Abstract

Carbon steel Coiled Tubing strings have been used in sour wellbore environments for many years. The use of Coiled Tubing in sour service has increased by job number, job complexity, pipe size and the stresses to which the pipe is subjected. A number of papers have been written on the steel chemistry and low cycle fatigue behaviour. These have been based on both theoretical and laboratory work. Great progress has been made in understanding the chemical and physical interaction of Coiled Tubing and hydrogen sulfide but little information is available from "real world" situations where the pipe has been worked in a variety of job types (acidising, gas lifting, drilling, etc) and where a large number of other factors may come into play (injector damage, well stimulation chemicals, sour inhibitor application techniques, erosion of inhibitors, CO2, high chloride water production, etc).

This paper will discuss the application of a Coiled Tubing technical specification (based on theoretical and laboratory work) in the operational world. Comparisons will be made between theoretical expectation and practical observation for 70 and 80 grade Coiled Tubing with regard to low cycle fatigue, pipe life and damage in sour environments.

Introduction

Coiled Tubing (CT) has been used successfully for over 20 years in sour well environments in Canada and elsewhere. However, the nature of the wells has changed which in turn has changed the demands placed upon Coiled Tubing. Accessing sour wells under live conditions to perform the usual range of interventions offered by CT is, if anything, an increasing requirement. To meet these changes the CT manufacturers, for their part, have responded by offering a greater range of yield strengths, diameters and wall thicknesses.

Coincident with these changes to CT have been changes in the requirements placed upon the industry in Canada by the regulatory authorities. Part of these requirements are outlined by MacArthur et al1.

The Alberta Energy and Utilities Board (AEUB) started to extend the concepts noted in the above paper when considering approvals for the underbalanced drilling of critical sour wells. Work was undertaken jointly by several industry experts to develop an Industry Recommended Practice (IRP) under the auspices of the Drilling and Completions Committee (DACC). Part of the difficulty with development of the IRP with respect to Coiled Tubing is that the material is used in such a way as to continually subject it to plastic strain reversals. This type of mechanical strain situation is not examined by the National Association of Corrosion Engineers (NACE) in preparation of their list of recommended materials for sour serviceability2. These issues are discussed in SPE54483 by Luft and Wilde3.

Additionally, sour environment testing of CT using the Bent Beam methodology, as specified by NACE testing procedures TMO17712, was performed by a Joint Industry Project (JIP) on several grades of CT4, as well as low cycle fatigue of samples exposed to a sour environment. Part of the outcome of the JIP's testing would suggest that there is no difference in susceptibility to sour environment for the differing grades of CT but all are equally affected when subsequently subjected to low cycle fatigue.

Further testing work continued pursuant to determining the suitability of CT for use in Critical Sour Under-balanced Drilling for the IRP and under what operating conditions. This work is summarized in SPE817235.

These previous bodies of work can not be directly translated for use in general workover applications as the work contained in both the JIP study and in SPE81723 relates specifically to CT which is exposed for extended periods solely to sour environments.

However, some information was extracted out of the testing that had been performed, in order to help formulate a purchase specification for CT which would be used in general workover well interventions.

This paper presents a review of how strings of Coiled Tubing used in interventions have performed after the introduction of the purchase specification and how a complete string management system has been developed.

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