This paper presents a general string design methodology that optimizes coiled tubing (CT) string makeup for extended-reach and high-pressure applications. The string design methodology includes three components, namely tubing force, pressure limit, and wall-selection criteria. The wall-selection criteria qualify the use of a given wall thickness for the string based on the consideration of string ratings and extended-reach potential, which relies on a new parameter called "compression ratio (?)". Through the analysis of tubular lockup behavior, the compression ratio is found to govern the lockup depth of tubing string in wellbores. Integrating the calculations of tubing force and pressure limit with the compression ratio-based wall selection criteria optimizes string makeup by minimizing the overall tubing weight while satisfying the requirements of string ratings and extended-reach. Design examples for both extended-reach and high-pressure applications demonstrate the effectiveness of this method.
Introduction
Coiled tubing (CT) operations in extended-reach or high-pressure wells are limited by many factors. Two of the major factors are the tendency to lock up,1 which prevents tubing from further advancing into an extended-reach well, and the likelihood to fail due to overload (pressure or axial load). For a given wellbore, several parameters determine how far a tubing string can penetrate into an extended-reach well, and how much pressure differential the tubing string can sustain without failure. These parameters are tubing grade (strength), tubing geometry (OD, wall thickness and ovality, etc.) and string makeup. Experience shows that for a given tubing grade and OD, the arrangement of string makeup can significantly affect the operational envelope of the tubing string. A sound string design could achieve the maximal operational potential the chosen tubing grade and OD can offer, while a poorly designed one would have an operational envelope much smaller than what the tubing grade can provide. Therefore, it is important to design a string makeup properly to maximize the use of CT in extended-reach and high-pressure applications.
In the past, there were two general approaches for string design. The first one is called the consistent safety factor method, which designs a tubing string based on a chosen safety factor. The other approach is called the constant overpull method, which designs a tubing string based on a chosen overpull force. These two approaches are in essence a tensile strength-based method. Since neither of them considers the effect of pressure resistance and buckling or lockup in the design process, they are inadequate for extended-reach or high-pressure applications.
Ref. 2 proposed a new approach to design CT string for high-pressure applications. It considers the CT's axial strength and pressure resistance to determine the maximum section length for a given wall thickness. The tubing string thus designed satisfies the string ratings requirement (burst, collapse, and overpull). The methodology discussed in Ref. 2 is very effective for designing a high-pressure string in a vertical well. However, it doesn't have enough information to design an optimal string makeup for extended reach.
The objective of this work is to expand the scope of Ref. 2 to include a general string-design methodology for extended-reach and high-pressure applications. To achieve this, we have studied the lockup behavior of tubing string and identified a new parameter called "compression ratio" that governs the lockup depth. We propose a compression ratio-based string design methodology to satisfy the string ratings and maximize the extended-reach capability while maintaining a minimal overall weight for a tubing string in a three-dimensional well.
