Stuides On Nucleation Induction Time And Growth Kinetics Of Hydrate Formation In Porous Media

As a new type of alternative energy with huge reserves,gas hydrate has attracted much attention for its exploration and exploitation and application of transportation technology.The natural gas hydrates in China are mainly deposited in the sedimentary layers of the the South China Sea Slope and permafrost regions of the Qinghai Tibet Plateau,and the reserves of natural gas hydrate in the South China Sea are estimated to be 700 million tons of oil equivalent.The kinetic characteristics of gas hydrate formation and reformation in porous media are of great significance for understanding the formation law of natural gas hydrates and the safe and efficient exploitation of natural gas hydrates.Based on this background,the kinetic characteristics of methane hydrate formation and reformation in porous media are systematically studied,and the nucleation induction time and growth kinetics characteristics of hydrate formation are emphatically explained in this paper.The kinetic characteristics of hydrate formation in porous media were studied by using methane hydrate formation test platform and nuclear magnetic resonance imaging system.The accurate measurement of the induction time of hydrate formation in porous media has been realized.The effects of temperature and pore characteristics on the kinetic characteristics of hydrate formation have been studied.The phenomenon of "memory effect" existing in the hydrate reformation is further studied.The real time observation of methane hydrate in the porous media is realized,the consistency of nuclear magnetic resonance imaging and pressure change method in measuring the induction time of hydrate nucleation is analyzed.The data and theoretical support are provided for the hydrate formation simulation in porous media.The methane hydrate nucleation induction time test system was set up,and a total of 44 groups of working conditions and 1106 induction time data points are obtained.The nonparametric test of the induction time data is carried out by frequency histogram,P-P diagram and K-S test,and the incomplete randomness of the hydrate nucleation induction time is found.It is obtained that the induction time conforms to the law of lognormal distribution.The quantitative analysis method of the influence of temperature,particle size,initial water saturation and "memory effect" on the distribution of induction time was put forward,and the influence mechanism of different factors on the distribution was revealed.The expected value of lognormal distribution of induction time is introduced into the induction time prediction model to improve the Kashchiev model.The kinetic parameter related to the effective diffusion coefficient of methane hydrate heterogeneous nucleation in porous media and the thermodynamic parameter related to the effective surface energy are corrected.The prediction of induction time for methane hydrate formation and reformation in porous media is realized,and the prediction accuracy of the model is greatly improved.On the basis of the multistep mechanism of hydrate formation,the formation step of"unstable molecular clusters" in the process of methane hydrate formation was simplied by using non-polar and hydrophobic characteristics of methane molecules,and the formation step of the "cage-like crystal unit cells" was simplied by combining the "residual structure" theory.The prediction of methane hydrate formation and reformation including the induction and growth period in porous media has been realized.The change law of each intermediate product and the rate constant of each step in the formation of methane hydrate can be accurately obtained,and the main controlling factors of the nucleation,reforamtion and growth period of methane hydrate formation in porous media are clarified.