Corrosion Failure and Electrochemical Corrosion Behavior of Coiled Tubing
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TECHNICAL ARTICLE—PEER-REVIEWED
Corrosion Failure and Electrochemical Corrosion Behavior of Coiled Tubing Liu Shaohu . Liu Yuanliang . Zhong Hong
Submitted: 5 March 2020 / in revised form: 22 July 2020 Ó ASM International 2020
Abstract Coiled tubing (CT) is known to be susceptible to corrosion due to the complexity of wellbore environment and limited material selection. Since the weld seam (WS) and the base metal (BM) of CT are different in composition and microstructure, WS material usually shows very different (lower) corrosion resistance compared with BM. To better understand the corrosion-related behaviors of CT materials and the mechanism behind them, the WS and BM samples were directly processed from a finished product of CT, and a series of electrochemical experiments were conducted in laboratory conditions to study the corrosion behaviors of both WS and BM materials. The results showed that the corrosion potentials of BM and WS under the condition of 60 °C and saturated CO2 increased by 57.1% and 41.6%, respectively, comparing with the condition of no CO2 presence. Also the corrosion current of the WS was found to be 14 times of the BM’s. A corrosion model was established assuming the principles of galvanic couple corrosion applied on the working conditions of CT. The surface morphologies of the samples were analyzed by the multi-physical field coupling method. The results showed a very good consistence between the corrosion model and the experimental data. Meanwhile, this model was also used to evaluate the impacts of the welding reinforcement, the area ratio of WS and BM, and the material defects on their corrosion rates. The results L. Shaohu (&) L. Yuanliang School of Mechanical Engineering, Yangtze University, Jingzhou 434023, People’s Republic of China e-mail: [email protected]; [email protected] L. Shaohu Z. Hong State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People’s Republic of China
indicated that the area ratio of WS and BM is in direct proportional to the corrosion rate, and the other two parameters showed very little effect. Keywords Corrosion Microstructure Coiled tubing Weld seam Galvanic couple corrosion
Introduction A single CT is thousands of meters in length, can be constantly stretched and bent, and is capable of being wrapped on the reel to enable rig up and rig down operations. It is widely used in drilling, well repair, well completion, oil recovery, gas recovery, and other scenarios [1, 2]. Comparing with the generally used oil pipe, CT has many advantaged features such as wider operating range, smaller occupation area, shorter lifting and descending time, quicker operation, higher efficiency, higher safety and reliability, etc. [3]. Therefore, it is often referred as ‘‘universal operating machine’’ in the oil and gas industry in China. In recent years, CT was gradually developed toward the trend of larger outer diameter, longer and higher strength to meet the demands of drilling much deep
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