Abstract

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.

Introduction

From 2002 to 2004 Coiled Tubing Drilling (CTD) activity for horizontal drilling in both underbalanced and overbalanced mode has increased considerably (approximately from about 3.000 m to 70.000 m per year1). As CTD became a reliable and cost efficient drilling technology, more operators began to consider this technology more often for reentry drilling and the CTD is now regularly considered for re-entry and UB drilling projects

Based on continuous activities, service providers and key operators developed guidelines to optimize operating practices and BHA design for various CTD applications. This paper will share some of these experiences and lessons learned with a focus on harsh drilling environments based on an e-line controlled CTD system (Figure 1).

Underbalanced Drilling / Multiphase Flow

One of the most challenging drilling environments is under balanced drilling. For underbalanced drilling with single phase flow light drilling fluids (e.g., crude oil, diesel and other oil-based muds) are often used. These fluids are highly reactive and damaging to all elastomer components and could limit the BHA's reliability and durability. In the project planning phase, it is critical that the various BHA components be matched to the specific drilling fluid. Multi-phase fluids, in particular a mixture of Nitrogen and the drilling fluid, can create elastomer damage due to swelling as well as high vibrations that could cause low hour failures on electronic components.

High Vibrations in multi-phase flow

Drillstring dynamics, hole cleaning and weight transfer problems are limiting factors in CTD applications. The reasons for drillstring vibrations are mainly due to the engagement of the bit with the formation and the contact of the BHA with the borehole wall. Those vibrations generate axial, lateral and torsional movements.

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