Simulated Experimental Study On Several Significant Problems Related To Marine Gas Hydrate

Marine gas hydrate is a potential important energy resource attracting the attentions worldwide. Therefore, how to correctly explore and evaluate the resource of marine gas hydrate is an urgent and significant problem to be solved. This paper aims to solve such problems related to marine hydrate research. Based on the low-temperature high-pressure techniques developed in our laboratory, several detection methods are discussed, including acoustic method, impendence method and Time-Domain -Reflector (TDR) method. A simple apparatus is specially developed to form gas hydrate, in basis of this, the gas content in hydrate formed in different conditions and the elemental geochemical anomaly in ambient environment during hydrate formation is studied in this paper. The main ideas are:1. Compositions, structures, properties, formation conditions, distribution feathers and marine gas hydrate resources as well as several detection techniques for marine gas hydrate are discussed concisely. The present situation and research tendency of experimental simulated study on marine gas hydrate is also introduced. Based on these, the aim and content of this study is presented.2. The phase equilibrium conditions of methane hydrate is studied in a spherical high-pressure vessel. The detection sensitivity is improved when the transmittance light strength ratio is used to detect the formation/dissociation of gas hydrate. The stability conditions of methane hydrate in pure water-methane system measured in this paper agree well with the classical phase equilibrium curves from literatures. In sediments, the formation/ dissociation process of gas hydrate is detected by changes of temperature and pressure in the vessel, the results show that the P-T conditions of methane hydrate in natural sand (0.28-0.9mm) is almost the same as those in the pure water-methane hydrate system, suggesting little effect on the stability conditions of gas hydrate by coarse sediments.3. Ultrasonic detection technique indicates that the velocity is not sensitive to the formation/decomposition of hydrate in a pure-water system. In loose sediments, sonic velocity changes sensitively during formation/dissociation processes of hydrate in the system, whereas the variation of wave amplitude is not significant in this case. In the sediment core, the velocity and amplitude of the compressional and shear waves increase as the porosity decrease, the sonic velocity sensitively reflects the variation of hydrate saturation in the marine sediment core.4. The preliminary results show that the TDR detection technique and the impedance detection method can detect the process of formation/dissociation of hydrate in laboratory. The TDR detection technique can determine water contents insediment core sensitively, that is to say that this method is sensitive enough to detect the hydrate saturation in the core. The impedance detection method is sensitive to the nucleation and micro-crystalline process of CO2 hydrate in the vessel, which is supposed to be useful in studying the hydrate nucleic mechanism in the future.5. Another simple experimental apparatus is designed to be easy to open, which uses the high-frequency vibrator and gains the national patent. There is an inner chamber in the high-pressure vessel, so that the synthesized hydrate can be taken out conveniently for direct gas storage determination in the hydrates, and the residual water can also be separated from the hydrate to determinate the ion concentrations. This apparatus is the experimental basis for gas storage determination and geochemical anomaly research for marine gas hydrate.6. Experimental study on gas storage in methane hydrate is carried out in the above apparatus. A new setup and technique is specially devised to measure gas volume during hydrates decomposing in a vacuum system. The results show that the determined E values are different in different conditions such as pressure, temperature, gas content in water and time, indicating gas storage in hydrate vary with its formation conditions. Compared with traditional gravimetric method, the volumetric method has a lot of advantages. It is easy to work and generally not affected by environmental conditions such as temperature and humidity. It is believed that this method is more useful to determine gas storage of natural gas hydrate in sediments, and will be widely used in the assessment of gas hydrate resources and the transport of natural gas.7. The formation process of marine gas hydrates is simulated in laboratory, and variations of ionic and isotopic concentrations in seawater are studied preliminarily. The results show that the effects of gas hydrate formation on the variations of ionic concentrations are not alike in different experimental conditions. The larger the gas consumption, the higher purity the hydrates, therefore, the salt-exclusion efficiency is stronger, resulting higher ionic concentrations in the residual solution, lower in the112hydrate solutions. The 8 O> 5D values in the solution are determined before and after each experiment, and the obtained isotopic fractionation factors of oxygen and hydrogen in natural seawater system are 1.0034 ~ 1.0063 and 1.018 — 1.036, respectively. SDS is an efficient surfactant, which can not only accelerate gas hydrate formation, but also probably affect the isotopic fractionation of oxygen and hydrogen.