Study on the gas-sealing performance and microstructure of a Sn-Bi alloy and cement composite plugging method for offshore abandoned wells

Conventional cement often fails to maintain integrity in high-temperature, corrosive offshore well abandonments, necessitating more corrosion-resistant plugging materials. Going beyond traditional single-material barriers, this study proposes a novel "cement–Sn-Bi alloy–cement" composite plug system to enhance wellbore sealing. Plugs with various alloy ratios were fabricated and evaluated using a custom gas-sealing apparatus and optical microscopy to investigate coupled composition and temperature effects. Results reveal divergent thermal sensitivities: a 30 °C temperature increase reduced average breakthrough pressures for Sn20Bi and Sn80Bi, but conversely increased them for Sn40Bi and pure Bi. Furthermore, the Sn60Bi plug exhibited a non-linear response due to its lower melting point, initially improving but subsequently degrading gas-sealing as temperatures rose. We conclude that gas-sealing performance is governed by alloy-temperature synergy, meaning optimal compositions must be strictly tailored to specific downhole conditions. By selecting ambient temperature and alloy ratio as variables to systematically investigate Sn-Bi alloys, the fundamental factors affecting the gas-sealing performance of composite plugging specimens were revealed.