| Natural gas hydrate is a kind of non-stoichiometric, ice-like clathrate hydrate, commonly known as "burning ice". It is a promising energy with huge reserves. Thus, developing safe and efficient technology for recovery is crucial. Depressurization is believed to be the most efficient and economical method. At present, the research of Natural gas hydrate recovery by depressurization under different types of storage is limited, thus it is a great of importance to research the depressurization dissociation behavior for different types of Natural gas hydrate storage.In this paper, considering the different types of Natural gas hydrate deposits, we mainly studied the effect of variable gas-water saturations on dissociation behavior of NGH by depressurization. According to the research, we designed a Magnetic Resonance Imaging (MRI) experiment system, and developed the corresponding experimental process. The methane hydrate (MH) was formed in the porous media, and through injecting water, we simulated the MH deposit with variable gas-water saturation before dissociation. At the same time, MRI showed the liquid water distribution in the whole experimental process, which was used to analyze the MH formation and MH dissociation behavior under variable gas-water saturations. The depressurization was applied to exploit the MH storage under variable gas-water saturations in this study, and we obtained the related parameter characteristics. During MH dissociation, four kinds of backpressures (2.8 MPa,2.6 MPa,2.4 MPa,2.2 MPa) were used to investigate the effects of pressures on MH dissociation behavior.The experimental results indicated that for the gas saturated porous media, the larger the pressure drop could make hydrate speed up decomposition through the differences of average MH dissociation rates under variable depressurization. After MH dissociation, the water distribution was different from the initial state in the porous media because of the movement of methane gas and water. For the low-water saturated porous media, we found that some liquid water flowed out of core barrel, which indicated that the path of gas output would be hindered by the water. Meanwhile MH dissociation pattern was effected by heat transfer, and MH dissociated first near the vessel axis where more liquid water concentrated. The larger depressurization range can enhance the average dissociation rate, while heat transfer affects the rate as well. For the high-water saturated porous media, water has a big liquidity during MH dissociation, which showed the greater hindrance on gas output. Also the variable depressurizations have little influence on dissociation pattern, and heat transfer is dominated. Though the difference of average dissociation rate is small, the effect of lager depressurization range is obvious, which agree with the first two conditions.The MH dissociation under high gas saturation porous media was compared with the dissociation under low water saturated and high water saturated porous media respectively, and we found that the increasing liquid water saturation could make MH dissociate faster. Meanwhile, the MH dissociation under low water saturated and high water saturated porous media has carried on the contrast with each other. It indicated that larger depressurization range could speed up MH dissociation. Liquid water saturation and depressurization range are two key factors for MH dissociation by depressurization. Increasing liquid water saturation can slow the rate of MH dissociation, and larger depressurization range can speed up the rate. Under different conditions, there are differences for the leading factors of MH dissociation rate. Comparing MH dissociation behavior by depressurization on high gas saturated, low liquid water saturated and high liquid water saturated porous media, we found that during MH dissociation, the higher the saturation liquid water had in porous media, the higher the liquidity water had, thus the greater hindrance was for gas output. |
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