The Effect of Corrosion in Coiled Tubing and Its Prevention

Abstract

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.

Experimental study on corrosion resistance of coiled tubing welds in high temperature and pressure environment

“…Liu Ming and Xue Yuna et al [ 14 , 15 ] studied the electrochemical corrosion behavior of CT in highly mineralized aqueous environment by means of potentiodynamic polarization and impedance spectroscopy, and quantitied the effect of immersion time on corrosion rate. Van arnam et al [ 16 – 18 ] studied the corrosion resistance of CT90 and CT100 by using linear polarization resistance method and tafel curve generation method. Bi Zongyue et al [ 19 ] analyzed the fracture failure of CT80 through physical and chemical performance inspection, microstructure analysis and energy spectrum analysis, and found mainly due to the combined effect of tensile stress, periodic plastic strain and acidic corrosion environment in the well.…”

Workovers in Sour Environments: How Do We Avoid Coiled Tubing (CT) Failures?

“…Corrosion starts the day CT is milled and spooled, unless a suitable corrosion-inhibition program is implemented (van Arnam et al 2000). For underbalanced-drilling operations in sour wells, performance testing of corrosion inhibitors demonstrated that there should always be an H 2 S inhibitor incorporated where CT is used (Luft 2003;McCoy and Thomas 2006).…”