Antistall technology (AST) is a mechanical downhole solution that aims to adjust the drilling torque automatically in real time. Originally, the tool was developed by Tomax AS for coiled-tubing applications where it has proven its ability to successfully reduce vibrations, motor stalls, equipment failures, and general wear, in addition to increasing the penetration rate and run length (Dagestad et al. 2006). The tool was then developed further based on the need for a similar solution for rotary drilling. The goal was to eliminate cutter-induced torque variations and string stalls in difficult formations and resultant harmful effects. Prototype AST tools were made in sizes ranging from 6¾ to 8¼ in. The tools were then run in test wells and later in field operations with a variety of tool configurations until the database, in addition to two controlled trials, counted 25 regular jobs—mainly on the Norwegian Continental Shelf. The paper describes in detail, both based on theory and on field experience, how the bit-induced torque fluctuations are significantly decreased to improve penetration, and how bottomhole-assembly (BHA) damage is prevented to increase run lengths.

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