| With the depletion of conventional energy, mankind is faced with the serious problem of energy shortage. As a widespread energy, natural gas hydrate (NGH) is deemed to be a new energy with large commercial exploitation value in the21th century because of its advantages of abundance resource, high energy density and clean combustion. At present, the main methods of NGH exploitation include depressurization, thermal stimulation and adding inhibitors, all of the three traditional methods may cause geological instability of the stratum where NGH exists, thus the traditional methods are constrained in the actual mining process. On the other hand, the massive discharge of greenhouse gas CO2has become a big environmental issue which human have to face, and CO2storage is an effective measure to reduce CO2emission and reduce the greenhouse effect. Based on the two points on the above, searchers suggest a new method to replace CH4in the hydrate by use of CO2. This method can not only avoid some traditional mining methods’advantages, but also provide an effective way to store CO2in a long time.In this dissertation, the memory effect between different structure hydrates is studied, the memory effect between different structure hydrates is confirmed, this result provides theoretical support for the replacement experiment. In different conditions of pressure and temperature, different saturated CH4hydrates are replaced withCO2, the influence of different experimental factors is explored. The research contents are as follows:(1) The domestic and international research on the replacement CH4in the hydrate with CO2is analyzed and summarized, the feasibility of replacement is proved, the advantages and disadvantages of replacement with different forms of CO2are indicated, and the key technical problems of replacement with different forms of CO2are stated. Besides, combining actual conditions of our laboratory, two different experimental systems which are suitable for this study are designed(2) Three different structure hydrates are formed by controlling the pressure and temperature. The result shows that the change rule of pressure and temperature is consistent regardless of the different conditions of pressure and temperature in the different hydrate formation processes. This conclusion proves that the hydrate formation follows the same rule, and has important reference value for the judgment of hydrate formation statues.(3) In the same condition of pressure and temperature, the repetitive experiments of three different structure hydrates are done with different water respectively. The experimental results show that the decomposed water, either from the hydrate itself or other structure hydrates, can reduce the induction time in the three hydrate formation processes. This conclusion not only confirms the existent of the memory effect of the same hydrate, but also proves the memory effect exists between different structure hydrates. Formation and decomposition reactions of different hydrates are contained in the replacement process, thus the result about the memory effect between different hydrates can provide important theoretical support for the feasibility of replacement.(4) Different saturated CH4hydrates are replaced with gaseous CO2in different conditions of pressure and temperature. Through analyzing the experimental result it can be concluded that the saturation of CH4hydrate and the replacement condition of pressure and temperature have important influence on the replacement efficiency, while the replacement time has little effect on the replacement efficiency.(5) The methods of replacement with gaseous CO2and decompression are combined, the replacement efficiency with combined method and that with single replacement method is compared. The result shows that the former is higher than the later, it proves the combined method is an efficient way to recover CH4from the hydrate. |
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