Local Diffusivity of CH4 in Clay Nano-Pore; Molecular Dynamics Study on the Effect of Interfacial Water

Effective shale gas production requires a better understanding of mass transport in reservoirs with very low porosity and permeability, which is typically less than 1,000 nano-Darcy. Since 20-80% of methane gas could exist as an adsorbed state in the porous media, numerous studies have focused on gas-mineral interfacial properties. Since it is difficult to observe the adsorbed phase directly, the thickness of which is typically a few nanometers from the surface, molecular simulations have been widely conducted as promising tools for detailed analysis. They have successfully reproduced a variety of interfacial phenomena, including adsorption behaviors, and revealed their mechanisms. However, the roles of in-situ water -dynamic behaviors in particular - are still under discussion. A better understanding of the water-gas-mineral interaction is expected to better interpret core-scale experiments and improve multi scale modelings such as LBM (Lattice Boltzmann Method) - MD (Molecular Dynamics) coupling or pore-network modeling. We performed MD simulations to calculate the local diffusivity of CH4 in nano clay pore systems. We show that water molecules decrease the diffusivity of CH4 on the surface, and it significantly depends on water-mineral interactions.