Research On The Multi-component Gas Hydrates: Structure Characteristics,formation And Dissociation Process

Multi-component gas hydrates are widely distributed all over the world.Compared with methane hydrate,the study of basic characteristics of multi-component gas hydrate,such as micro-structure,formation and dissociation kinetics,thermodynamics and micro-distribution,are scarce.It is of great significance to study the basic characteristics of multi-component gas hydrates in exploration,development and utilization of multi-component gas hydrate resources.In this thesis,multi-component gas hydrate is systematically studied using experimental simulation method,combined with advanced experimental techniques such as X-ray diffraction,solid-state nuclear magnetic resonance(SSNMR),Raman spectroscopy,X-ray computerized tomography(X-CT),and the main understandings are listed as follows:The lattice parameters of gas hydrates are related to the size and content of guest molecules: for single component gas hydrates,the van der Waals diameter of guest molecule is positively correlated with the lattice parameters of corresponding hydrates.The cell size of binary component gas hydrates is mainly controlled by the content of the guest molecule with larger size.Similarly,the lattice parameters of complex component gas hydrates are generallyshrinking as the total contents of smaller ones increase.Temperature has a significant effect on the lattice parameters of hydrates.The change of lattice parameters of both structure I(methane hydrate)and structure II(complex component gas hydrate)hydrate with temperature accords with the corresponding quadratic function.Quantitative nuclear magnetic resonance(NMR)experiments show that the almost all the larger guest molecules occupy the large cages of structure II hydrate,while the smaller molecules have a lower cage occupancies in large cages than that of type I hydrate.To some extent,the different cage occupancies of guest molecules can explain the reason why the lattice parameters of hydrate are controlled by guest molecules with larger size.Raman spectra of gas hydrates with single,binary and complex components are compared and analyzed.Raman spectra of guest molecules in different cage structures and their attribution are determined.It is concluded that cage distributions of methane molecule cannot be determined by using the spectrum peaks of methane only.Judging the structure type of complex components hydrates needs synthetically analyses of the Raman characteristic peaks of all the guest molecules.In this thesis,the Raman spectra of eight groups of complex gas hydrate samples are basically the same,which coincide with the Raman spectra of structure II hydrate.The occupancies of methane in the large cage are generally low,and the sediment and particle size range show little influence on the Raman spectra of hydrate.The microstructural characteristics of natural gas hydrate samples in the South China Sea and permafrost regions of Qilian Mountainare determined.Based on the simulation experiments,the lattice parameters of hydrates in Shenhu area and Pearl River Mouth Basin in the South China Sea are obtained,as well as the cage occupancies and hydration numbers.The thermal expansion characteristics of natural gas hydrate samples in Shenhu area are measured for the first time,and the crystal structures of hydrates collected in different sea areas are compared and analyzed.Raman spectra of hydrate samples from different boreholes and buried depths in Juhugeng drilling area of Qinghai Province are basically identical,which coincides with the spectral characteristics of structure II hydrate.The guest molecules are mainly composed of light hydrocarbons such as methane,ethane,propane and butane.Methane mainly distributes in the small cages of hydrate with low filling cage occupancies.Nitrogen is commonly found in the samples,and hydrogen sulfide is detected for the first time in hydrate collected from permafrost region of China.The aggregation process of multi-component hydrate presents "Interface Dominance" phenomenon.The interface(gas-liquid interface,reactor wall and sedimentary medium)in the reaction system provides favorable conditions for the rapid nucleation and mass accumulation of hydrates.The permafrost core provides "interface advantage" for the rapid formation of hydrates,and obviously shortens the process of hydrate nucleation and growth.Gas composition,solution system and deposition medium show no significant effect on the dissociation conditions of multi-component hydrates under constant volume conditions.The dissociation process of multi-component gas hydrate is accompanied by the redistribution of different components among gas,liquid and solid hydrate phases.The stability condition of multi-component hydrate determined under constant volume condition is actually the stability condition of a dynamic equilibrium.The pressure change during vacuum dissociation of quartz sand + methane hydrate,natural gas hydrate samples collected in Qilian Mountain permafrost and Shenhu area of the South China Sea generally presents the characteristics of "fast-slow-fast",which accords with the phenomenon of "self-preservation effect" of hydrate.The micro-distribution of multi-component hydrates in porous space(quartz sand,alumina sphere and consolidated Berea sandstone)shows remarkable dynamic evolution characteristics with the change of hydrate contents(saturation).In the initial stage of hydrate formation,the hydratesin pore space are mainly in suspension mode,while in the middle stage of reaction,suspension and contact coexistence are dominant,and in the late stage,contact mode is mainly dominant.Fracture-type(Qilian Mountain cores)hydrate mainly grows along the fracture direction,and its formation and distribution characteristics are obviously controlled by the mass transfer process along the fracture.The results of this study can provide new enlightenment for the study of gas hydrate reservoir formation and future development in the permafrost region of Qilian Mountain.