Study Explores Mechanisms of Surface Damage on Coiled Tubing

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 194254, “Study on Mechanism of Coiled Tubing Surface Damage in Injector Heads,” by Zhihong Zhou, Guofeng Zhang, Fayong Yuan, Tang Wang, Yunwei Gao, and Weijia Wang, Jianghan Shale Gas Development Technical Service Company, prepared for the 2019 SPE/ICoTA Well Intervention Conference and Exhibition, The Woodlands, Texas, USA, 26–27 March. The paper has not been peer reviewed. Although several kinds of mechanical damage cause coiled tubing (CT) failure, longitudinal plowing marks (LPMs) account for a large portion of such damage. Understanding the mechanisms of LPM damage on CT surfaces will help reduce the occurrence of this type of damage. In this paper, the authors study the stress status of CT string held by gripper blocks, the axial force distribution of gripper blocks, and the longitudinal resonance of CT strings in vertical wells. Introduction The overwhelming majority (greater than 80%) of CT failures have been classified into four categories: corrosion, mechanical damage, string-manufacturing issues, and human error. Of these, failures caused by mechanical damage constitute almost 30% of all CT failures. Further study of these types of mechanical damage reached the following two important conclusions: LPMs, which are injector-related, account for 46% of mechanical damage observed on CT strings from 2006 to 2017. Recent periods that have seen dramatic increases in mechanical damage coincide with plug milling and higher-grade-CT operations. The authors propose that the pressure in the CT string plays a role in mechanical damage. Contact Status Between CT and Gripper Blocks The gripper blocks carried by chains are used to hold CT strings in place. The gripper blocks are divided into single-pitch and two-pitch categories. The engagements between the CT string and the gripper blocks are sometimes suboptimal. The contact area between the gripper and CT string is only at the end-point of the arc, and the contact stress is much higher there than at the point at which the gripper blocks grip the original CT string. According to the von Mises yield criterion, if the von Mises stress is larger than the yield stress at any point, the material at that point will yield and plastic strain will be produced. Distribution of Gripper-Block Lifting Forces in the Injector Commonly, several pairs of gripper blocks engage the CT string to provide the friction forces needed for deployment and retrieval of the tubing into and out of the wellbore. How the axial loads of these pairs of gripper blocks are distributed is essential to safe operation and avoidance of CT damage. To obtain the axial load distribution of all pairs of gripper blocks, a mechanical model is created by the authors and discussed in the complete paper.