Unpredictable coiled tubing (CT) service life is not acceptable in the CT industry and is not tolerated by the customer. Experience shows that service life becomes unpredictable without adequate tubing maintenance programs that include corrosion prevention. Therefore, monitoring and maximizing CT service life requires effective corrosion control on both inside and outside CT surfaces from the time the tubing string is milled until it is retired. Corrosive degradation of CT can result from contact with the atmospheric environment, pumped fluids, and production fluids. Corrosion, especially localized corrosion, must be prevented because it can greatly affect tubing life by initiating premature fatigue cracking and growth during cycling. Additionally, corrosion can reduce usable tubing strength and pressure integrity. This paper discusses effective preservation and inhibition programs instituted at the tubing mill, service centers, and in the field. In addition, electrochemical corrosion-rate data for as-milled and cycled CT is presented. Linear-polarization resistance and Tafel-curve generation were used to derive general corrosion-rate data for new and cycled CT-90 and CT-100 in various common oilfield fluids, including a stimulation fluid. These tests suggest that CT corrosion tendency is not significantly accelerated as a result of cycling, except at high temperatures. Additionally, the paper presents high-pressure fatigue data from tests performed on CT-90, comparing fatigue life in water and in inhibited 15% hydrochloric acid (HCl). Introduction Standard grades of CT (defined in API Specification 5LCP and covered in API Recommended Practice 5C7) are manufactured from low-carbon steels with limited alloy content. The alloy content generally does not increase the tubing's resistance to typical corrosion that occurs in oilfield environments. Although the tubing grades are considered weathering steels because they contain copper and other elements, these alloys do not significantly increase resistance to aqueous corrosion or other forms of corrosion typical of in oilfield operations. Corrosion can begin the day the CT is milled and spooled unless a suitable corrosion-inhibition program is implemented. If the tubing temperature reaches the dew point and moisture condenses on the tubing, rusting can initiate during the tubing's first night of existence. In this situation, hydrated iron oxides, Fe(OH)2 and Fe(OH)3, are formed. During a drying period, the hydrated oxides will lose water and form hematite (Fe2O3) and magnetite (Fe3O4). Additionally, the presence of contaminants, such as chlorides, sulfates, or carbon dioxide (CO2) will increase corrosion. Almost every environment to which the CT is exposed can be a potential source of corrosion. Corrosion can be encouraged by the atmosphere, produced fluids, injected fluids, and water used to flush the tubing after a job. However, CT can be protected from significant corrosion if proper maintenance procedures are followed. Examples of CT Corrosion-Related Failures The service life of a CT string can be greatly affected by corrosion. Many operators know that corrosion is the root cause of a significant portion of premature tubing-string failures. In 1999, service companies reported1 that 24 to 51% of all failures were caused by corrosion.

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractThis paper will review distinctive corrosive wellbore environments that can be detrimental to the performance of (High Strength Low Alloy) carbon steel coiled tubing which is typically used for workover/intervention and completion applications. The paper will demonstrate how these corrosive environments form the technical push and potential market pull for the development of coiled tubing suitable for specific corrosive applications.The product development of a corrosion resistant coiled tubing will be reviewed, indicating initial design input, verification data depicting corrosion tests, mechanical properties; inclusive of strength, hardness, full body low cycle fatigue and surface property attributes. The product range depicting diametrical and wall thickness ranges will be reviewed.The paper will include manufacturing issues such as forming, welding and inspection techniques. The paper will conclude with a review of the strings manufactured to date with application case histories.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractButt welding of coiled tubing performed routinely in both new and used tubing extends life or increases application versatility of existing strings. Welds are commonly made to replace overly fatigued sections, remove mechanical damage, extend string length, and attach special application strings or tools to work reels and repair tubing imperfections. Properly designed and executed butt welds provide the same load bearing properties of the surrounding tubing. The weld integrity is verified by non-destructive testing, insuring sound joints. The fatigue properties of the weld must be understood and properly managed. This has resulted in an extremely successful record for butt welds placed in coiled tubing for field application.The paper documents the steps required to insure the best weld quality is being placed into a string of coiled tubing. Data on properties of welds from manual and machine welds, in both new and used tubing verify the load carrying capability of the welds. The down rating of fatigue life and potential corrosion implications are handled through continuous string management. The field experience of butt welds performed in both factory and field environments are reviewed. Results indicate the clear cost effectiveness of currently available, properly installed butt welds combined with systematic monitoring by the relevant service companies.

