Applications of X-ray Diffraction in Natural Gas Hydrate Research

Natural gas hydrates are clathrate hydrates formed from gas molecules (e.g. hydrocarbon gases and non-hydrocarbon gases such as CO₂ and H₂S) and water molecules under high pressure and low temperature, with three types of crystal structure, such as cubic (Ⅰ), rhombus (Ⅱ) and hexagonal (H). The structural characteristics and variation rule of clathrate hydrate is significant for understanding the formation mechanism, micro-kinetic, phase transformation and identification of gas hydrate. X-ray Diffraction (XRD) is an analytical technique that is used to analyze the crystal micro-structure (i.e. atom orientation) based on the diffraction pattern produced by X-ray irradiating the crystal or some non-crystal substances. When XRD is applied in gas hydrate research, not only can it provide the important information such as structure type and lattice parameter of gas hydrate, but it can also be used to observe the micro-kinetic process of hydrate formation and dissociation. Several applications of XRD in natural gas hydrates research, such as structural characteristics, in situ observation of gas hydrates formation/dissociation, and hydrate sample identification on field are reviewed in this paper. It is well known that the crystal lattice of structure Ⅰ and Ⅱ is approximately 12.0×10^-10 m and 17.3×10^-10 m, respectively, and approximately 12.2×10^-10 m and 10.0×10^-10 m along a and c axis for structure H. Therefore, the crystal structure type can be easily identified with the accurate lattice parameters measured by XRD. Now, the XRD technique has been already conducted in identifying and obtaining the structural information of gas hydrate samples recovered from marine and permafrost region overseas. In addition, the structure transformation and influencing factors of gas hydrate can be investigated based on the lattice parameter variation of gas hydrate, which was measured under different conditions. The experimental results show that all the mixture composition, the volume and diameter of guest molecules as well as the temperature can affect the lattice parameter and structure of gas hydrate. In situ XRD technique under high pressure is also used to measure the variation of diffraction peaks during gas hydrate formation/dissociation, and eventually to investigate the kinetic processes of gas hydrate formation and dissociation. The results show that two stages were observed during gas hydrate formation/dissociation, the fast process of hydrate formation/dissociation on the gas-liquid (solid) surface and the gas molecules diffusion process in the liquid (solid). The last stage controls the whole reaction speed. At present, although the XRD technique is widely used in gas hydrate research abroad, it is just beginning in China. It is believed that the XRD technique can be used in gas hydrate research to resolve the basic scientific problems such as structure type identification and lattice parameter measurement, and will provide a powerful technological support for gas hydrate relevant theory research once it combines with other analytical techniques like NMR, IR and X-CT.