Mitigating gas migration with eutectic bismuth alloy plugs

Gas migration is an important concern to address in oil wells, especially in those that are to be abandoned. Stopping a gas leakage is not a simple task, and this can be particularly detrimental when setting a cement plug, as the migrated gas can undermine the integrity of said plug. In this context, and considering the recent attention given to metal plugs, we investigate the capability of bismuth plugs in shutting off gas leakages. The bismuth alloy employed to form the plug is to be melted downhole, and thus its proper solidification may be compromised if a leakage is underway. We test the sealing capability of two bismuth alloys – the eutectic bismuth-tin and the eutectic bismuth-tin-indium – along with two pipes – acrylic and steel. Results indicate that the bismuth plug can seal off the inner space of the pipe as long as the alloy used is eutectic; otherwise, a channel is created if the alloy is only near eutectic, effectively permitting that the leakage still continues. This sealing capability has been verified for both eutectic alloys tested, although it was also observed that voids are still present within the plugs themselves, potentially undermining their mechanical properties. Furthermore, we also observed that a slow and controlled cooling approach reduces the volume of inner voids, thus mitigating the plug degradation caused by the migrating gas. Further work should verify how these voids impact the hydraulic shear bond strength of the plug. From a microstructural perspective, it seemed that the solidification cooling rate plays a key role in the grain size of the material, which significantly impacts the microstructure of the alloy and, consequently, its mechanical properties. Considering the findings discussed in this work, we recommend that a two-plug approach is employed in leaking wells: a first, eutectic plug would shut down the leakage at the cost of its strength, and then a second – eutectic or non-eutectic – plug would be placed under no leakage ensuring the desired strength and sealability requirements. This study further highlights the potential of bismuth alloys to enhance safety and efficiency in plug and abandonment (P&A) operations by mitigating gas migration issues.