TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCoiled tubing (CT) is widely used in well intervention as a practical and cost-effective means of servicing wells. Over the years, the actual flow through CT has been a point of discussion and theory. Testing has been conducted to promote a better understanding of what happens inside the CT. In recent years, the use of computational fluid dynamics (CFD) software has provided greater insight into actual CT flow patterns, including fluid-flow velocity profiles and secondary flow regimes. These flow patterns can be studied in a straight pipe or curved (CT) condition with variable flow rates and variable fluids. CFD could help us understand the phenomenon of erosion, including particle path and migration through the tubing. It might also lead to a greater understanding and efficient design of friction pressure gradients. After CFD is proven and established, an alternative to full-scale testing or other predictive methods may be possible.This paper discusses the flow phenomenon of CT using CFD, particularly over the tubing guide. This is an area of concern because the tubing configuration changes from straight to bent to straight again. Details of fluid flow for both the straight and curved sections have been examined to evaluate flow-velocity profiles and flow patterns. Fluids investigated have included water, gel, and slurry.

The coiled tubing (CT) industry continues to operate in deeper and higher-pressure wells.1 Under these demanding service conditions, performance and reliability of the tubing are essential. This paper discusses field service and other events that led to an aggressive plan to introduce new inspection techniques that will help assure a higher level of performance of coiled tubing strings. The enhanced inspection techniques being instituted include inspection by the tubing manufacturer of the incoming strip and continuous phased-array ultrasonic inspection of the seam weld. In addition, the traditional through-hole calibration standards were upgraded with more realistic discontinuities to help ensure that the produced tubing is defect-free. The paper also discusses the important requirements of string management and maintenance procedures. A key to the success of this ongoing program has been the synergistic activity between the service company and the tubing manufacturer. Introduction Recently, the CT industry has begun to perform service work on deeper and higher-pressure wells. It is not uncommon to rate tubing for use at 10,000-psi. Previous experiences with high-pressure, high-temperature (HPHT) strings, however, have strongly indicated that additional testing of coiled tubing is required to ensure the integrity of the tube wall throughout its service life. Upon review of the issues, it was decided that both short-term and longer-term solutions and actions were required. Likewise, string management and tubing maintenance issues were recognized as essential components in completing the mission of providing reliable tubing to the CT operators. This paper outlines the steps taken by Quality Tubing, Inc. (QT) and Halliburton Energy Services, Inc. to assure tubing integrity from manufacture through the life of the tubing. Experiences Highlight Need for Enhanced Inspection Involvement with several string problems over the last year highlighted the need for increased assurance of seam weld quality as well as a need for higher assurance of tube body integrity. This assurance is vitally needed, of course, for HPHT strings but the case can also be made that all work strings deserve total integrity assurance. The primary incident that suggested that enhanced inspection of milled strings was needed occurred in August 2000 in Aberdeen. A leak was detected during pressure testing of a 1.750-in. OD QT-1000 string (12392) in preparation for an HPHT job. The string was relatively new (roughly 8% fatigue life utilization at the leak point, Fig. 1). It should be noted that leakage was not apparent after pressure testing at 5,000 psi but was only identified after testing at pressures in excess of 10,000 psi. The original mill hydrotest pressure was 11,800 psi, at which time there was no leak. A formal investigation concluded that the leak was the result of small weld-line imperfections in the longitudinal seam weld. Evidence collected during the failure analysis suggested that the string contained lack-of-fusion defects after milling,2as opposed to initiation-of-weld cracking during service work.

Coiled tubing (CT) is used as a cost-effective means of servicing wells for well intervention. Over the years, the fluid flow through CT has been discussed and theorized. Testing has been conducted to promote a better understanding of what happens inside the CT. In recent years, the use of computational fluid dynamics (CFD) software has provided greater insight into actual CT flow patterns, including fluid-flow velocity profiles, secondary flow regimes, and erosion. CFD helps to understand the CT fluid flow phenomenon, which can lead to effective tubing management. CFD has proven to be an effective alternative to full-scale testing.This work compares the results of CFD modeling of erosion to actual field data. It compares predicted values, actual values, and computed values from the CFD solutions. CFD is used to provide correlations for the predictive model without full-scale testing, which greatly reduces the cost of development. Fluids investigated include slurry pumped from an actual job and full-scale testing.

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. AbstractA Tapered OD Coiled Tubing System (TODCTS) enabling the deployment of multiple outer diameter (OD) coiled tubing sections in a single string is being developed. 1 By employing larger OD tubing sections near the top of the string and smaller OD sections near the bottom of the string, the tension along the string length is reduced while maintaining sufficient flow capacity for well intervention operations. The TODCTS can enable intervention service in deeper wells or installation of multiple OD velocity strings with the diameters optimized based on well production requirements. This paper describes the development plan, system and component design, and the results of analyses and tests conducted to verify that the the tapered connector and surface equipment meets the requirements necessary to deploy the connector and adjoining strings of different ODs.