Origin of Self-preservation Effect for Hydrate Decomposition: Coupling of Mass and Heat Transfer Resistances
Gas hydrates could show an unexpected high stability at conditions out of thermodynamic equilibrium, which is called the self-preservation effect. The mechanism of the effect for methane hydrates is here investigated via molecular dynamics simulations, in which an NVT/E method is introduced to represent different levels of heat transfer resistance. Our simulations suggest a coupling between the mass transfer resistance and heat transfer resistance as the driving mechanism for self-preservation effect. We found that the hydrate is initially melted from the interface, and then a solid-like water layer with temperature-dependent structures is formed next to the hydrate interface that exhibits fractal feature, followed by an increase of mass transfer resistance for the diffusion of methane from hydrate region. Furthermore, our results indicate that heat transfer resistance is a more fundamental factor, since it facilitates the formation of the solid-like layer and hence inhibits the further dissociation of the hydrates. The self-preservation effect is found to be enhanced with the increase of pressure and particularly the decrease of temperature. Kinetic equations based on heat balance calculations is also developed to describe the self-preservation effect, which reproduces our simulation results well and provides an association between microscopic and macroscopic properties.
Evolution of bubbles in decomposition and replacement process of methane hydrate
Abstract Nanobubbles have been proved to play an important role for the extraction kinetics of CH4 hydrates. Existing investigations on nanobubbles mainly focus on the hydrate melting. However, the mechanism and effect of bubble evolution during the replacement of CH4 hydrates by CO2 are still elusive. Therefore, molecular dynamics simulations on melting and replacement of CH4 hydrate are performed with the same initial system size, pressure and amount of CH4 hydrates. It is proved the two processes move layer by layer but with different features. Moreover, despite the different composition of gas phases, behaviours of nanobubbles are similar during the two processes except that the gas film is not formed during the replacement. In addition, a quantitative analysis of molecules in each nanobubble during the decomposition is conducted preliminarily. Also a remarkable effect of mixed nanobubble on the replacement is revealed based on the analysis of bubble behaviour. It is found that CH4 molecules are surrounded by some dense CO2 molecules in the mixed bubble during the replacement. Moreover, CO2 molecules in the solution can assist the bubble formation of CH4 molecules at an initial stage and prevent their further escape and reoccupation by encompassing them afterwards.
Experimental study of brine injection and depressurization methods for dissociation of gas hydrates
The two most promising techniques for producing natural gas from hydrate reservoirs are depressurization and brine injection. This paper examines the dissociation characteristics of methane hydrates during these processes. A correlation for the rate of hydrate dissociation during brine injection as a function of salinity, brine temperature, brine injection rate, pressure, and hydrate/brine surface area is presented. Depressurization experiments how that hydrate dissociation result in a decrease in the rate of pressure decline and contributes significantly (15% to 70%) to the cumulative gas production.