| The high-quality development of China’s economy requires large amounts of clean natural gas resources.As an unconventional natural gas resource,natural gas hydrate has a huge reserve and is expected as an alternative energy for future.Therefore,the commercial development of natural gas hydrate is of great significance to ensure future energy security.However,over 90%of the world’s gas hydrate resources are in the clayey silts.To achieve the efficient development of natural gas hydrate,the dissociation conditions and kinetics of natural gas hydrate in clayey silts must be studied.In this thesis,an experimental method for the determination of dissociation conditions of gas hydrate in porous media was established.Experiments on methane hydrate formation and dissociation were carried out to preliminarily investigate the dissociation characteristics of hydrate in clayey silts.For clayey silts,as the temperature increases,the hydrate in small pores will dissociate first,and the hydrate in the larger pores will dissociate subsequently.At last,the hydrate dissociation curve will concide with that of bulk hydrate.With the increasing of initial pressure and the reduction of water content,the proportion of hydrate in nanopores will increase.For the water-saturated system,the rapid formation of hydrate on the surface of clayey silts may lead to a sharply reduction of permeability.In this way,a mass transfer barrier which prevents the further hydrate formation was formed.And during the step-heating process,the reformation of hydrate was observed.An experimental device was built up for the determination of dissociation conditions of methane hydrate in clayey silts.The results showed that both the grain size and the mineral composition of clayey silts sediments can affect the dissociation conditions of hydrate,and the influence of minerals is especially significant.Based on the fact that the dissociation conditions of hydrate in natural sediments are affected by several factors,a commercial water activity meter was adopted to determine the water activity in different systems,and the experimentally determined water activity was combined with the traditional hydrate thermodynamic model.By introducing the experimentally determined water activity,a new method for predicting the dissociation conditions of methane hydrate in clayey silts was proposed.The predictions were then validated by using the experimental data,and the relative errors of the predicted hydrate dissociation conditions for different systems were less than 12%.A device for the depressurization experiments of methane hydrate in clayey silts was built up.In this thesis,we firstly determined the gas production distribution of methane hydrate in clayey silts by step-depressurizing.Subsequently,the dissociation kinetics of methane hydrate in different sediments was studied by depressurizing to a fixed pressure.The results showed that hydrate in quartz powder will dissociate at one pressure during the step-depressurizing process,while a segmental dissociation of hydrate under different pressures was observed for the montmorillonite and South Sea sediments.The hydrate dissociation rate increased significantly with reduction of system pressure in all experiments.The hydrate in quartz powder of small particle size and salt component will dissociate faster.When the driving force of fugacity difference dominates the kinetics process of hydrate dissociation,the gas production curves almost coincide,and the grain size of sediments has little effect on the hydrate dissociation kinetics.For montmorillonite and South Sea sediments,hydrate will dissociate faster in sediments with lower water content at the same pressure.For the prediction of dissociation kinetics of hydrate in clayey silts,based on Kim-Bishnoi model,by introducing the equivalent diameter D and the surface area correction factor ψ to characterize the influence of clayey silts on hydrate dissociation,a prediction model on the dissociation kinetics of hydrate in clayey silts was established.This model was then validated by using the experimental results,and the predicted results were in good agreement with the experimental data.Except for the South China Sea sediments with water content of 30%,the prediction errors were all less than 10%. |
