Experimental Study On The Formation And Decomposition Characteristics Of Natural Gas Hydrate In Silica Sands

Gas hydrate is a potential abundant clean energy,widely distributed throughout the world,mainly exist on the sea bottom sediment pore and permafrost region.The development and utilization of natural gas hydrate can solve the present situation of the shortage of natural gas and optimize the energy structure in our country,also can ease the world energy crisis.Now there is no mature technology for gas hydrate mining,the actual mining of gas hydrate need a long preparation time,the investment and the risk is big,so the countries around the world has developed simulation mining research of natural gas hydrate.The study of hydrate decomposition in laboratory should first obtain samples of hydrate,so the research of hydrate formation kinetics rule in porous media is very necessary,and to generate enough hydrate in a relatively short time is the premise of decomposition experiment.Now the dynamics research of natural gas hydrate in porous media mainly focus on the influence of porous media type,initial pressure,experimental temperature in the generation process,the conjoint analysis of decomposition with generate process is little,and lacking intuitive understanding of generation and decomposition regulation.This paper is based on the above issues.In this paper,the phase equilibrium conditions of CH4 gas hydrates formed in pure water was measured to determine the temperature and pressure conditions of kinetic experimental.Then we conducted the natural gas hydrate formation experiment in porous media(silica sands)system with different water saturation,different particle size,different gas inlet mode,different concentration of SDS and different bed height,and researched regular of hydration conversion rate,gas consumption and generation rate in different conditions.When the hydrate samples was obtained,we conducted decomposition experiment by heating,decompression,and heat injection method,the decomposition rate and gas production were studied.At last,we observed gas hydrate formation and decomposition in pure water-silica sands system and SDS solution-silica sands system by high-pressure visible microscopic system,and observed the process of hydrate formation and decomposition by micro level.The conclusions of this study were summarized as follows:(1)In hydrate formation experiment,the conditions of low saturation system had a higher gas consumption and generation rate,by the perspective of water conversion rate,70% saturation was the optimal saturation;The gas consumption and hydrate generation rate of small particle size(0.18 mm~0.25 mm)was significantly higher than the large size(0.3 mm~0.9 mm);The below gas inlet mode could accelerate generation rate and short the reaction time,but the overall gas consumption had little difference with the up inlet mode;Adding SDS could obviously short the reaction time,and the hydrate generation deceased with the bed depth increased.(2)In hydrate decomposition experiment,There was a plateau during the heating decomposition process,the start time of decomposition was almost same,and the starting point was phase equilibrium point of this situation;In constant pressure decomposition,we found that the lower the constant pressure was,the greater the gas production was,and SDS system had a higher gas production and decomposition rate;In heat injection decomposition,we found that hydrate can generate secondly if the decomposition gas didn't exhaust in time,and the second generation phenomenon of SDS solution was more obvious,hydrate content had increased after heat injection.(3)In the microscopic observation experiments,we found that in the silica sandswater system hydrate growth on the wall from down to up,the gas-solid interface crystallized slowly,in heating decomposition process,hydrate on the wall was firstly decomposition,the bed height had no obvious change during the whole experiment process;in silica sands-SDS system,hydrate generated rapidly,spread for gas phase along the wall,the transparency of hydrate layer was low,the heating decomposition produced a large number of bubbles and the upper bed elevated due to bed rupture,directly depressurization decomposition had obvious damage to bed,bed height increased obviously after decomposition.