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 nonconventional 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 18 1 8 -in. ϫ 22-in. section twenty days ahead of the planned AFE.

Historically, casing while drilling (CWD) has not been largely deployed in Brazil land operations due to inconsistent results achieved in the past and also due to the lack of a clearly identified technical need. In some onshore fields located in the Northeast of Brazil, total or partial mud losses in many shallow surface sections, make those sections very expensive to drill. Apart from the large volume of mud required when lost circulation occurs, which heavily impacts the operation logistics, and the cost associated with remedial actions such as cement plugs and lost circulation materials, HS&E risks are typically the major concern. To validate a technically viable casing while drilling solution with a non-retrievable bit, a series of three surface hole sections were drilled through this severe lost circulation zone. This paper will describe the planning, implementation, results and lessons learned from the first three surface non-retrievable CWD onshore operations done in Brazil. These wells were drilled by land rigs equipped with a kelly and the operations were performed with specialized 12 1/4-in non retrievable casing bits and 9 5/8-in surface casing. It is important to mention that to perform these operations did not require any modification to the rigs. The results showed that the casing while drilling technique has considerable applicable in surface sections that are Polycrystalline Diamond Compact (PDC) drillable and where fluids losses make conventional drilling practices difficult or expensive. However, in those applications the casing bit design must be tailored to drill efficiently through the medium to hard tophole carbonates, to ensure reaching the desired casing point.