Comparative Evaluation of Bismuth-Silver and Bismuth-Tin Alloys for Plug and Abandonment

As an alternative to cement, the feasibility of bismuth-tin (BiSn; contains 58 wt% Bi and 42 wt% Sn, abbreviations are not in stoichiometric ratio) as a low melting-point plug has been tested by Zhang et al. (2020) for rigless plug-and-abandonment (P&A) service of offshore wells. Similar to BiSn, bismuth-silver (BiAg; contains 97.5 wt% Bi and 2.5 wt% Ag, abbreviations are not in stoichiometric ratio) also exhibits desirable properties compared with Portland cement. However, because of its greater melting point, BiAg has potentially a wider application than BiSn, especially in deep formations. In the present study, we investigate the feasibility of BiAg alloy for P&A. The bond quality of the alloy-shale cores is evaluated through shear, tensile, push-out, and permeability tests, and compared with those of BiSn alloy-shale and cement-shale cores. To avoid phase change-induced shale damage at elevated temperature while setting BiAg plugs, water was first extracted with supercritical carbon dioxide (CO2). For shear and tensile tests with pinhole-anchored BiAg, the ultimate strength and modulus were measured as a function of anchor points at different temperatures (21, 80, and 110°C). For the push-out tests, shale samples of smooth, rough, and pinholed surfaces were prepared with the BiAg alloy plug. In general, we find that, without anchors, bond failure precedes shale failure. Results for cement-shale cores are also reported for comparison. We contrast the performance of BiAg and BiSn alloys at 21, 65, 80, and 110°C to determine the crossover temperature for deployment suitability.