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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 (

Nowadays, the combination of natural gas hydrates exploitation and CO2 storage is considered of excellent application. In this work, we explored reservoir reformation technology to enhance gas recovery from the unsealed water-saturated hydrate-bearing deposits in a large-scale tridimensional simulator. Firstly, recurrent CO2 injection into the overburden was to gradually form CO2 hydrate and reduce its permeability. Then traditional depressurization operations were conducted to compare the influences of overburden thickness and additional reservoir reformation on the evolution of pressure distribution and the gas/water production. The results showed that reservoir reformation could effectively improve the mining efficiency, reaching ∼2–4 times of gas production and ∼3–5 times of gas–water ratio compared with the direct depressurization. After a long-term reformation, CH4 recovery ratio was increased from 24.5% to 84.9%. Additionally, we further demonstrated the complete spatiotemporal evolution mechanism of the initial CH4 hydrate and artificial overlying CO2 hydrate during depressurization. A large amount of CO2 was ultimately stored in the depleted reservoir in the form of CO2 hydrate and the CO2 storage ratio was up to 79.5%. Therefore, this reservoir reformation approach is greatly promising for high-efficient and safe exploitation of marine hydrate and CO2 storage.

CO2 replacement is considered as a promising method for the simultaneous development of natural gas hydrate and CO2 sequestration. The addition of small molecular gases, such as N2 and H2, into the injected gas can increase the gas recovery ratio and prevent CO2 liquefaction. Based on previous studies, this work presents methane hydrate exploitation using the CO2/H2 continuous injection-production mode. The mechanism combines gas sweep with CH4/CO2 replacement. A series of experiments were carried out to optimize the injected gas composition and flow rate, which have a significant effect on the rate of CH4 hydrate decomposition, amount of CO2 sequestration, and cost. The compositions of the injected gases had little effect on the recovery rate when a relatively higher flow rate was employed. A balance between CH4 production and CO2 sequestration was established when the CO2 mole fraction was slightly

The exploitation of unsealed marine hydrate deposits is of greater commercial value, but also more challenging because of the risks of the invading of huge body of sea water and subsidence of the seabed. In this work, we proposed a new method to recover natural gas from the usealed hydrate deposits saturated with water by pressure-retaining gas (CO2 + H2) injection. A series of experimental simulations was performed to prove its feasibility through establishing a highly simulated unsealed marine hydrate-bearing sediments system. The results demonstrated that overlying water invading was almost completely inhibited for the pressure-retaining approach while unimpeded for the simple depressrization process. Further, we designed a temporary depressurization process before gas injection to establish a flow field for gas in the sediments. This combined operation effectively enhanced the migration and spread of the injected gas, eventually increasing gas recovery ratio from 25% to 60% while the produced water/gas ratio remains as low as 12.6 ST kg/m3. Additionally, it is proved that the balance between CO2 sequestered and CH4 recovered could be achieved, which was an excellent support for reservoir stability. The combination method realized a higher CH4 concentration in the produced gas and a lower injection-production ratio, which greatly reduced the costs of gas injection and subsequent treatment of produced gas. For comparison, similar simulation work was also performed with respect to sealed gas-rich hydrate-bearing sediments. Overall, this temporary depressurization aided pressure-retaining gas injection approach is very promising for high efficient and safe exploitation of marine hydrtates.

Gas injection, as a method for natural gas hydrate exploitation, has attracted much attention, because of its advantages of low energy consumption and non-destructive characteristics. The gas recovery can be significantly enhanced by adding small-molecule gases such as N2 or H2 to the injecting gas mixture. To evaluate the independent effects of the small-molecule gases on the CH4 hydrate decomposition behaviours, the Raman spectroscopy, gas chromatography, and a CCD camera were adopted. The experimental results show that the CH4 decomposition pressure was increased by injection of small-molecule gases such as N2 or H2. The addition of small-molecule gases in the gas phase led to the change of the hydrate phase equilibrium, which made the CH4 hydrate become more unstable. No N2/H2 hydrates Raman peaks were obtained throughout the experimental process, which indicates that the N2/H2 molecules did not enter the hydrate cages and did not participate in the hydrate formation. Moreover, it suggests that the injected small-molecule gases promote the decomposition of CH4 hydrate by changing the fugacity of CH4. H2 has a more significant effect on the decomposition of CH4 hydrate than N2. This is probably due to the larger diffusion coefficient of H2 in the hydrate phase than that of N2. This kind of promotion effect is beneficial to the production of CH4 gas from hydrate.

The self-preservation is considered as an advantageous property for natural gas hydrate transportation and storage. However, it may also bring serious troubles for well drilling and hydrate exploitation. In this work, various factors affecting the self-preservation effect of CH4 hydrate were investigated by using HP μ-DSC. The results indicate the presence of porous sediments does not influence the anomalous self-preservation region of CH4 hydrate. The CH4 hydrate dissociation rate increases with the decreased initial water content and quartz sand particle size as a whole. However, the self-preservation could be still found in a very low initial water content (10 vol%) and small-particle sediments (25–38 μm) condition. On the other hand, an enhanced self-preservation effect and excessive pressure phenomenon were unexpectedly found in bentonite and kaolin. The hydrate can still maintain high metastability with hardly any decomposition after the pressure was released to atmospheric pressure. In situ Raman and CCD camera were used for the further study of the mechanism. We speculate the interaction of hydrogen bonds between bentonite and hydrate might be the main reason for this abnormal phenomenon. The knowledge gained in this work is significant for the comprehension of CH4 hydrate dissociation in porous sediments below ice point, and provides a better understanding of the self-preservation mechanism.

In recent years, wireless sensor networks (WSN) have attracted a great deal of attention in the research community. This is due to the applications that will be enabled once wireless sensor networks are in place. Wireless sensor networks are emerging as a promising solution for various types of futuristic applications both for military and public. Security in wireless sensor networks is very crucial as these networks are expected mainly to operate in the hazardous and hostile environments. Efficient key management could guarantee authenticity and confidentiality of the data exchanged among the nodes in the network. Depending on the application requirements, the WSN may operate in star topology, in which the communication is established between sensors (i.e. nodes or devices) and a single central controller, called the WSN coordinator. In order to make the power consumed as efficiently as possible, a typical WSN architecture is first described and then energy strategies and model are specified on account of the hardware constraints of the sensors in the paper. Furthermore, energy estimating method is formulated and deduced. At last, computer simulation based on the above architecture and model gives the analysis illustrations and the results. Experiments and simulation show how energy savings of 50% can be achieved and can delay the deadline of the WSN system. This energy model can also be used for power management in any system characterized by different levels of power consumption in various stages of shutdown.