Study Of Heat Transfer Characteristic And The Effects On Exploitation For Natural Gas Hydrate Sediments

The natural gas hydrate is the one of the most potential clean energy in the worldwide view. The development of exploration and exploitation of hydrate has come to a stage of breakthrough in China. The heat transfer of hydrate-bearing sediments has a significant effect on the high efficiency of gas production from hydrate. The improvement of the gas production efficiency has been a key problem of gas hydrate exploitation. Under this background, the control mechanism and change rule of the thermal conductivity of hydrate sediments, heat transfer factors of sediments and the heat transfer during the dissociation process were studied.To study the thermal conductivity characteristic of hydrate sediments, this paper developed an in-situ experimental measurement system and calculation method of effective thermal conductivity of hydrate sediments under the high pressure and low temperature conditions. The regularity of effective thermal conductivity of hydrate sediments with different factors were obtained and analyzed. A fitting correlation of effective thermal conductivity for hydrate sediments was developed. The fitting correlation could effectively predict the thermal conductivity of THF hydrate sediments and water saturated sediments.The different heat transfer factors of hydrate sediments during the gas production has been systematically studied, it is concluded that the thermal conductivity is dominant factor for the hydrate dissociated in a closed pattern with thermal stimulation. The experimental results proved that production temperature, thermal conductivity of sediments determined the efficiency of exploitation. The study showed that the high value of thermal conductivity of porous media would improve the dissociation of hydrate at the late stage of gas production. The high value of thermal conductivity of porous media could improve the efficiency of gas production on the aspect of enhancing temperature of gas production, obtaining high rate of gas production and more accumulated gas, suppressing the phenomenon of ice formation. Moreover, through the calculation and analysis of Ste, it is concluded that the sensible heat of hydrate sediments controls the gas production efficiency at the fast depressurization stage. The higher Ste could improve the decomposition of hydrate, increase the production temperature and suppress the phenomenon of ice formation. For the condition of Shi>30%, the effects of sensible heat of sediments on gas production weakened. In addition, this study further proved that the Qov become a dominant heat transfer factor at the late stage of gas production for the depressurization method. The improvement of gas production from Qov increases with the growth of hydrate saturation. Note that, the higher Qov induces the up-migration of free water in the sediments and decreases the efficiency of gas production.To establish the internal relationship of the heat transfer change rule during hydrate dissociation process and the gas production efficiency, the heat transfer control mechanism for the three different gas production method were first analyzed and compared based the efficiency of gas production. The experimental results proved that the sensible heat of sediments and the suppression of ice formation are the key factors for the gas production efficiency of depressurization. The energy efficiency, hydrate saturation and permeability are the dominant factors of efficiency of gas production. The combined method yield impressive efficiency of gas production, the experimental results concluded that the combined method would have the best application prospect. Furthermore, the change rule of effective thermal conductivity of methane hydrate sediments during the different stage of gas production were obtained and analyzed. The rate of gas production and hydrate saturation controls the decrease of the effective thermal conductivity of hydrate sediments after the dissociation of hydrate. A measurement method of local heat transfer coefficient in hydrate sediments during the hydrate dissociation process was developed. This study obtained the coordinated change rule of heat transfer coefficient in the hydrate sediments with gas production efficiency. This study proved that the phase change, rate of liquid migration is the dominant driving force of heat transfer coefficient. At the fast depressurization stage, the heat transfer coefficient increases with rate of gas production accurately. Based the heat transfer coefficient, this study found that the heat transfer coefficient increases with Shi first and then decreases.