The structural transition of methane-ethane gas hydrates is generally observed during the forming process; however, it has seldom been reported during the dissociation process. Study on the dissociation behavior of methane-ethane hydrate below ice point has important implications on gas storage and transportation. It was also be helpful for the natural gas hydrate production by depressurization in permafrost zones. The dissociation of a series of methane-ethane hydrate samples at atmospheric pressure and temperatures below ice point (272.15-269.15 K) was performed, and the influence of gas composition and temperature on the structural transition was examined using in situ Raman spectroscopy. The hydrate structures were found to transition from structure I to structure II over a methane composition range of 50-68 mol%. The hydrates remained as sI or sII type compounds, and no structural transition occurred during the dissociation when the methane content in methane-ethane gas mixture was decreased to a certain amount (< 50 mol%) or increased to a higher value (>= 70 mol%). Further investigation showed that the occurrence time of structural transition reduced with an increase in the methane concentration under the same decomposition temperature. Furthermore, hydrate dissociation was retarded upon decreasing the temperature in this temperature range (272.15-269.15 K). The mechanism of the structural transition occurring in gas hydrate decomposition was proposed.

The structural transitions in the forming process of methane and ethane gas hydrate were reported previously, this phenomenon has also been observed during the dissociation process of methane and ethane gas hydrate. In our earlier work, the self-preservation and structural transition of 68 mol.% CH4+32 mol.% C2H6 hydrates during dissociation below the ice point have been studied. In this work, the dissociation process of a series of methane + ethane hydrate samples at atmospheric pressure and temperatures below the ice point (272.15 K-269.15 K) was analyzed using in suit Raman spectroscopic, suggesting the hydrate structures transfer from structure I to structure II over a methane vapor composition (yCH4) range of 50 mol.%-68 mol.%. Further investigation shows that the time of structure transition is reduced with the increasing yCH4 at the same decomposition temperature. Meanwhile, hydrate dissociation is retarded with decreasing temperature in this temperature region (272.15 K-269.15 K). The result of this work can be applied to hydrate exploitation in permafrost zone and seabed. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.

In this work, the gas production behaviours from synthesized methane hydrate reservoir by the combined method of ethylene glycol (EG) pre-injection and depressurization was investigated in a medium size three-dimensional reactor. The results indicated that the production period could be significantly shortened by this combination method. The characteristics of EG migration were largely influenced by injection rate, which caused different gas production rates in the follow-up process. In addition, EG efficiency could be enhanced by adjusting EG injection rates. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.

The structural transition of methane-ethane gas hydrates is generally observed during the forming process; however, it has seldom been reported during the dissociation process. Study on the dissociation behavior of methane-ethane hydrate below ice point has important implications on gas storage and transportation. It was also be helpful for the natural gas hydrate production by depressurization in permafrost zones. The dissociation of a series of methane-ethane hydrate samples at atmospheric pressure and temperatures below ice point (272.15-269.15 K) was performed, and the influence of gas composition and temperature on the structural transition was examined using in situ Raman spectroscopy. The hydrate structures were found to transition from structure I to structure II over a methane composition range of 50-68 mol%. The hydrates remained as sI or sII type compounds, and no structural transition occurred during the dissociation when the methane content in methane-ethane gas mixture was decreased to a certain amount (= 70 mol%). Further investigation showed that the occurrence time of structural transition reduced with an increase in the methane concentration under the same decomposition temperature. Furthermore, hydrate dissociation was retarded upon decreasing the temperature in this temperature range (272.15-269.15 K). The mechanism of the structural transition occurring in gas hydrate decomposition was proposed.

Aiming at increasing the exploitation efficiency of natural gas hydrate via decreasing the yield of water, a new idea "reservoir reformation" is proposed in this work, i.e., reforming natural gas hydrate reservoir by constructing an artificial impermeable overlying CO2 hydrate cap. Fine feasibility of this idea has been proved in this laboratory scale work. After reformation by CO2 injection to the permeable overburden, a confined environment with an impermeable CO2 hydrate cap is constructed successfully for depressurization operation. The cap can maintain mechanical stability for enough long time until the finish of production process. With the protection of the artificial CO2 hydrate cap, the production efficiency is greatly improved and the water yield is decreased remarkably. This work is of energy and environment double significance although lots of deeper and wider work should be done in future. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.

To enhance the exploitation efficiency of natural gas hydrate by decreasing the yield of water, a novel "reservoir reformation" concept is proposed that involves the reformation of a natural gas hydrate reservoir by constructing an artificial impermeable overlying CO2 hydrate cap. The feasibility of this concept has been demonstrated in this laboratory-scale experiment. After reformation by injecting CO2 emulsion into the permeable overburden, a confined environment with an impermeable CO2 hydrate cap is successfully constructed for depressurization operation. The cap can maintain mechanical stability until the end of production process. With the protection provided by the artificial CO2 hydrate cap, the production efficiency was greatly improved to 83.3% and the water yield is remarkably decreased. Moreover, the optimal CO2/H2O volume ratio of the emulsion for forming the desired CO2 hydrate cap was confirmed to be 1:1. The formation of CO2 hydrate cap can also protect the geological stability of depleted methane hydrate zones and seal a large amount of CO2, which is of both energetic and environmental significance; however, intensive and extensive research should be conducted in the future.