Research On Self-preservation Mechanism And Thermokinetics Behavior Of Gas Hydrate Dissociation

Either developed or developing countries have a increasing demand on energy resources in the 21 st century. However,the reserves of traditional energy resources(oil and natural gas) are not enough to keep pace with the development needs. This demand that developing new energy resources to replace traditional energy resources is more the more obvious. Because of the advantages such as large storage and less pollution, natural gas hydrate is considered to be the greatest potential energy resource. Natural gas hydrate is an ice-like crystalline compound, formed by water and natural gas molecules under certain temperature and pressure conditions, also called clathrate hydrate or "flammable ice". Since the gas forming hydrate is mainly methane,it is also called methane hydrate. Natural gas hydrates distribute widely in marine continental margin sediments and permafrost environments, and the overwhelming majority of hydrate reservoir discovered existed in the former.So far, the study of exploiting natural gas hydrate is still on the step of experimentation. Natural gas hydrate is thought to become a new medium for energy storage and transport for its stability below 273K at atmospheric pressure. Pepole thought that the stability is caused by a ice layer which forms from the dissociated gas hydrate; the ice layer coats the gas hydrate, sealing its further dissociation, and the effect is called self-preservation of gas hydrate. To research the temperature dependence of dissociation and self-preservation of gas hydrate, a X-ray diffraction was used to observe the dissociation process of gas hydrate in situ. In the experiments, we clarified the dissociation process of CH4 hydrate at atmospheric pressure with temperatures between 148 and 253 K. In situ measurements of CH4 hydrate dissociation using X-ray diffraction indicated that dissociation has two regimes. Ice growth is relatively slow in the second regime; furthermore, between 168 and 198 K, the hydrate decomposition follows a diffusion process that indicates that the hydrate is coated by an ice layer.To understand the self-preservation during dissociation process of gas hydrate further,a molecular dynamics method is used to simulate the dissociation process of gas hydrate I at some temperatures. From the simulation, we found that the state of H2O in ice layer affected the mass transfer coefficient directly. The state of H2O in ice layer was similar with that in ice when the temperature was lower than freezing point. At the time, as the mass transfer coefficient was high, the diffusion coefficient of H2O was low; So CH4 can't flee the ice layer easily, then the further hydrate dissociation was blocked. The state of H2O in ice layer was liquid when the temperature was higher than freezing point. At the time, as the mass transfer coefficient was low, the diffusion coefficient of H2O was high; So CH4 can flee the ice layer easily.We analysed the difference of dissociation processes and self-preservation of gas hydrate using different depressants. It was found that hydrogen bond was formed between oxygen in depressants and H2O which form the cage structure of gas hydrate clathrates. The hydrogen bond decrease the stability of gas hydrate clathrates, as a result, the cage structure break up. For the reason, CH4 from dissociation hydrate can diffuse easily, then self-preservation slow down to some extent.It is found that porous medium has an important place in affecting the stability of gas hydrate and self-preservation. The paper researched the stability of gas hydrate in porous medium; the effect of boundary conduction in dissociation process of gas hydrate in porous medium was also studied through the experimental method. It was found that temperature was the determinant element of hydrate dissociation rate if the permeability of porous medium was high, in other words, heat transfer was the determinant element of hydrate dissociation rate.A thermodynamics model and a kinetics model were built to analyse the thermokinetics behavior. Energy consumption in hydrate exploitation with thennostimulation method was analyzed. It was found that the energy consumption in hydrate exploitation included five parts: energy consumption in hydrate dissociation, energy consumption in temperature rising of rock, energy consumption in temperature rising of hydrate, energy consumption in temperature rising of formation water and energy consumption in temperature rising of water and natural gas from dismembered natural gas hydrate. Energy consumption in hydrate dissociation and temperature rising of hydrate was a little part of the total energy consumption. So improving utilization rate of energy consumption in hydrate exploitation with thennostimulation method is a major project today.A governing equation of pressure and temperature in hydrate exploitation with thennostimulation method was given at the conditions of adverse reactions in hydrate dissociation, mass conservation law and low of energy conservation. And pressure, temperature and gas production rate in hydrate exploitation were analyzed with numerical simulation method. And a useful energy consumption model of exploiting gas hydrate with thermostimulation method was also built.