fax 01-972-952-9435. AbstractSixteen chrome coiled tubing (16Cr CT) was introduced in the spring of 2003 and over 200 strings have been put into field use as velocity strings. Following preliminary testing, two 16Cr CT reels were deployed at Prudhoe Bay, Alaska to evaluate feasibility as an intervention workstring. The two reels performed a variety of standard CT applications on a daily basis. Observations and data were gathered to determine operating guidelines, applicability, and limitations. The field trial indicated that 16Cr CT can be deployed in the field with only minor operational modifications.16Cr has superior abrasion resistance in 13Cr production tubulars and little CT surface (external) wear was seen during the field trial. The second reel developed a pinhole failure earlier than expected; however, analysis of the adjacent material indicates that 16Cr has increased low cycle fatigue life when compared to standard carbon steel CT. Additional testing is ongoing, and it is felt that the conditions resulting in the failure can be mitigated to avoid future premature failure. This paper documents the lab and field trial results. Standard operating procedures for 16Cr CT are described that provide easily implemented guidelines. 16Cr has applicability as an intervention workstring, particularly in corrosive environments and in areas where abrasive 13Cr production tubulars must be endured.Recognition is extended to the BP Alaska Wells Team who contributed to making the 16Cr CT field trial a success. Special recognition is made to

QT-16Cr coiled tubing was introduced as a commercial product in the spring of 2004. This high strength corrosion resistant alloy product was developed to offer a cost effective coiled tubing solution for both injection and secondary production applications in wet CO2 environments where carbon steel products may not be suitable. This paper focuses on the results of two years of field applications and laboratory testing with the intent of defining the suitability and limitations of QT-16Cr as a completion string subjected to varying concentrations of CO2, H2S, chlorides and pH. The combination of actual fluid and gas analysis from wells where QT-16Cr has been employed will be compared to autoclave testing which simulates downhole conditions with applied stress on the tubing. This paper also addresses the manner in which QT-16Cr has a positive effect on production system economics using several S. Texas wells as examples. A review of field tests utilizing QT-16Cr as work strings will be covered. Attributes will be reviewed inclusive of abrasion characteristics when run inside 13 chrome production tubulars and low cycle fatigue performance data. Both operators and service providers that have potential applications for corrosion resistant alloy coiled tubing should have interest in this update and overview regarding the performance of QT-16Cr coiled tubing. Developing The Case (QT-16Cr) For many years specific requirements for corrosion resistant coiled tubing have been requested with manufacturers making efforts to produce tubing from exotic materials such as Titanium, Nickel alloy 625 and Beryllium Copper. Although these efforts were valiant, the material and manufacturing limitations coupled with lack of supporting system economics led to their early demise. In early 2000 it was clear that niche applications requiring exotic alloys with fluctuating commodity pricing would not sustain coiled tubing manufacturing efforts. At this time many gas producers were embracing the advantage that coiled tubing had to offer as "velocity" or "siphon" strings for reestablishing the productivity of gas wells. With the system economics favorable for velocity applications, many operators began multiple well coiled tubing completions creating a substantial demand on coiled tubing manufacturers. It was apparent that many of the aging gas wells were developing environments non-suitable for the relatively thin walled carbon steel coiled tubing. The primary Achilles heel being the age-old problem; the encroachment of water along with partial pressures of CO2, resulting in carbonic acid. This increased demand was the driver for the development of QT-16Cr. The need for a CRA coiled tubing became apparent when many older gas wells producing CO2, commonly referred as sweet corrosion, combined with direct water production and/or the introduction of condensed water created as the gas traveling to the surface passes through its critical condensation temperature corroded standard coiled tubing stings. These conditions generate carbonic acid which can create extremely corrosive environments for carbon steel coiled tubing as depicted in figure 1.