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).
