A review of natural gas hydrates and its pipeline transportation technologies in deep water

Abstract

Gas hydrates are non-stoichiometric crystalline compounds that consist of host water molecules to form cages through hydrogen bond and hydrate formation gas molecules to fill inside, and thus stabilize the crystal lattice by weak van der Waals forces. Hydrate formation gas molecules, also called as guest molecules can be methane, ethane, propane, carbon dioxide and so on which are suitable in sizes for being adsorbed in water cages. Natural gas hydrates in deep water are the unconventional natural gas resources with wide development prospects. Pipeline transportation is the key point to realize the development and utilization of natural gas hydrate resources. During the transportation, the pressure and temperature in the pipelines would change continuously which lead to the complex gas-liquid-solid three-phase flow coupled with natural gas hydrates formation and decomposition. In this paper, research progress in four main aspects were analyzed which included the distribution conditions of natural gas hydrates resources in the world, the methods of exploitation, the kinetics of natural gas hydrates formation and decomposition and the gas-liquid-solid multiphase flow. Based on the four aspects, the safety of natural gas hydrate pipeline transportation in deep water was analyzed. Due to the special thermodynamic conditions of the natural gas hydrate transportation pipelines in deep water, it is common to observe the growth, dissociation, aggregation, deposition and other such kind of behaviors of hydrate particles. The flow regulation of this gas-liquid-solid multiphase is truly complicated and hardly well understood by us. What's more, the interaction of hydrate particle and multiphase flow increases the difficulty of predicting crucial flow parameters, such as flow patterns and pressure drop. Later research should insist on the combination of experiment and theory. More work should focus on the following three parts. The first part is establishing the formation and decomposition kinetic models for multi-component gas hydrate particles while taking the characteristics of mass and heat transfer in the natural gas, water and hydrate interfaces during the pipe flow into consideration, and involving the effects of gas, liquid flow rates and flow patterns on mass and heat transfer. The second part is to consider the forces on hydrate particles microscopically and the carrying capacity of the fluid comprehensively. Meanwhile, analyzing the aggregation and flow features and the deposition regularities of hydrate particles in pipelines. The last one is to establish a theoretical gas-liquid-solid multiphase flow model which includes the growth, decomposition, aggregation and deposition of hydrate particles. This model is supposed to reflect the coupling effects of hydrate particles' growth or decomposition with the flow rate, flow pattern and pressure drop of the pipelines. Consequently, the development of hydrate particles and the regularities of multiphase flow could be described quantitatively. This study provides the theoretical and technical foundations for the design, operation and management of natural gas hydrate transportation pipelines in deep water.