RESEARCH PROGRESSES ON THE INFLUENCING FACTORS OF NATURAL GAS HYDRATES PHASE TRANSITION PROCESS IN POROUS MEDIA

Natural gas hydrate(NGH) is regarded as one of the most important clean energy in the 21^st century. It is found that NGH is mostly stored in the porous media of submarine sediments, and some in permafrost zone. Safe and efficient utilization of NGH is important for solving current energy crisis. However, the NGH production is a complex process with many impact factors, especially the phase transition in porous media during exploitation. By comprehensively analyzing previous research results, we generalized the phase transition process of NGH into the change of equilibrium temperature and pressure, transition rate, transition stability, transformation efficiency and distribution pattern, which are significantly influenced by the characteristics of porous media. These influencing factors can be divided into pore diameter and particle size, wettability, thermal conductivity, temperature and pressure, water and gas conditions in porous media. We summarized the influence and its mechanisms as follows: 1)The pore diameter and particle size control the equilibrium temperature and pressure, the small particle size can accelerate the hydrate formation rate and decrease the transformation efficiency. 2)Wettability can determine the contact form between the hydrate and the porous media. The surface of the medium with strong hydrophilicity can speed up the formation rate of NGH. 3)The rate of NGH phase transition increases with the enhancement of the thermal conductivity. 4)The rate of NGH phase transition is positively related to the driving force of pressure and temperature, but this trend has not seen when the driving force of temperature and pressure is too large. 5)The water and gas conditions influence the equilibrium temperature and pressure, transition rate, transition stability and distribution pattern. With extensive literatures reviewed, this paper points out several key problems in current researches, gives some potential research methods and comes up with promising research directions, which is important for the extraction and exploration of NGH.