The combustion characteristics of both pure propane hydrates and the mixtures of hydrates and quartz sands were investigated by combustion experiments. The flame propagation, flame appearance, burning time and temperature in different hydrate layers were studied. For pure propane hydrate combustion, the initial flame falls in the “premixed” category. The flame propagates very rapidly, mainly as a result of burnt gas expansion. The flame finally self-extinguishes with some proportion of hydrates remaining unburned. For the hydrate-sand mixture combustion, the flame takes the form of many tiny discontinuous flames appearing and disappearing at different locations. The burn lasts for a much shorter amount of time than pure hydrate combustion. High porosity and high hydrate saturation is beneficial to the combustion. The hydrate combustion is the combustion of propane gas resulting from the dissociation of the hydrates. In both combustion test scenarios, the hydrate-dissociated water plays a key role in the fire extinction, because it is the main resistance that restrains the heat transfer from the flame to the hydrates and that prevents the hydrate-dissociated gas from releasing into the combustion zone.

In marine sediments, seawater influences the phase behavior of natural gas hydrate. As a porous medium, the water distribution and physical properties of montmorillonite are influenced by the salt ions in seawater. In this work, the bound-water content in, and crystal structure of, montmorillonite is measured to investigate the effect of salt ions on the water distribution in montmorillonite. It can be determined from the results that the bound-water content in montmorillonite decreases as the salt-ion concentration increases. Salt ions affect the intercalation of water molecules in montmorillonite, and they then inhibit the expansion effect of montmorillonite. Next, the phase behaviors of methane hydrate in montmorillonite with NaCl solution are investigated using high-pressure micro-differential scanning calorimetry. The phase behavior of hydrate in montmorillonite with NaCl solution is discussed. In montmorillonite with NaCl solution, the phase equilibrium temperatures and the conversion rate of methane hydrate both decrease with increasing NaCl concentration. The results show that methane hydrate in montmorillonite is influenced not only by the phase-equilibrium effect of salt ions, but also by the formation effect of the salt ions on the bound-water content in montmorillonite.

As a typical unconventional energy reservoir, natural gas hydrate is believed to be the most promising alternative for conventional resources in future energy patterns. The exploitation process of natural gas hydrate comprises a hydrate phase state, heat and mass transfer, and multi-phase seepage. Therefore, the study of heat transfer characteristics of gas hydrate is of great significance for an efficient exploitation of gas hydrate. In this paper, the research methods and research progress of gas hydrate heat transfer are reviewed from four aspects: measurement methods of heat transfer characteristics, influencing factors of heat transfer in a hydrate system and hydrate-containing porous media systems, predictive models for effective thermal conductivity, and heat transfer mechanisms of hydrate. Advanced measurement techniques and theoretical methods that can be adopted for the heat transfer characteristics of gas hydrate in the future are discussed.

As one of the most important future clean energy sources, natural gas hydrate (NGH) is attracting widespread attention due to the vast reserves available and high energy density. How to extract this source in a safe, efficient, and environmentally friendly manner has become the key to the commercial utilization of its resources. This paper reviews the recent advances in the study of the fundamental reservoir properties of offshore NGH, summarizing the methods and technologies for testing the sedimentary properties of reservoirs, analyzing the characteristics in reservoir mechanics, electrics, thermodynamics, and fluid dynamics, and discusses the influence of reservoir fundamental properties on NGH exploitation. The aim is to provide guidance and reference for research on the exploitation of NGH in different target exploitation areas offshore.

A gas-lifting production method was firstly proposed to transport the methane-water mixture from natural gas hydrates deposits through marine vertical pipe in this work. Aiming at UBGH2-6 site, SH7 site and GMGS2-8 site, the gas-lifting performance of methane-water mixture in the vertical pipe was investigated by numerical calculation. The potential of Natural gas hydrates (NGH) self-eruption production induced by the gas-lifting process under ideal conditions was also studied based on the energy analysis. The calculation results indicate that the gas-lifting method has great advantage in avoiding the secondary hydrates formation in marine vertical pipe and reducing energy consumption. The gas-lifting process in the vertical pipe is testified to be spontaneous in UBGH2-6 site and SH7 site during the initial 4000 and 1000 days, respectively, which indicates the energy consumption for methane-water mixture transportation is saved. Sufficient heat supply for the hydrate dissociation is crucial for the NGH self-eruption production. Sensitivity analysis indicates that the water-gas ratio has more significant influences on gas-lifting performance in the vertical pipe compared to the flow rate. With the decrease of water-gas ratio, the bottomhole pressure decreases rapidly. Thus, the reduction of water production is effective to improve the gas-lifting performance.