Deepwater drilling often requires simultaneous hole-enlargement-while-drilling (HEWD) to improve project economics and deliver the required wellbore geometry. World-wide applications have shown that certain formations have caused damage to under-reamer cutting structures through premature cutter wear, particularly in abrasive sections with high formation strength. To mitigate cutting structure damage the operator and service provider need to collaborate to ensure the under-reamer cutting structure is designed for superior stability, PDC cutters are engineered for thermal stability and abrasion resistance, BHA configuration is optimized and drilling parameters are carefully selected and managed. Advanced BHA modeling and real-time vibration measurement are the enabling technologies for breakthrough HEWD performance.

Working off the coast of Brazil, an operator was preparing to HEWD two hole sections (17½??×20?? and 14¾??×16½??) through hard and abrasive formation, which would be encountered at shallower depths than usually expected. An offset well had significant difficulty attempting to HEWD a 17½??×20?? section, wearing out two reamer cutting structures. HEWD was not attempted on the offset's 14¾?? section because of the difficulty with HEWD on the 17½??×20?? section. Offset lithology data suggested the formations were composed of soft shale and hard sand formations, with a few limestone stringers.

To solve the challenges on the new well, the operator and service provider implemented a comprehensive process to identify hazards and evaluate the solutions, which included a unique approach to BHA design and strict parameter management through the problem formations. This paper will discuss the following contributing factors to the successful HEWD operation:

  • Extensive pre-well planning and communication

  • Advanced under-reamer cutting structure design and cutter technology

  • Dynamic BHA modeling and optimization of unique BHA design

  • Drilling parameter testing and selection

  • Utilization of an operational road map to aid well-site personnel in properly managing drilling parameters

The holistic optimization process, along with the operator's best practice, resulted in successful HEWD operations for both hole sections drilling to the targeted depth without premature failures. The cutters on the under-reamers were only mildly worn and the measured stick-slip and vibrations were significantly lower than on offset wells.


In January 2009, the operator began an extensive offshore exploration program in Brazil as operator of the Transocean Deepwater Discovery. The dynamically positioned ship is capable of drilling in water depths of up to 10,000 feet. BP holds interests in 10 blocks offshore Brazil including seven as operator. The company has joined Petrobras on six of these blocks to explore the country's promising offshore areas.

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