Abstract

A joint industry project (JIP) among ExxonMobil Development Company, Shell Canada Limited and BJ Services Company Canada was initiated approximately 2 years ago to extend the prior research conducted into the serviceability of coiled tubing (CT) for under-balanced sour well drilling and work-over operations, to the higher strength (90 to 110 ksi SMYS) grades. A significantly different and unique laboratory testing protocol is employed in the present JIP research. The methodology entails full-body tubing specimens that have been immersed in a sour solution of varying severity followed by testing in a bend fatigue machine to determine the low cycle fatigue performance of high strength CT materials degraded by prior exposure to the sour environment. In addition, there are several important aspects of possible CT degradation from exposure to sour wells that were not adequately investigated previously. These include the incubation times for cracks to form due to hydrogen induced cracking (HIC) or sulphide stress cracking (SSC), the tensile strength integrity of coiled tubing strings in which HIC and/or SSC have formed, the relative resistance of different CT strength grades to HIC and SSC, the benefits of H2S corrosion inhibitors and the effects of different sour environment severities and external mechanical damage. Detailed metallographic examinations of the CT material characteristics that define their relative susceptibility to failure under sour conditions were also performed. Although the JIP research is still in progress, this presentation will reveal many of the significant results obtained to date with emphasis on CT operational implications and considerations. The newly obtained test results are also being exploited for the preparation of Alberta Industry Recommended Practices (IRP-21) currently under development in Canada.

Introduction

The oil and gas industry has enjoyed many years of sour well interventions with relatively few failures. This success is likely due to a variety of factors including time of exposure to sour conditions, adequate corrosion protection programs, inherent resistance to 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 coiled tubing (CT) materials while exposed to sour downhole conditions, has been the focus of research at BJ Services (BJS) for several years[1,2,3,4,5]. The majority of this research entailed experimental investigations under the auspices of joint industry projects (JIP) utilizing various laboratory testing procedures and apparatus for both coupon and full-body CT test specimens. The testing protocol included both standard and custom designed fixtures comprising slow strain rate testing (SSRT), axial low cycle corrosion fatigue (LCCF), double cantilever beam (DCB), NACE hydrogen induced cracking (HIC) and proof ring tests, acoustic emission (AE) measurements of crack incubation times, constant load tests (CLT) on full body specimens for sulfide stress cracking, bend fatigue machine (BFM) tests on sour CT fatigue specimens and a custom designed hydrogen diffusion vacuum cell for full body CT specimens. Extensive metallographic examinations were performed to investigate the 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[4,5]. 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 that under sour conditions. The 80 grade sour CT limit served the company well but imposed an undesirable limitation on sour well interventions for which higher strength coiled tubing strings were required. Also, the LCCF results indicated a drastic reduction in sour bend fatigue life that was only in the order of 15% achievable under similar loading in air.

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