Qualifying Bismuth Plug Technology in Mozambique

Abstract

_ Over the past 10 years, bismuth plugs have been used more often to solve complex downhole integrity issues and to permanently abandon wells, both on and offshore. As bismuth technology is relatively new to the market, it often needs to be qualified in-country before use on a larger scale. This is a significant barrier to the adoption of new technologies in general within the industry and not unique to the use of bismuth as a permanent sealing barrier. Bismuth and bismuth-based alloys have unique properties that allow them to be used as a sealing material that has the potential to change the industry. It has a specific gravity of 10 which means it is 10 times denser than water. In molten form it can displace any fluid in the wellbore. Not only is it dense but in its molten form it also has a viscosity like water, which means it can flow easily and through perforations, cracks, crevices, or any micro annuli without the need of any pumping equipment. The most interesting property however is its ability to expand on solidification. BiSN uses proprietary thermite heaters to melt bismuth-based alloys downhole, which then flow into the annulus through perforations or section milled windows taking the shape of volume they are filling up. The alloy on solidification then expands (like ice) which creates the gas-tight seal against the surface it is expanding. These alloys not only create a solid mechanical barrier but are also highly corrosion resistant to H2S, CO2, acids, and other wellbore fluid that might be present, thereby creating a barrier that can last forever or be classified as “eternal.” Compared to existing traditional sealing technologies, a bismuth plug requires a significantly shorter seal, resulting in less materials, section milling, and time needed for deployment. For example, a traditional cement plug might need to be hundreds of feet in length where a bismuth plug can be set at as little as 6 ft. Field Test—Context, Challenges, and Methodology In May 2023, BiSN was contracted to fulfill a rigless plug and abandonment (P&A) campaign for Sasol in Mozambique. Before proceeding, BiSN and Sasol worked together on a well in Mozambique to qualify BiSN bismuth technology as a permanent abandonment solution in the country. The well chosen for the seal test had several complicating factors including the need to set the seal in open hole through perforations and the thin caprocks located in the region. The overall goal of the field test was to isolate gas-bearing silt from shallower formations by forming an effective seal. To accomplish this, the bismuth plug was required to be set with a 10-m shale window identified by Sasol as a suitable location to set a regional seal to essentially restore the caprock. Previous explorations had revealed areas of patchy cement from earlier P&A attempts.