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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 (

A three-scale in situ observation technique combining the optical microscope, confocal microscope, and Raman spectroscopy was proposed to gain insights into the thickening growth kinetic of hydrate film formed on a gas bubble suspended in water. The evolution of the morphology and mass transfer channels of the hydrate films formed from pure CH4 or the mixture of CH4 and C2H6 during thickening growth was investigated. Results demonstrated that the morphological evolution of hydrate films during thickening growth depended on the initial morphology of the hydrate film formed in the lateral growth. The sI CH4 and sI CH4–C2H6 hydrates had the same ultimate morphology (e.g., a large number of well-defined polyhedral granular hydrates), while the sII CH4–C2H6 hydrates appeared massive. It was clearly observed that the gas pores on the hydrate film gradually vanished with the growth of hydrate film, which suggested that the predominant mass transfer channels on the film changed from the gas pores to the lattices....

The CO2 replacement technique is considered as a promising approach for both gas hydrate recovery and CO2 sequestration. Based on this technique, the continuous CO2/H2 injection-production mode had been studied in our previous work. However, there are limitations on this continuous mode, such as lower CH4 recovery and CO2 sequestration ratios, lower CH4 concentration in produced gas and higher injection-production ratio caused by the fast breakthrough of injected gas. Here we proposed a so called semi-continuous injection-production mode, in which continuous injection-production process is interrupted periodically by stopping injection and production operations for letting injected gas diffuse sufficiently, delaying its breakthrough and increasing its effective sweep region. A series of experimental simulations were performed with respect to this mode in a three-dimensional simulator with a volume of 10.6 L. The results indicated that for the injected gas with low CO2 concentration, the CH4 recovery and concentration in the produced gas could be enhanced dramatically by the semi-continuous gas injection method. Additionally, the fast decomposition stage of CH4 hydrate could be retained separately in the replacement process, thereby effectively improving production efficiency. However, the corresponding CH4 concentration in the produced gas decreased and the injection production ratio increased. Notably, the process combining the CH4 steam reforming with cyclic injection-production was first simulated. The results showed an excellent CO2 storage capability and CH4 hydrate recovery ratio, which was mainly controlled by the gas composition of the re-injected gas. Through systematically comparing and analyzing the CH4 concentration, recovery ratio, production efficiency and injection-production ratio, it is demonstrated that the semi-continuous injection-production mode is superior to the continuous one. The results obtained in this work are of significance for guiding future NGHs exploitation.

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.

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.

Measuring the dissolution rates of CO2 hydrates in marine environments is urgently needed because CO2 hydrate is a powerful means in CO2 ocean sequestration as well as a product of CO2-CH4 gas replacement in hydrate exploitation. The stability of CO2 hydrates is a significant index to evaluate the feasibility of gas hydrate technologies. It is generally agreed that the dissolution of hydrates is diffusion-controlled rather than reaction-controlled. In this work, the dissolution rates of CO2 hydrates in sediments under a variety of experimental conditions were calculated by the CO2 fluxes and mass balance. The in situ CO2 concentrations in static and flowing water layers were measured by Raman spectroscopy. Firstly, the dissolution rates of CO2 hydrate in sediments in different diffusion stages were 314 to 3.5 cm/year and 194 to 1.2 cm/year in static deionized water and brine layer, respectively. The CO2 hydrate underwent a rapid dissolution stage in the beginning of the experiment. Then the dissolution rate slowed down because of the decreasing concentration gradients. The difference of dissolution rates between deionized water and brine is related to the CO2 solubility in different solutions. CO2 hydrates covered with sediments dissolved much slower than hydrates dissolved in direct contact with water layers due to the increased diffusion boundary layers and the decreased diffusion coefficients. Additionally, the dissolution rates of CO2 hydrate that exposed to a variety of flow rates showed a strong velocity dependency. In this work, the dissolution rates of CO2 hydrate under flowing water layers ranged from 5.6 to 365 cm/year with volume flow rates ranged from 1.01 to 8.57 ml/min.

