Today, there is a need for higher-strength coiled-tubing (CT) grades with better resistance to severe environments and better fatigue performance in both the tube body and the bias weld. A complete redesign of CT technology and its manufacturing process has recently been carried out. Testing has shown that the fatigue life of the new CT grades exceeds that of currently available high-strength grades. Additionally, the bias-weld fatigue life has improved significantly. Sulfide-stress-cracking (SSC) resistance of the new CT grades is considerably better than that of conventional CT grades with the same strength.

Introduction

Significant changes in CT operating conditions have been driven by the adoption of multistage fracturing in long horizontal wells for shale oil and gas completions. In these, CT is used in well-­completion operations, typically perforating the toe of the well, then milling zonal-­isolation plugs used in fracturing, and then cleaning debris from the wellbore.

Stress and strain levels in the CT used for well-completion operations are typically much greater than those experienced in traditional intervention operations. These operating conditions have identified performance limitations of the CT as it is currently manufactured. One limitation is the yield strength of currently available CT grades. Another limitation is the reduced fatigue life of the bias welds relative to the adjacent base tube, particularly in high-strength-grade tubes. A third limitation is the resistance to SSC, which generally deteriorates as the strength of the tube increases. The objective of this new CT-technology development was to provide a higher-strength tube with better bias-weld performance and improved SSC resistance. This required a full metallurgical understanding of current CT materials and of the manufacturing process, in order to identify improvements. For a discussion of past and current CT technology, manufacturing, and validation methodology, please see the complete paper.