Pore–Scale Study of Effects of Hydrate Morphologies on Dissociation Evolutions Using Lattice–Boltzmann Method

Abstract

The natural gas hydrate, plentifully distributed in ocean floor sediments and permafrost regions, is considered a promising unconventional energy resource. The investigation of hydrate dissociation mechanisms in porous media is essential to optimize current production methods. To provide a microscopic insight in the hydrate dissociation process, we developed a Lattice Boltzmann (LB) model to investigate this multi–physicochemical process, including mass transfer, conjugate heat transfer, and gas transport. The methane hydrate dissociation is regarded as the reactive transport process coupled with heat transfer. The methane transport in porous media is modelled by the generalized LB method with the Bhatnagar-Gross-Krook (BGK) collision model. The mass transfer from hydrate to fluid phase is described by the hydrate kinetic and thermodynamic models. Finally, the conjugate heat transfer LB-model for heterogeneous media is added for solving the energy equation. In the numerical experiments, we primarily investigated the effects of different hydrate distribution morphologies such as pore–filling, grain–coating, and dispersed on the hydrate dissociation process. From simulations, we found that in general, the dissociation rate and the methane average density rapidly approached the maximum value and then decreased with fluctuation during the dissociation process. This trend is due to that the endothermic reaction heat decreased the temperature, resulting in decelerating the dissociation. The average temperature decreased to minimum value instantaneously as hydrate started to dissociate. After the minimum value, the average temperature would increase slowly, accompanied by the thermal stimulation and hydrate consumption, displaying a valley shape of the temperature curve. We also found that the whole dissociation process and permeability–saturation relations are significantly affected by the hydrate morphologies. Under the same hydrate saturation, the dispersed case dissolves the fastest, whereas the grain–coating case is the slowest. Furthermore, we proposed a general permeability–saturation relation applicable for three cases, filling the gap in the current relative permeability models. The LB model proposed in this study is capable to simulate the complex physicochemical hydrate dissociation process. Considering the impacts of thermodynamic conditions (P,T), we investigated their influences on the coupled interaction between dissociation and seepage under three different morphologies and proposed a general permeability–saturation relationship. The results can be applied as input to adjust parameters in the continuum model, and provide instructions for exploring clean energy with environmental considerations.