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Evaluation of Bismuth-Tin Alloy for Well Plug and Abandonment

As an alternative to cement, bismuth-tin alloy (BiSn) containing 58-wt% bismuth (Bi) and 42-wt% tin (Sn) has been proposed for plug and abandonment of offshore wells. Alloy seals are advantageous over cement because they can be significantly shorter, reducing well-abandonment cost. Besides being able to deliver alloy pellets and melt them in situ, adhesion with common lithologies found in hydrocarbon reservoirs is also necessary. In addition, within the likely extremes of stress variability, the bond should be sufficiently strong to withstand the differential pressure across the plug. In this paper, we present a series of tests conducted in our laboratory for evaluating the alloy/rock bond. To overcome the intrinsic lack of chemical affinity and the resulting nonwetting nature of the alloy, we propose capillary intrusion into the porous-network of the rock. Above the capillary entry pressure, penetration distance by intrusion is controlled by available alloy volume. As a function of intrusion, we measure the quality of the bond through shear and tensile tests. Results for rocks, cement, and cement/limestone cores are also reported for comparison. Isolation quality is evaluated through permeability tests, performed both parallel and perpendicular to the macroscopic bond interface. For shale, we propose a surface treatment before plug placement that also prevents plug displacement under differential pressures. Strength is best achieved by straddling permeable media and shale. For deployments within tubing or casing, similar bond-strength evaluations can be conducted.