Study On Gas Hydrate Phase Equilibrium Condition And Simulation Experiment And Testing Technology

Natural gas hydrate,also called clathrate,is a kind of crystalline solid,which may form wherever water and natural gas molecules exit under suitable conditions (suitable temperature,pressure gas saturation,water salinity,Ph value etc). The white gas hydrate looks like ice; its structure is also similar to that of ice. Its building blocks consist of a 'guest'gas molecule surrounded by a cage of water molecules. Under the condition of low temperature and suitable pressure,the gas molecule and water molecules are stably combined together by Van de Waals Force. In the natural environment,methane is the most frequently found 'guest'gas in gas hydrate. The gas hydrate which contents 99% methane is called methane hydrates. In the natural environment,gas hydrates largely distribute in such areas as the slopes of continent and island,continental margins,polar continental shelf,sea and the deep lakes. In standard condition,the breakdown of a unit volume of methane hydrate produces about 164 unit volumes of methane. It is assumed that the worldwide amount of methane in gas hydrates contain about twice the amount of carbon held in all fossil fuels on earth,which means gas hydrate is an very important potential future resource. At the same time,methane is a potent greenhouse gas. According to some researchers,the greenhouse effect of methane may be ten to twenty times of CO2. In addition to those,gas hydrate is also a factor closely related to geohazard. Therefore,gas hydrate study is becoming a hotspot in international geological society in resent years. Simulation experiment study of gas hydrates is of great importance for the exploitation of gas hydrates. The key problem in this study is the effectiveness of the testing methods. Designed by Qingdao Institute of Marine Geology,the first professional gas hydrate simulation laboratory was established in November 2002. Equipped with various apparatus for optics,ultrasonic,temperature and pressure experiment,it aims to serve for the exploration and exploitation of marine gas hydrates. We observed the formation and decomposition of methane gas hydrates by measuring the changes of light transmissivity in pure water and methane system. We got satisfied result. The phase equilibrium diagram we got from the experiment was quite similar to the international classic one,which proved that our lab system was feasible and reliable. All marine gas hydrates occur in the sediment. It has been found that in different sediments,the P-T equilibrium conditions of gas hydrates were quite different due to the difference of the porosity and other parameters. So it is very important to conduct the gas hydrate P-T equilibrium condition testing using gas hydrate bearing sediment in the lab. So far,there is no good method for simulating the formation and decomposition of gas hydrate in the sediments. This paper is about our experimental study of the formation and decomposition of gas hydrate in the sediment by P-T method. Through long time experiment,we found that the phase equilibrium condition of the water-methane system in natural sand was basically in accordance with that of the pure water-methane system. We made a great progress in the technique of ultrasonic testing. In the experiment with pure water-methane system,we found that sound velocity and amplitude varied following a rule with the formation or decomposition of the gas hydrates. But the result was not good due to the influence of the system temperature. We found when the frequency was 112Khz,the variation of the amplitude was sensitive to the gas hydrate formation and decomposition. The amplitude rose as the large quantity of gas hydrates appeared,and it went down when gas hydrates decompounded. Thus we can measure the amplitude change to detect the formation and decomposition of gas hydrates. When we did experiment with gas hydrate containing loose sediment,we used both ultrasonic and P-T methods. The experiment showed that the velocity was very sensitive to the gas hydrate formation and decomposition. The velocity increased as the gas hydrates formed and decreased when they broke down. This result was consistent with what we anticipated. We also found that the frequency changed regularly,from 112KHz before the formation of gas hydrate to 107KHz,and became 112KHz again after the gas hydrate decomposed. Hysteresis was showed in the experiment. The characteristic frequency of the gas hydrate in our experiment should be 107KHz. Resistance method is also an effective way for the study of the mechanism of gas hydrate formation in the sediment. Based on the theoretic discussion,this paper summed up the experiences of several testing methods for gas hydrate simulation experiment. To conduct the resistance method,we developed the equipment by ourselves and did the experiment with the CO2 and CH4 gas hydrate bearing sediment. And we got some good results. We measured the impedance differences during the gas hydrate formation and decomposition process. The data indicated that thermal procedure and thermal dynamic...