New approaches in avoiding gas hydrate problems in offshore and deepwater operation

Abstract

Oil industry is facing with challenging gas hydrates and flow assurance issues in deepwater developments. The situation is not any better for Brown fields as a result of increasing water cut. The other factor which is playing an increasing role is product quality and environmental concerns, demanding reduction in chemical usage. The current industry practice for hydrate prevention is injecting hydrate inhibitors at the upstream end of pipelines based on the calculated or measured hydrate phase boundary, water cut, worst pressure and temperature conditions, and the amount of inhibitor lost to non-aqueous phases. In general, systematic ways of controlling and monitoring along the pipeline and/or downstream to examine the degree of inhibition are very limited. Monitoring changes in the pipeline pressure drop is inadequate to provide reliable indicator for hydrate formation and deposition. Therefore, early hydrate warning and online hydrate monitoring techniques are demanded to optimise inhibitor dosage, reduce the risk of gas hydrate formation/deposition and the cost of mitigating the blockage in subsea pipelines. The primary part of this thesis is to develop a new approach for early warning system and monitoring against initial hydrate formation. It is known that the formation of hydrates changes the water structure, which is claimed to remain in the aqueous phase for a period of time even after the dissociation of gas hydrates. This change of water structure is hypothesized to be in the form of water memory. Therefore, two hydrate early warning techniques are investigated based on the presence of water memory resulted from hydrate formation. The techniques investigated in this thesis are dielectric properties and onset of ice formation. In this thesis, the new approach demonstrate that dielectric properties at microwave frequencies has the potential to be used as a downstream and online analysis for detecting the initial hydrate formation and/or presence of hydrate particles and/or changes in water structure due to hydrate formation. Characteristic of onset of ice formation by freezing method for water samples with and without hydrate water memory shows that samples with water memory nucleate faster than that without water memory. It is concluded that the new approach described above have potential to be developed for early warning and online hydrate monitoring. The results are very encouraging and could potentially change the industrial approach to gas hydrate control strategy. Low Dosage Hydrate Inhibitors (LDHIs) have been applied in the field to prevent gas hydrate problems by delaying gas hydrate nucleation and/or growth and to prevent agglomeration of hydrates from growing larger enough to plug the flowline. However, the mechanism of hydrate formation and inhibition is still not well understood. It is believed micro-scale investigation could provide vital clues. The second aim of this thesis is to investigate the inhibition mechanism of Low Dosage Hydrate Inhibitors (LDHIs) by visual observation of gas hydrate formation, growth, and morphology by means of high-pressure glass micromodels, multichanel flow conduits, and glass capillary tubes. Extensive novel data and knowledge was generated from these techniques. The finding of this study shows that various hydrate morphologies formed in the presence of different KHIs for Natural Gas Hydrate and Methane Hydrate. It was concluded that these techniques provides a new data to supplement the lacking of knowledge on the kinetics of gas hydrate inhibition and morphologies.