Historically, deepwater offshore Brazil underreaming operations in large hole sizes (bigger than 20 in.) were always considered a significant challenge due to the presence of hard, abrasive interbeded formations in these sections. These challenges were typically associated with premature underreamers cutting structure damage and with bottomhole assembly (BHA) or drillstring catastrophic failures. These situations have caused extra costs to operators to implement remedial solutions, such as enlarging a pre-drilled hole or performing expensive fishing operations.

Downhole vibrations, along with underreamers' cutting structure durability had been the main villains in this type of operation. Recently, many options have been tried by operators and service companies including the selection of various drill bit types to improve drilling dynamics, and the use of non-conventional BHAs with underreamers in tandem to enhance the system's longevity. In spite of these efforts, the problems have remained.

A total systems approach was implemented to overcome these problems in a particularly challenging application offshore Brazil, where in the closest offset seven underreamer runs were needed to enlarge 850 meters. As the goal was maximizing the likelihood of drilling and underreaming the entire section in a single run, the under-reamer was equipped with a specialized heavy-duty cutting structure design. The design consisted of an optimized blade profile with increased cutter density and the latest newest technology in PDC cutters with improved impact and abrasion resistance. Along with the fit-for-purpose underreamer design, it also utilized a concentric expandable stabilizer on top of the underreamer to reduce downhole vibrations and a real-time drilling dynamics service to monitor downhole torque, weight on bit and vibrations to adjust drilling parameters as needed during the operation.

This concept was introduced for the first time in a big-bore project in Brazil. Proper planning and integration between the operator and service company resulted in drilling and underreaming 1,164 meters in a single run, with a total of 304 circulating hours. This accomplishment finalized the 1818-in. × 22-in. section twenty days ahead of the planned AFE.

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