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Abstract A unique combination of a matched, steerable, positive-displacement motor and a long gauge PDC bit has recently been introduced. The system was initially designed to address problems of high vibration which were known to be causing downhole tool failures. As the design elements of the system were developed it was realised that many of the problems being attributed to downhole vibration were related to hole quality issues such as borehole spiraling. A new system was designed to alleviate this problem. The new system, known as SlickBore™, used a specially-designed steerable motor and a long gauge PDC bit that, together, retain the steerability of a conventional motor/bit system. SlickBore improved overall drilling efficiencies such as hole cleaning, excessive torque and drag, and poor ROP when sliding. Extensive field tests and subsequent commercial applications have clearly demonstrated the increased efficiencies of this system.
The majority of the initial runs with SlickBore were completed in clastic sequences in the North Sea, Gulf of Mexico, and Far East. When introduced to the Middle-East, the system's performance in the predominantly carbonate sequences of the Arabian Gulf was unknown. Subsequent runs, however, have proved the system's benefits also apply to these formations.
This paper will discuss the theories behind the design of the system and provide examples of the improved performance seen in the carbonate formations of Abu Dhabi.
Introduction The development of this new drilling system known as SlickBore™ began initially as an investigation into the causes of downhole vibrations which were having an adverse effect on downhole tool reliability.1 A reduction in vibrations was achieved as well as an overall increase in drilling rates. Significantly it was seen that it had been achieved with no increase in the power delivered to the bit, i.e. without increased weight-on-bit, or torque. This increase in drilling efficiency was believed to be due the fact that with SlickBore, much less power was being wasted by the generation of vibration or "non-constructive bit behaviour" such as stick-slip, side cutting etc.
Reducing Downhole Vibration
Downhole BHA vibrations are known to be a common cause of the failure of downhole tools such as drilling mud motors and MWD/LWD tools. When these tools fail the resulting trips to replace them can significantly add to overall drilling costs. A study into the causes of downhole vibration was carried out using software algorithms to design BHA's and determine their critical rotary speeds. This data was then validated in the field by the use of MWD vibration monitoring subs. These transmitted warnings of high vibration in real-time but also recorded more detailed information data in the tool memory for more precise post-run analysis.
By using these tools it was possible to reduce vibration. What the data showed was that the vibration was often a result of bit whirl and/or self-exiting vibration from mud-motors. Vibration was particularly bad when back-reaming or circulating off bottom. As both of these scenarios are commonly used to aid in hole-cleaning it followed that an improvement in hole cleaning would reduce the requirement for requiring these high-vibration situations.
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