Study Of Hydrate Formation And Dissociation In Site Conditions

Natural gas hydrates (NGH) is a kind of new clean energy with high efficiency and huge amounts deposits. In this study, a set of experimental device has been built up to find the formation mechanisms and thermodynamic characters of gas hydrate in natural sediment. The effects of pore size, salinity and different gas component on the formation and dissociation of hydrate in sediment have been measured. The results can provides quantitatively thermodynamic character and explain the formation mechanisms, distribution, state of stability of NGH, and provide important date for the investigation and exploration about NGH.The results indicate that the induction time of gas hydrates formation in porous media is shorter than that in bulk-water. The decrease in the pore size, induced by decreased glass bead size, leads to the enhancement in the equilibrium pressure as the salinity and the temperature are kept the same, in other words, lower temperature and higher pressure are required for the formation of methane hydrate in glass beads. While the increase of salinity makes the equilibrium pressure increase too as the pore size and the temperature are kept the same. Through the experiment of gas hydrate formation and dissociation in simulated sediments, phase equilibrium of gas hydrate composed of methane, ethane and propane has been obtained. It can be concluded that the equilibrium at the condition of decreased methane concentration needs a lower pressure and a higher temperature.The effects of anions and cations on the phase equilibrium of methane hydrate in porous media were studied by using the orthogonal experimental design. Analysis of variance demonstrated that only Mg2+ had significantly influence on the equilibrium point of methane hydrate. The influences of all halogen ions on methane hydrate phase equilibrium are similar, which can be explained by the similar mean ionic activity coefficients of sodium halides at experimental concentrations. The influences of three normal anions concentration on the equilibrium point of methane hydrate were all significant.Two kinds of hydrate formation cases were obtained due to the different initial P-T conditions of CO2. The results indicated the present of glass bead leads to the enhancement in the equilibrium pressure as the salinity and the temperature are kept the same. The increase of NaCl concentration caused the enhancement in the equilibrium pressure as the pore size and the temperature are kept the same. The effects of NaCl solution on hydrate equilibrium can be neglect when temperature was lower than ice points. This can be explained with that the vapor pressure of solution was equal to that of solid phase (ice) when the equilibrium temperature gets to ice point. Comparison of methane and CO2 equilibrium conditions shows the lower temperature and higher pressure are required for the formation of methane hydrate.An improved model was proposed to predict the hydrates equilibrium in marine sediment environment. According to the mechanical equilibrium relations to H-L-V system, interfacial energy between hydrate and liquid was corrected by the function that is expressed with temperature and electrolyte concentration when electrolyte was in pore water. The activity of water is calculated using the Pitzer model and the interfacial energy between liquid and gas is solved using the Li's method. The model was used to predict methane hydrate equilibrium in bulk water and in porous media containing electrolyte or not. The prediction results show a good agreement with the experimental data.Since Magnetic resonance imaging (MRI) is an effective tool for hydrate investigations, a series of experiments were carried out to study CO2 hydrate formation in porous medium using it. The results indicated that the MRI data could visualize hydrate formation and was good agreement with the pressure change. The hydrate formation rate was also quantified using MRI data. The result also shows there are multi-sites for CO2 hydrates initial formation and dissociation in the experimental condition. The effects of heating rate on the hydrate dissociation were also investigated in this work.