Sour-Well Serviceability of Higher-Strength Coiled Tubing
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 107014, "Sour- Well Serviceability of Higher-Strength Coiled Tubing," by H.B. Luft, T. Padron, and E. Kee, BJ Services Co. Canada Ltd., and E. Szklarz, Shell Canada Ltd., prepared for the 2007 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, The Woodlands, Texas, 20–21 March.

A joint-industry project (JIP) was initiated approximately 2 years ago to extend the research conducted into the serviceability of coiled tubing (CT) for underbalanced sour-well drilling and workover operations to higher-strength [90- to 110-ksi specified-minimum-yield-stress (SMYS)] grades. A significantly different and unique laboratory testing protocol is used in the present JIP research. The methodology calls for full-body tubing specimens that have been immersed in a sour solution of varying severity followed by testing in a bend-fatigue machine (BFM) to determine the low-cycle fatigue performance of high-strength CT materials degraded by exposure to a sour environment.

Introduction

Over many years, sour-well interventions have been performed with CT with relatively few failures. This success likely is the result of a variety of factors including time of exposure to sour conditions, adequate corrosion-protection programs, inherent resistance to hydrogen sulfide (H2S) damage by different CT materials, continuing research into the response of CT materials to sour conditions, and generally good CT-management procedures to minimize the risk of failure. A better understanding of the response of CT materials while exposed to sour downhole conditions has been the focus of research for several years. The testing used both coupon and full-body CT test specimens. The testing protocol included standard and custom-designed fixtures comprising slow-strain-rate testing (SSRT), axial low-cycle corrosion fatigue (LCCF), double-cantilever beam (DCB), Natl. Assn. of Corrosion Engineers (NACE) hydro-gen-induced cracking (HIC) and proof-ring tests, acoustic-emission (AE) measurements of crack-incubation times, constant-load tests (CLTs) on full-body specimens for sulfide stress cracking (SSC), BFM tests on CT sour-fatigue specimens, and a custom-designed hydrogen-diffusion vacuum cell for full-body CT specimens. Extensive metallographic examinations were performed to investigate failure mechanisms, fracture characteristics, and relationships to CT material properties and environmental severity.

The initial JIP considered only CT-strength grades of 70 and 80 ksi and concluded with a recommendation that sour-service CT strings be limited to a maximum strength of 80-ksi yield. This was based on SSRT strain-to-fracture limitations for CT of higher strength using conventional A606/607 modified materials. Bend-fatigue lives of CT materials were estimated on the basis of LCCF measurements by assuming that the ratio of axial- to bend-fatigue life in sweet environments applied equally to sour conditions. The 80-grade sour CT limit served the JIP companies well but imposed an undesirable limitation on sour-well interventions for which higher-strength CT strings were required.

The purpose of the full-length paper is to share the major interim findings of the second JIP, involving tests conducted over the last 1 1/2 years on higher-strength CT90, CT100, and CT110 grades. The majority of these data involves actual low-cycle fatigue tests on 7-ft-long CT specimens that were submerged in a sour solution for 4 days before fatigue loading to failure.