Abstract

Conventional PDM (Positive Displacement Motors) are widely used in coiled tubing drilling jobs. PDC bits are often used with the PDM's because of their longer life, increased ROP, and increased steerability. But conventional PDM-PDC bit systems often create hole problems and high vibrations that result in casing running problems, downhole equipment failure and lost drilling time.

A new steerable assembly has been designed that provides significant advantages over conventional steerable assemblies. The new system comprises of uniquely designed pin-down mud motor with a specially designed long-gauge PDC bit in a matched drilling system. To date, field results from the new system have shown to improve hole quality and increase ROP. Other advantages include less hole spiraling and tortuosity, reduced vibration, better steerability and hole cleaning, increased bit life, and improved PDM and MWD reliability. These benefits are expected to have considerable implications for coiled tubing drilling applications.

Introduction

For many years, North Sea directional wells have challenged service providers to improve drilling equipment reliability. Despite advanced planning capabilities that now include real-time vibration sensors, vibration remains a primary cause of tool failure in directional wells. To improve hole quality and reduce vibration, BHAs designed to reduce vibration were investigated. Already widely recognized for providing excellent "gun-barrel" hole quality in vertical wells, long-gauge bits have not been widely used in directional applications due to a common perception that such designs are incompatible with steerable PDMs (Positive Displacement Motors). Therefore, an effort was undertaken to design a system that retains the attributes of the steerable motor, i.e., directional control, while providing the "hole quality" benefits of long-gauge bit utilization.

The resulting SlickBore™ drilling system incorporates a pin-down steerable PDM and box-up long-gauge PDC bit specifically matched as a drilling system. The system exhibits excellent directional control and can be used to turn and to build, drop, and hold angle, much like a standard steerable assembly.

In both field application and controlled testing, use of the new system significantly reduced downhole vibration, as evidenced by the measurements provided by downhole vibration sensors. A number of other benefits have been demonstrated from field experience in the North Sea, Canada, and Southeast Asia. These additional benefits include better steerability and hole cleaning, reduced drag, less hole spiraling and tortuosity, and greater mud motor and MWD reliability. Specifically, it was found that the benefits of improving hole quality were compounded by increased ROP in both sliding and rotary drilling modes while reducing the time required for many non-drilling activities such as casing installation, circulating and back-reaming. This provided a reduction in total drilling time and improved equipment reliability and performance.

These benefits not only have significant advantages in standard directional drilling applications but also may have considerable implications for extended-reach drilling and coiled tubing drilling applications.

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