Modelling and Experimental Work Applied to Natural Gas Pipelines
CO2 corrosion is a general problem in the industry and it is expensive. The focus of this study is an oil gas production related problem. CO2 corrosion is observed in offshore natural gas transportation pipelines. A general overview of the problem is presented in chapter 1. The chemical system consists mainly of CO2-Na2CO3-NaHCO3-MEG-H2O. Sodium is injected in the pipelines as NaOH in order to pH-stabilize the pipeline to avoid corrosion and MEG is injected in order to prevent gas hydrates. There are a great number of models available in the literature which may predict CO2 corrosion. These models are not very accurate and assume ideality in the main part of the equation. This thesis deals with aspect of improving the models to account for the non-ideality. A general overview and extension of the theory behind electrochemical corrosion is presented in Chapter 2 to 4. The theory deals with the basic thermodynamics of electrolytes in chapter 2, the extension and general description of electrolyte mass transport in chapter 3, and the electrochemical kinetics of corrosion in chapter 4. A literature overview of CO2 corrosion is shown in chapter 5 and possible extensions of the models are discussed. A list of literature cites is given in chapter 6. The literature review in chapter 5 shows how FeCO3 plays a main part in the protection of steel. Especially the solubility of FeCO3 is an important factor. Chapter 7 discusses and validates the thermodynamic properties of FeCO3. The study shows that there is a discrepancy in the properties of FeCO3. Sets of consistent thermodynamic properties of FeCO3 are given. A mixed solvent electrolyte model is regressed in chapter 8 for the CO2-Na2CO3-NaHCO3-MEG-H2O system. Parameters of the extended UNIQUAC model is fitted to literature data of VLE, SLE, heat excess and validated against heat capacity data. The model is also fitted to experimental data produced and shown in chapter 8 for SLE in the Na2CO3-NaHCO3-MEG-H2O system. The application of the above model is shown in chapter 9. Here the thermodynamic correction factors are calculated. These show how the diffusion process in CO2 corrosion models deviate from the ideal case. Conclusion and suggestion for future work are presented in chapter 10 and 11.