This research presents the results of an experimental study of five parameters identified as having a significant influence on Top of the Line Corrosion: partial pressure of CO2, condensation rate, gas temperature, organic acid concentration and gas velocity. A comprehensive analysis of the effect of each of these five parameters on the type of corrosion is performed. In addition, considerations regarding the importance of the condensation process are discussed.
The transportation of fluids in pipelines is a critical step in oil and gas production. When it comes directly from the well, the fluid is usually unprocessed and multiphase and can be a mixture of oil, solids, gas and water. The presence of water leads to considerable corrosion problems on the internal walls of the pipelines. The phenomenon of interest in this study is the transportation of wet gas and, more precisely, the Top of the Line Corrosion (TLC) that occurs when significant heat exchange is present between the pipelines and the surroundings (frozen land, deep-sea water). The unprocessed vapor flowing through the pipe condenses on the cold walls, forming a thin film and/or droplets of liquid. The liquid can contain corrosive species such as organic acids and dissolved corrosive gases (such as carbon dioxide or hydrogen sulfide). Therefore the condensation of wet gas can lead to a very corrosive environment. The first case of TLC observed in the field was reported in the sixties in a sour gas field in France1. Since then, numerous cases have been reported, mostly offshore2,3,4,5 but also on several occasions onshore6,7. As it is reported, TLC occurs exclusively in stratified flow regime, at low gas velocity, and in sweet or sour environments. The condensation rate and the presence of organic acid seem to be controlling parameters. In the past twenty years, TLC has been the subject of intensive research. Olsen and Dugstad8 conducted a systematic experimental study on parameters influencing TLC in sweet conditions. They found that the competition between FeCO3 film formation kinetics and the condensation rates controlled the extent of the corrosion attack. At high temperature (70ºC) and low condensation rate, a dense and protective FeCO3 is favored to form rapidly. At high condensation rate, the saturation in FeCO3 is more difficult to obtain due to the rate of fresh water renewal. In 1999 Gunaltun4 added more insight into TLC mechanisms by defining three main zones in the pipeline: - The bottom of the pipe where the corrosion is uniform and where the rate can be lowered with the use of inhibitors. - At the sidewall of the pipe where the condensed water drains to the bottom. The corrosion is also uniform but inhibitors are not efficient. - At the top of the line where a protective iron carbonate layer can be formed in certain cases. Inhibitors are not effective and localized corrosion occurs. The condensation rate was once again identified as a controlling parameter in TLC and the concept of critical condensation rate grew stronger.
