower temperature, 20 o C results in finer microstructure of the BiSn alloy and consequently in 67,3 MPa yield strength comparing to 41,64 MPa at 90 o C. 113,53 bar the hydraulic pressure for the plug at 20 o C and 50,9 bar for the plug at 90 o C. • The microstructure is explained by analyzing the phase diagram and using SEM and EBSD techniques.