Study And Application Of Analytical Method For Gas Compositions Of Natural Gas Hydrate

Natural gas hydrate, with important strategic significance and great economicvalue, is recognized as one of the alternative energy sources throughout the world.The molecular and isotopic composition of the guest of gas hydrate can providevaluable information, such as the origins and sources of gases, the mechanism of gasmigration and accumulation process, and the control factors of gas hydrate formationand decomposition. The experiment and simulation method is believed to be a criticalaspect of the gas hydrate exploration and exploitation. To establish series analyticaltechniques and to carry out related experimental simulation is very important forrecognizing the gas type, source and origin, and revealing the “formation” and“decomposition” behavior of gas hydrates in the natural environment in China. Theseresearch achievements will be helpful to promote continuously the development ofgas hydrate exploration and exploitation.In this paper, an analytical method for molecular and isotopic composition ofhydrate-bound gases was established, which has been successfully applied in thedetermination of natural gas hydrate samples from South China Sea and QilianMountain Permafrost, and in experimental studies on formation-dessociation ofmarine gas hydrate. The main results are listed as follows.A gas chromatographic analysis technique with parallel detectors of flameionization detector and thermal conductivity detector （GC-FID/TCD） was establishedby one column, one capillary splitter and two detectors. The method can be used todetermine16gases （C₁^C₆alkanes, CO₂, H₂S, O₂+N₂） of natural gas hydratessimultaneously by one injection. The analysis parameters, such as capillary column,temperature program, column flow, inlet temperature, detector temperature, and thereference&makeup flow of TCD were optimized. Under the optimal conditions,good linearities were obtained in the experimental concentration ranges for the16gases with correlation coefficients （r²） of0.99903-0.99998. The detection limits ranged from0.0003mol/mol to0.046mol/mol with the relative standard deviations（RSDs, n=6） of1.62%-5.05%. Meanwhile, a gas chromatography-isotope ratio massspectrometry （GC-IRMS） method for determination of carbon and hydrogen isotopeof gas hydrate-bound gases （C₁^C₅, CO₂） was also established. The precision ofδ¹³C-（C₁^C₅, CO₂） and δD-（C₁^C₅） is in the range of0.05‰-0.15‰and1.10‰-1.89‰, respectively, and with relative standard deviations （RSD, n=6） of0.20%-0.57%and0.71%-1.74%, respectively. These methods have been proved to besimple and practical, and suitable for the measurement of gas molecular and isotopiccompositions in the natural gas hydrates from marine, permafrost and laboratorysynthetic samples.Some pre-treatment techniques for measurement of molecular and isotopiccompositions of hydrate-bound gases were studied. These techniques include thepreserve method of hydrate samples, the practical applicability of different gasdecomposition and collection methods （i.e. headspace, syring, drainage）, and thestorage effect of different storage vessels （i.e. aluminum air bag, glass bottle withbutyl rubber plug）. The results indicate that the best temperature for gas hydratesample storage is less than-100℃under atmospheric pressure. The headspacemethod and syring method can be widely used in hydrate-bound gas’ decompositionand collection, while drainage method is not suitable for hydrate samples containingCO₂. Therefore, we should prefer to use glass bottle with butyl rubber plug for gasstorage other than air bag. In addition, it is suggested that the optimum time scale forthe analysis of molecular and isotopic compositions in5and60days, respectively.The characteristics of the molecular and stable isotope composition ofhydrate-bound gases from South China Sea and Qilian Mountain Permafrost areinvestigated comparatively. The results show that the gas hydrate samples from SouthChina Sea mainly contain methane （99.38%-99.69%）, less amount of C₂, and littleamount of C₃. The gases are characterized for dominant microbial origin of C₁withsmall amounts of thermogenic C₂. By contrast, the gas hydrate samples from QilianMountain Permafrost contain not only a quite amount of C₁^C₅alkanes, but also acertain amount of C₆,CO₂etc.,suggesting a thermogenic origin of gases. In this paper, a gas hydrate simulation experimental apparatus equipped withonline gas chromatography was used to do experiments about the “formation” and“decomposition” behavors of complex gas hydrates in the pure water, sea water, andseawater-sediment systems. The results indicate that the formation rate of gas hydratein the three systems is in the order of “pure water> sea water> seawater-sediment”under the same temperature and pressure, whereas the decomposition rate is on thecontrary. During the hydrate formation, the mole fraction of each gas component willbe changed between the free gas phase and the hydrate phase, with similardifferentiation rules: free gas> original gas> decomposition gas for C₁,decomposition gas> original gas> free gas for CO₂, C₂and C₃, and decompositiongas <original gas for C₄and C₅. However, there is almost no isotope fractionation ofcarbon and hydrogen between free gas and decomposition gas with the fractionationfactor around1.000.