Experimental And Theoretical Investigation Of Top Of The Line Corrosion In CO2 Gas And Oil Environments

Top of Line Corrosion (TLC) occurs in the transportation of multiphase wet gas flow pipelines due to the condensation of water when there is a significant temperature difference between the produced fluids and the surrounding environment. If the gas/liquid flow is stratified, saturated water vapour condenses on the inside walls of the pipeline and forms small water droplets which become saturated with acidic gases e.g. CO2, H2S and CH3CO2H leading to severe corrosion.
In the oil and gas industry, a standard method for corrosion control is an injection of the corrosion inhibitors. The conventional corrosion inhibitors are non-volatile liquids and cannot easily reach the upper surface of the pipeline where the aggressive condensed water is formed and remain at the bottom of the pipe. Therefore, this mitigation technique is not successful for TLC prevention because the delivery of the corrosion inhibitor is governed by the flow regime. For better understanding the mechanism of CO2-TLC and to predict the corrosion risk and develop improved corrosion mitigation techniques, an experimental and theoretical research on the influence of the environmental parameters on the corrosion at the top of the line is needed.
This study provides a comprehensive experimental investigation into the effect of condensation rate, surface temperature and iron carbonate saturation on TLC and FeCO3 precipitation rate on carbon steel (X65) in a CO2 environment. TLC is found to be governed by surface temperature regardless of water condensation rates (WCR) at a surface temperature below 32°C, whereas for temperatures above 32°C the WCR influences the TLC, precipitation rate and scale film formation. On the basis of the experimental data, a new empirical model to predict the TLC rate in the absence of FeCO3 films is proposed. The post process observations on the morphology of the corrosion product films and the feature of the localized pits on the steel surface provide a useful insight into the mechanisms involved in the TLC processes.
To enhance our understanding of water condensation mechanism at top of the pipeline during gas and oil transportation, Zhang model (Corrosion, 2007.Vol 63, No.11) was implemented and developed at different flow conditions.
In this study, a mathematical model for the condensation of water vapour was developed for the prediction of the water condensation in a top of the line scenario at stagnant gas velocity. In this model, the heat and mass conservation equations are linked to account for the effect of all important heat and mass transfer resistances. A numerical method is proposed to solve the non-linear equation system and predict the condensation rate. The mechanistic model was validated with experimental results. The experiments are conducted in the unique setup developed to simulate the top of line corrosion at static conditions. The comparison of the experimental and the model results showed that the model was able to predict the condensation rate at static conditions reasonably well.
A mechanistic of the top of line corrosion has been implemented, which covers the processes of condensation, dropwise condensation, chemical reactions, electrochemical reactions at the steel surface and diffusion through condensed droplets. The model was validated against experimental data and used then to explore the influence of key parameters.