The Effect of Bismuth Plug Expansion on Cement Sheath Integrity

Abstract In the North Sea, around 2000 off-shore wells are scheduled to be plugged-and-abandoned over the upcoming decades, requiring new cost-effective solutions. Among those, bismuth alloy plugs are raising a major interest, as they have high sealing abilities and require less material volume than cement plugs. Bismuth is well-known for its expansion during solidification as its main sealing mechanism. In this work we investigate whether this expansion may damage the annular barrier material set behind the casing. In this study, we examine a laboratory-scale well section composed of a sandstone rock (diameter 15 cm and height 20 cm), a 2mm thick casing, and a temperature-resistant silica-Portland G cement blend placed in the annulus and cured at 110°C for one week. The setup has been designed to allow for bismuth alloy placement in the casing and curing at relevant field conditions: 160°C and 60 bar. With an X-ray Computed Tomography (CT) scanner, we take 3D pictures of the sample with a resolution of 100 µm to evaluate the development of any cracks. To assess the impact of bismuth expansion on cement and separate it from temperature and pressure effects, we conduct X-ray imaging at various stages. Initially, we image before placing bismuth where we examine the cement and rock for existing flaws, like voids and cracks, right after the cement cures. Next, we expose the sample to 160°C and 60bar with water inside the pipe to check whether the annulus cement can withstand the testing conditions, followed by imaging. In the third phase, we pour the Bismuth alloy into the pipe and leave it to cool at standard conditions. At this stage, the alloy is not expected to expand much in the radial direction. Following these initial scans, we remelt the alloy inside the pipe at 60bar and 160°C using an in-house designed test cell, and then take CT images after 8 hours of curing under 60 bar, and one after 4 days and another after 4 weeks. Finally, we perform a final CT scan after melting out the bismuth alloy to verify the integrity of the annulus cement. In this study, we address a major concern related to bismuth alloy plugs, specifically, the impact of their expansion on the pre-existing well barrier materials such as the cement in the annulus. The findings of this study will be used to refine our experimental methodology at later stages in this extensive research and support the qualification process of bismuth alloys as an alternative sealing material in P&A operations.