Evolution Process of Natural Gas Hydrate Deposits in Water-Dominated Systems

摘要

In order to improve the operation safety of the multiphase flow in the subsea pipelines and facilitate the development of deep-sea oil and gas resources, the evolution process of different hydrate deposits has been studied based on high-pressure reactors and focused beam reflectance measurement (FBRM). The results show that larger hydrate formation (6% vt hydrate concentration) can form layered deposits but smaller (3% vt hydrate concentration) corresponding to the suspended slurry, and the reduction rate of the fixed bed thickness is greater than the moving bed. Microscopic images show that the downward transportation of pore water dominates the evolution process when the moisture content is higher than 87%. The evolution mathematical model of hydrate deposit thickness which is based on Darcy's law is in good agreement with the experimental results, proving the properties of porous media in hydrate deposits. The migration rate and permeability of porous hydrate particles are positively related to the particle size, both showing a linear relationship at the microscopic scale. The deposits with a uniform particle size distribution are more difficult to stratify and thus more suspended, which is conducive to the safety of pipeline transportation compared with the layered deposits. On these bases, the mechanism and mathematical model of hydrate deposit stratification and evolution are established.