Flow assurance in pipeline in presence of hydrate / Sheida Shahnazar Nezhad Khalesi

Khalesi, Sheida Shahnazar Nezhad (2015) Flow assurance in pipeline in presence of hydrate / Sheida Shahnazar Nezhad Khalesi. Masters thesis, University of Malaya.

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Abstract

At low temperature and high pressure thermodynamic conditions, if certain guest molecules are trapped in water cages, non-stoichiometric ice-like particles named as “hydrate”, can be formed. Hydrates formation and the associated slugging or clogging of transportation pipeline are major concerns to oil and gas industry. Safety concern associated with transport line blockage perhaps is the most significant reason for understanding hydrate. Over the history of gas and oil transportation there have been many equipment damage accidents or even personnel injury due to hydrate plug formation. The most common preventive tool for hydrate occurrence is injection of thermodynamic inhibitor such as mono-ethylene glycol (MEG) and methanol as the most utilized chemical. However, during transportation and storage of natural gas, the throughout prevention of hydrate formation is often impractical or too expensive to be accomplished. Thus, engineers are shifting their attention to managing hydrate particles in flow rather than completely preventing their formation based on this fact that hydrate risk management is considerably more economical than trying to keep flow condition out of hydrate formation region. Hydrate risk management by the slurry flow method is an approach which was recently proposed to industry. Therefore, the theoretical works which are done in this area are extremely limited. Hence, flow assurance partners of offshore projects are not willing to use “Cold Flow Technology” yet which is a method that allows the formation of hydrate while assuring its flow ability. Therefore, it seems vital to improve the hydrate slurry flow models in order to encourage industry to utilize this technique as a less expensive and more environmental friendly method. This study discussed available models and compared their various solutions. In this project, gas-slurry one dimensional flow is coupled with hydrate kinetic model in order to investigate the flow behavior of liquid-solid-gas in pipeline. First, two models are developed for estimating hydrate equilibrium condition and is compared to some of the most popular techniques. This part is of great importance because it reveals the exact length of pipeline where hydrate forms. As the next step, two phases are assumed: the first phase is continuous oil with dispersed water particles in it, and the second phase is gas. The conversion of gas molecules to hydrate solid particles is modeled via the most suitable hydrate crystal kinetic model. Several kinetic models are developed over the past decades, both mass transfer and heat transfer limited models. The most suitable one should be picked based on the case studies and be coupled with flow equations. The two-phase flow model is solved by numerical techniques and predicts the pressure drop of the pipe zone where the flow condition falls beyond hydrate curve and its formation occurs. Then the velocity decrease and pressure drop is calculated and the flow is characterized under hydrate formation condition.

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