Gas transport and diffusion coefficients in a coupling coal system of matrix and nano-fracture: A molecular simulation study

Abstract

Handling the gas transport mechanisms in fractured media is vital for the exploitation of coalbed methane (CBM). The gas storing and transport capability in coals relies to a great extent on the nano-structure. In this research, a combined matrix and nano-fracture system based on the double-porosity model was constructed. We aimed at providing molecular-level insights into the gas transport behaviors through simulation methods. The results demonstrated that the average density of methane in nano-fracture was obviously beyond that in matrix. Meanwhile, there was a significant equilibration time-lag between matrix and fracture. Gas molecules distributed in the nano-fracture had high diffusion rate. In addition, the adsorption layers were formed in the transition area between matrix and fracture with the increased densities. The gas molecules distributed in coal matrix randomly moved in pores. By contrast, the methane in nano-fracture and adsorption layers moved directionally along Z-axis and YZ plane, respectively. The simulated self- and transport diffusion coefficients were calculated, and the results turned out to coincide well with the experimental data. The diffusivity component parallel to fracture played a major role in the whole diffusion. There was a strong pressure dependence of diffusivity in the case of a small channel width. For the impact of surface geometry, the molecular velocity in the cylindrical system increased dramatically compared with that in the cubic block. The molecular simulation could be an effective means to correctly characterize the micro-transport properties of CBM molecules.