Abstract

Residual bend exists in every coiled tubing string. When the coiled tubing is placed inside a wellbore, its initial configuration is not straight, but has a certain residual bend. This paper's objective is to understand explain how this residual bend ion coiled tubing affects its extended reach.

This paper presents a new modeling technique to account for the effect of residual bend on coiled coiled-tubing post post-helical buckling behavior and maximum penetration. The initial configuration of the coiled tubing is assumed to take be the form of a helix along the wellbore. By using the energy method, a set of solutions are obtained for the helix configuration and the radial contact force underthe conditions of applied axial force and initial residual bend. These solutions are used to analyze axial load transfer and extended extended-reach capability of coiled tubing on horizontal wells. It is shown that that in a horizontal well, with a relatively small residual bend radius, the existeance of residual bend significantly reduces the maximum depth the coiled tubing can reach. Results of this work can be used to enhance job design for extended extended-reach applications.

Introduction

Residual bend exists in every coiled coiled-tubing string. During storage and transportation, a coiled coiled-tubing string is plastically deformed (bent) as it is spooled on a reel. During operations, the tubing is unspooled (bent) from the reel, and bent on the gooseneck before entering into the injector and the wellbore. After leaving the injector, the coiled tubing has a residual bending radius typically in the order ofranging from 150 to 400 inches. After entering into the wellbore, the initial configuration of the coiled tubing is not straight;, but it retains a certain residual bending curvature. The objective of this paper is to understand explain how this residual bend affects coiled coiled-tubing post post-helical buckling behavior and lockup.

The subject of buckling and lockup for long tubing strings has been researched extensively in the oilfield industry over the years. [1–11] In his pioneering work on the helical buckling of a long tubing string in the wellbore, Lubinski first established the relationship of between the pitch of the helix and the applied forces.[1] Thereafter, most of the works researchhavebeen focused on determining the critical buckling loads of a long tubing string in a horizontal or inclined wells.[2–5] A few others researchers looked at the effect of torsion on helical buckling [6–9] or the effect of connectors.[9] Mitchell[10] further established the relationship between the radial contact force and the applied axial force on a helically buckled string, which has since been widely used to evaluate the effect of helical buckling on lockup depth.

These works typically assumed a straight tubing string. Qiu et al[11] developed a new model to evaluate the effect of the tubing's initial configuration (curvature)on sinusoidal and helical buckling. In their work, it is They assumed that the initial configuration fo of the coiled tubing is assumed to bewas sinusoidal with the coiled tubing lying on the low side of the wellbore. It wasThey concluded that the initial configuration hads an significantessential [a critical] effect on the least axial force requiredto [that would] produce the coiled-tubing helical buckling in the wellbores. [This sentence is ambiguous.]

In this paper, a new modeling technique is presented to account for the effect of coiled coiled-tubing residual bend on post post-helical buckling and lockup. The initial configuration of a coiled coiled-tubing string is assumed to take the form of a helix. By using the energy method [you should explain what "energy method" is or cite a reference],, the formulae for helix pitch and radial contact force is derived with the consideration of, accounting for the initial residual bend and the applied axial force. These relationships are used to derive the axial load transfer along the coiled tubing with residual bend. The axial load transfer solution allows us to evaluate the effect of residual bend on coiled tubing helical buckling and lockup. It is shown that in a horizontal well, the effect of residual bend could significantly reduce the coiled tubing maximumal reach. Results of this work can be used to help in design coiled coiled-tubing jobs design for extended-reach applications.

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