Condensation of water on rust surfaces, and possible implications for hydrate formation in pipelines
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The threat of climate change has put the spotlight on the increase in carbon emissions and possible methods to attenuate it. As a consequence of this, oil and gas companies have started to consider both how to reduce emissions and use long-term sequestration of produced CO2. Statoil is working on a project which explores the possibilities of storing CO2 in geological formations beneath the sea to mitigate the climate change, as well as to investigate the possibilities for using CO2 gas in Enhanced Oil Recovery (EOR). This project will involve pipeline transport of CO2 gas on the bottom of the North Sea. Because of non-zero water content of this CO2, water can drop out as a liquid within the pipeline. The presence of liquid water and a strong hydrate former like CO2 can lead to hydrate plugging the pipeline under certain conditions. Companies such as Statoil spend large amounts of money on preventing hydrate formation within the pipelines, for example by injecting glycols. Their best practice" approach commonly uses dew-point temperature curves to determine water dropout conditions. Recent work by Martin Haynes and Jan Thore Vassdal indicates that there is another driving force for water dropout in pipelines since water can adsorb on rusty pipe surfaces before it condenses within the gas phase. If this threat is valid, ignoring it could ultimately could cost the industry a lot of money. The purpose of this study was to investigate how condensation of water on rust surfaces can affect kinetic and thermodynamic characteristics of hydrate formation in a pipeline. This was investigated by Molecular Dynamic simulations on a microscopic level. A system containing hematite, hydrate, CO2 and water was built and simulated using different force field models, in order to properly model the molecular interactions in the system. These simulations managed to reproduce a number of the system properties successfully. The water-wettening nature of hematite was observed early in the simulation, as water adsorbed on the hematite surface in two highly structured layers. If this finding holds true in the case of real carbon dioxide pipe transport, this would indicate that the use of dew point curves to determine water-drop out is an outdated method, as the water would condense on the hematite surface before it condenses within the CO2 phase. Ascertaining the affinity of water would require estimating the chemical potential of water adsorbed on hematite. The adsorbed water layer prevented the CO2 molecules from approaching the hematite surface making it impossible to observe any correlations between hematite and CO2 in the course of the simulation. This indicates that individual CO2 molecules may not been able to approach a wet hematite surface and adsorb on it. Another interesting effect of including the hematite crystal was the accelerated diffusion of CO2 into water in the presence of hematite. On the other hand, it also appeared that including the hematite crystal in the aqueous phase also resulted in a lower solubility of CO2. The hematite crystal exhibited no clear-cut preference for orientation, as the crystal continued to translate and rotate throughout the simulation. Although observations showed that the crystal approached both of the phases in the simulations, no permanent orientation could be identified. The results obtained for the hydrate crystal were promising. The surrounding...