Experimental Study On Impedance Characteristics And Dissociation Kinetics Of Gas Hydrate In Coal

Coal and gas outburst is a complex dynamic phenomenon during the process of coal mining,which performs as a sudden ejection of high-pressure two-phase flow of gas and coal into the roadway space in a very short period,which will cause serious underground accidents.The formation of hydrate in coal seam can reduce the gas pressure in coal seam and weaken the outburst energy of two-phase flow of gas and coal,so that the occurrence of such accidents can be prevented effectively.The key to prevent the occurrence for coal and gas outburst is to clarify the dynamic characteristics of hydrate formation and dissociation in coal.In this paper,the formation and dissociation processes of gas hydrate are studied by using the high-pressure experimental device of formation and dissociation of gas hydrate with impedance measurement function,and the influence of temperature and saturation on gas hydrate dissociation and gas desorption in coal is investigated emphatically.It provides a more solid theoretical basis and practical guidance for the prevention of coal and gas outburst.According to the five dissociation temperatures(293.15 K,298.15 K,303.15 K,308.15 K and 313.15 K)based on different geological conditions in China,the kinetic experiments of hydrate formation and dissociation in coal-THF system are conducted.The results show that the impedance change can qualitatively describe kinetic characteristics of hydrate formation and dissociation in coal.At the same dissociation temperature,the abrupt change point of pressure and impedance in gas hydrate formation processes in coal-THF system is hydrate nucleation point.Pressure and impedance suddenly change at the same time,which imply methane molecules and tetrahydrofuran(THF)molecules enter hydrate phase at the same time.In the first stage of hydrate dissociation process: the impedance value for the coal-THF system gradually decreases with time and the system pressure does not change significantly,which implies THF hydrate is dissociated first.In the second stage of hydrate dissociation process,the impedance value increases at first and then decreases with time,and the pressure increases gradually with time.At this stage,THF hydrate reformation occur and then it is dissociated gradually.The higher the dissociation temperature,the faster the hydrate dissociation rate.When the dissociation temperature increases to 303.15 K,if it continues to rise,the hydrate dissociation rate will be less affected by dissociation temperature.Only at 293.15 K,the hydrate is not completely dissociated.The gas recovery rate ranges from 46.4% to 52.5%,and the final system pressure ranges from 1.8 MPa to 1.9 MPa.According to the five saturation conditions(20%,40%,60%,80% and 100%),the kinetic experiments of hydrate formation and dissociation in coal-THF system are conducted.The experimental results show that the change of impedance can reflect the distribution of liquid in coal.The dissociation rate of hydrate in coal firstly increases and then decreases with the increase of saturation.The effect for the increase of saturation on gas production recovery is not significant in a certain range of saturations.According to the five saturation conditions(20%,40%,60%,80% and 100%),the kinetic experiments of hydrate formation and gas desorption in coal-THF system are conducted.The experimental results indicate that forming hydrates in coal can effectively prolong the gas desorption time.With the increase of saturation,the instantaneous desorption of gas in the hydrate-bearing coal body shows a trend of increasing first and then decreasing;There was no positive correlation between the increase of saturation and the change trend of gas desorption time and gas cumulative desorption amount.