Numerical Analysis of Experiments on Thermally Induced Dissociation of Methane Hydrates in Porous Media

Numerical Investigation into the Development Performance of Gas Hydrate by Depressurization Based on Heat Transfer and Entropy Generation Analyses

The purpose of this study is to analyze the dynamic properties of gas hydrate development from a large hydrate simulator through numerical simulation. A mathematical model of heat transfer and entropy production of methane hydrate dissociation by depressurization has been established, and the change behaviors of various heat flows and entropy generations have been evaluated. Simulation results show that most of the heat supplied from outside is assimilated by methane hydrate. The energy loss caused by the fluid production is insignificant in comparison to the heat assimilation of the hydrate reservoir. The entropy generation of gas hydrate can be considered as the entropy flow from the ambient environment to the hydrate particles, and it is favorable from the perspective of efficient hydrate exploitation. On the contrary, the undesirable entropy generations of water, gas and quartz sand are induced by the irreversible heat conduction and thermal convection under notable temperature gradient in the deposit. Although lower production pressure will lead to larger entropy production of the whole system, the irreversible energy loss is always extremely limited when compared with the amount of thermal energy utilized by methane hydrate. The production pressure should be set as low as possible for the purpose of enhancing exploitation efficiency, as the entropy production rate is not sensitive to the energy recovery rate under depressurization.

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

An Experimental Investigation on the Kinetics of Integrated Methane Recovery and CO2 Sequestration by Injection of Flue Gas into Permafrost Methane Hydrate Reservoirs

Large hydrate reservoirs in the Arctic regions could provide great potentials for recovery of methane and geological storage of CO₂. In this study, injection of flue gas into permafrost gas hydrates reservoirs has been studied in order to evaluate its use in energy recovery and CO₂ sequestration based on the premise that it could significantly lower costs relative to other technologies available today. We have carried out a series of real-time scale experiments under realistic conditions at temperatures between 261.2 and 284.2 K and at optimum pressures defined in our previous work, in order to characterize the kinetics of the process and evaluate efficiency. Results show that the kinetics of methane release from methane hydrate and CO₂ extracted from flue gas strongly depend on hydrate reservoir temperatures. The experiment at 261.2 K yielded a capture of 81.9% CO₂ present in the injected flue gas, and an increase in the CH₄ concentration in the gas phase up to 60.7 mol%, 93.3 mol%, and 98.2 mol% at optimum pressures, after depressurizing the system to dissociate CH₄ hydrate and after depressurizing the system to CO₂ hydrate dissociation point, respectively. This is significantly better than the maximum efficiency reported in the literature for both CO₂ sequestration and methane recovery using flue gas injection, demonstrating the economic feasibility of direct injection flue gas into hydrate reservoirs in permafrost for methane recovery and geological capture and storage of CO₂. Finally, the thermal stability of stored CO₂ was investigated by heating the system and it is concluded that presence of N₂ in the injection gas provides another safety factor for the stored CO₂ in case of temperature change.

Microscopic measurements on the decomposition behaviour of methane hydrates formed in natural sands

In this work, the decomposition behaviour of methane hydrate in porous media was investigated microscopically using powder X-ray diffraction, cryogenic scanning electron microscopy and in situ Raman spectroscopy. The effect of grain sizes on the decomposition of methane hydrate was measured. The results showed that bulk hydrates could exist stably at 223 K and atmospheric pressure because of the self-preservation effect. However, hydrate formed in sands was relatively easier to decompose because it had a higher equilibrium pressure compared with bulk hydrate at the same temperature. In this case, there would be a higher decomposition driving force. Interestingly, the complete decomposition time for hydrate formed in sands did not decrease with the decrease in particle size. The shortest decomposition time was observed for the sands with the particle size range of 38-55 μm, which was less than 30 minutes. Moreover methane hydrate was found to decompose faster in the porous medium containing 3.5 wt% NaCl, which suggested that there was almost no self-preservation effect. In situ Raman measurements showed that the integrated intensity ratio of methane in large and small cages (A L/A S) did not change during the decomposition process, suggesting that the methane hydrate crystal units decomposed as an entity in sands. This study provided important data as a basis for drilling fluid technology in hydrate mining.