The focus of this work is to optimize coiled tubing string design for greater horizontal reach in unconventional wells using a more rapidly tapered section than historically available. The string design will encompass a single outer diameter with a tapered wall thickness, resulting in a tapered inner diameter.
Comparison of string properties will include modeling a traditional straight wall (mono-wall) string design compared to a traditional continuous taper (longer tapered sections) and two rapidly tapered sections. The tapering helps limit the weight of the final string, making ground transportation within North America more feasible using common reel sizes.
Outcomes of the modeling comparing the four string designs will be presented using commercially available software for tubing forces analysis, fatigue modeling and fluid dynamics. This modeling will be used to create a database of theoretical results for typical field applications and parameters with the aim of comparing results to actual field data, when available. At the time this manuscript was authored, production of a string with the preferred design is expected to be possible near the end of 2015.
By utilizing a tapered design, improved low-cycle fatigue life can be achieved in tapered designs compared to straight wall designs of the minimum wall thickness of the tapered design. The tapered design generally has more desirable mechanical limits compared to the straight wall designs while experiencing limited reduction in friction pressure (due to the tapered inner diameter).