Understanding Passive Layer Formation for Further Corrosion Management in Gas Production Pipes

Corrosion is a critical issue during the development of a gas field, especially wet gas or retrograde gas field. Corrosion affects the management system of a field and further impacts the amount of investment. Therefore, accurate prediction of corrosion rate is needed to plan an effective preventive action before going further to the development phase. One of the important parameters that should be noticed to create an accurate prediction is the formation of the passive layer. In CO₂-H₂S environment, there will be three possibilities of passive layer: FeS, FeCO₃ or no passive layer. In this study, we create mathematical models to determine the formed passive layer in each segment of the gas production tubing and pipeline. The model is built using Faraday's Law and Thermodynamic approach to account the passive layer formation at different temperature, pH, corrosion rate and partial pressure of CO₂ and H₂S. From the simulation, it was found that there were three boundary conditions: no scale-FeS boundary, no scale-FeCO₃ boundary and FeS-FeCO₃ boundary. The first two boundaries evolved over a time as the concentration of Fe²⁺ ions was increasing. However, FeS-FeCO₃ boundary remained steady as it was not affected by the addition of Fe²+ ions. Using sample case study, few variations were noticed at production pipeline and tubing. It was caused by the gas composition, which contained high CO₂ and very low H₂S. Boundary conditions only changed slightly over two days period. (paper)