Today's drilling environments require a CTD bottom hole assembly (BHA) be designed to perform in harsh environment. These include drilling in Underbalanced conditions (UB), challenging formations and high temperatures. This paper details findings from Coiled Tubing Drilling (CTD) operations in Alaska, Algeria and Sharjah.

These challenging operations required a complex CTD BHA that should be capable of withstanding temperatures in excess of 150°C (300° F), severe vibrations, over-speeding of motors, high-performance positive displacement motors (PDM's), nitrogen intrusion in BHA and PDM elastomer as well as many other challenges.

As a result, the CTD BHA was engineered to meet these various challenges. For example, in order to reduce (or better manage) BHA dynamics under extreme vibration levels, a multi-axis vibration sensor was used to optimize the BHA with weight and flexible bars. This permitted the driller to monitor vibration levels in real-time and adjust drilling procedures and parameters (WOB, mud flow, use of circulating sub, etc.) to reduce excessive vibrations. The BHA also had to address telemetry issues and, on the Sharjah nitrogen-drilled wells, an E-line CTD BHA was employed as it was the only way to transmit MWD/LWD data in the multiphase mud flow.

The paper will also describe some of the special BHA features like multi-cycle circulating sub use, ECD/pressure management, high-speed motor use and more. And the results of these efforts will be documented with performance and vibration graph comparison.

In the previous two years, drilling processes on the Sharjah and Alaska wells were significantly improved for increased footage per day and lower overall well costs. As a result, these projects have been recognized as economic successes and the findings from these wells can be applied to optimize the drill string and drilling parameters for enhanced performance in other challenging environments.

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