Field Study of Corrosion Inhibition Effect on Corrosion of Low Flow Velocity Gas Flowlines Under CO₂ Environments

The effect of flow velocity on corrosion inhibition of low carbon steel gas flowlines was investigated utilizing: corrosion inhibitor injection rates, OLGA¹ transient simulation model for water presence identification, historical operational data and in line inspection (ILI) data. The gas flowlines included in the study shared a common condition of producing operations at variable flow velocities below 10 ft/s and continuous chemical treatment in a CO₂ environment.

The study concluded that CO₂ corrosion was effectively mitigated at low flow velocities even at different operating temperature/pressure, variable amount of produced water and different CO₂ partial pressure by ensuring that enough corrosion inhibitor was injected, provided there were no operational upset conditions.

Carbon Dioxide (CO₂) corrosion of carbon steel (sweet gas corrosion) poses a serious problem in oil and gas production, since most used materials are low alloyed carbon steels, which are not corrosion resistant. Basically, the main aspects of CO₂ corrosion are well known today, and the parameters are temperature, CO₂ partial pressure, pH value, and flow velocity [1-2].

CO₂ dissolves in brine to form an acidic solution that corrodes carbon steel equipment. A vast effort has been devoted to combating this problem through materials selection and application of inhibitors. The selection of materials and the corrosion control strategy has a major influence on the life cycle cost of production facilities. In many cases, the selection of carbon steel and the use of chemical inhibitors for corrosion control represent the most economical option. However, the economic success of this scenario depends on understanding all factors affecting CO₂ corrosion and on the selection of the proper corrosion inhibitors. Factors affecting CO₂ corrosion include fluid chemistry, temperature, flow characteristics, steel composition and operating conditions [3].

One of the common modes of operation of inhibitors is adsorption of the inhibitor molecules at the metal-solution interface [4]. This adsorption changes the electrical potential between the metal surface and the corrosive media. This variation of electrochemical potential reduces, or in a best case avoids corrosive attack [5]. The effect of an inhibitor is mostly limited by flow velocities [6]. At low flow velocities, which lead to stratified streams, problems arise due to the permanent separation of the phases, with accumulation of water at the bottom [7].