Numerical simulation of CH4 recovery from gas hydrate using gaseous CO2 injected into porous media

As for the replacement of CH4 by CO2 from CH4 hydrate-bearing sediment, the replacement rate strongly depends on the diffusion of gas through the hydrate layer, so it is necessary to investigate the diffusion-limited transportation as well as the mass and heat transfer in the porous media. In this paper, a two-dimensional numerical model combining thermodynamic equilibrium and kinetic model through effective fugacity was proposed to address the above issues when the total pressure and temperature are in the hydrate stable zone in poor water (<5%) sediment. Water existence location (sealed under the hydrate layer or distributes freely on the surface of the hydrate layer), the formations of mixed hydrate, and the storage of CO2 in hydrate through the reaction with water were taken into account. Then, this model was used to study the replacement process in a lab-scale core sample. The results showed that a high diffusion coefficient of mixed gases and low permeability can lead to both a relatively large replacement percentage and CO2 storage efficiency. An increase of initial pressure is only beneficial to the formation of CO2 hydrate but it has little influence on the replacement percentage, while an increase of initial temperature leads to a high replacement percentage and low CO2 storage efficiency. Water is adverse to the replacement while benefiting the formation of CO2 hydrate when it is freely distributed in pores. But when the water is sealed under the CH4 hydrate layer, it barely has an impact on the replacement. Besides, a little amount of water can be produced under low initial pressure. CO2 storage efficiencies are always larger than replacement percentages. The largest value of CO2 storage efficiency can exceed 50%, while that of the replacement percentage is less than 40% under present conditions.