Reaction Kinetic Characteristics and Model of Methane Hydrate Formation in Porous Media

摘要

Most of the natural gas hydrates on Earth are buried in shallow formations under deep water. Comprehensively understanding the reaction kinetic characteristics of gas hydrate in porous media is very beneficial to the deep exploration of the hydrate accumulation in nature. In this paper, the formation process of CH4 hydrate in porous media was simulated physically, using a reactor that is operating at high pressure and low temperature. The hydrate phase equilibrium and reaction kinetic characteristics at different temperatures, pressures, sand grain sizes, and clay contents were assessed. Based on the determination of relevant hydrate kinetic parameters, a novel mixing-flux hydrate reaction model was proposed, which can be used for numerical simulation of gas hydrate accumulation. The experimental results show that the porous media can make the phase equilibrium of CH4 hydrate shift to the right under the capillary effects on the gas and hydrate phases. Low temperature and high pressure can provide a large driving force for hydrate formation, but large clay content and small sand grain size usually give a negative effect on the CH4 transfer in the porous media. It often leads to a slow hydrate formation rate and hard distinction of pressure drop between hydrate nucleation and growth stages. Based on the experimental results, the hydrate nucleation kinetic parameters were regressed, and the activation energy (E-a), as well as the reaction frequency factor (k(fo)), of hydrate growth were fitted to be 75.45-90.85 kJ/mol and 8.72 x 10(8)-6.02 x 10(11) mol/(m(2) kPa day), respectively. In the numerical simulation of hydrate accumulation, the hydrate formation process can be described by coupling the low-flux reaction and the high-flux reaction, which consume the CH4 dissolved in water and the free CH4 gas in pores, respectively. This novel mixing-flux hydrate formation model is suitable for the flexible and practical hydrate accumulation simulation, which can consider various gas sources and transfer states in the hydrate reservoir.