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

Considering the limitations of a single method for hydrate recovery, the combination of the traditional methods receives more attention nowadays. In this work, the gas production behavior from a synthesized methane hydrate reservoir by the combined method of ethylene glycol (EG) injection and depressurization was investigated in a medium-size three-dimensional reactor. The hydrate-bearing sediments were prepared according to the temperature and pressure conditions of the South China Sea reservoir. The results indicated that the production period could be significantly shortened by the combination method. After comparison of different EG injection conditions, it is found that the migration characteristics of EG solution in the reservoir were quite different in the EG injection stage, which caused different gas production rates in the follow-up process. Moreover, the increase of the EG concentration or EG injection volume could enhance the gas production rate. From analysis of the evolution of the low-tempe...

Using the different diffusion rates of gas mixture components in hydrate crystals is a promising new method for the separation of gas mixtures. The diffusion behavior of hydrogen (H2), methane (CH4), and carbon dioxide (CO2) in tetrahydrofuran (THF) hydrate was investigated experimentally by exposing a THF hydrate layer to H2, H2 + CH4, or H2 + CO2 gas or gas mixtures and monitoring the penetration of the molecules into the hydrate by Raman spectra. The experimental results demonstrate that only H2 molecules can penetrate the entire 5 mm thick hydrate layer, while the THF hydrate is resistant to CH4 and CO2 molecules on the experimental time scale employed. The minimum partial pressure for H2 to diffuse through the THF layer is 2.9 MPa, and the diffusion coefficient of H2 in the THF hydrate layer was determined to be 6.1 × 10–12 m2/s via time-resolved Raman collection. This is the first observation of the sieving behavior of a massive hydrate layer toward different gas molecules. This work also suggests t...

The morphology and structure of hydrate films of pure methane and methane+ethane mixtures were studied by suspending a single gas bubble in liquid water. The methane+ethane gas mixtures, containing 20 mol% or 81–89 mol% methane, were used to form hydrate films with different crystal structures. Polyhedral structures appeared on the background of uniform polycrystalline hydrate films formed by gas mixtures with a high methane mole fraction of 0.81–0.82. Some of the polyhedral structures are holes, and others are separated structure II polyhedral hydrate crystals. However, for pure methane, a gas mixture with a lower CH 4 mole fraction of 0.2, or a gas mixture with a higher CH 4 mole fraction of 0.89, the morphology is uniform through the whole hydrate film. The crystal structures in the hydrate film were determined using in situ Raman spectra. The results show that both pure methane and the gas mixture with a lower CH 4 mole fraction of 0.2 form a hydrate film with structure I. The single polyhedral crystal appeared in the background of the uniform polycrystalline hydrate film formed by gas mixtures with a high methane mole fraction of 0.81–0.82 as a structure II CH 4 -C 2 H 6 double hydrate crystal. The hole is mainly gas with an initial small amount of structure I hydrate, and that hole will be filled with structure II hydrate after a certain time. The experimental methods and the new findings presented in this work should be significant for the further research of hydrate growth kinetics.

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.

The hydrate structure type and dissociation behavior for pure methane and methane-ethane hydrates at temperatures below the ice point and atmospheric pressure were investigated using in situ Raman spectroscopic analysis. The self-preservation effect of sI methane hydrate is significant at lower temperatures (268.15 to 270.15 K), as determined by the stable C-H region Raman peaks and AL/AS value (Ratio of total peak area corresponding to occupancies of guest molecules in large cavities to small cavities) being around 3.0. However, it was reduced at higher temperatures (271.15 K and 272.15 K), as shown from the dramatic change in Raman spectra and fluctuations in AL/AS values. The self-preservation effect for methane-ethane double hydrate is observed at temperatures lower than 271.15 K. The structure transition from sI to sII occurred during the methane-ethane hydrate decomposition process, which was clearly identified by the shift in peak positions and the change in relative peak intensities at temperatures from 269.15 K to 271.15 K. Further investigation shows that the selectivity for self-preservation of methane over ethane leads to the structure transition; this kind of selectivity increases with decreasing temperature. This work provides new insight into the kinetic behavior of hydrate dissociation below the ice point.

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.