Shale gas in type II kerogen. Transport properties of shale gas and its individual components were studied in 24 type II kerogen models comprising a variety of porosity characteristics (e.g. limiting pore diameter, accessible pore surface etc.). Shale gas was modelled as a mixture of methane, ethane, carbon dioxide and nitrogen having molar fractions of 0.85, 0.7, 0.4 and 0.4 respectively. The kerogen structures had been previously constructed using a well-established protocol based on isothermal isobaric (NPT) MD simulations. The porosity of the model structures was characterized with a newly developed algorithm, quantifying the number of pores in the structure, their degree of percolation, the maximum and limiting diameters, and the % porosity. The structures were loaded with the model gas by means of stochastic MC simulations at the grand canonical ensemble (GCMC) so that the confined gas is in equilibrium with the bulk gas phase at the same conditions. MD simulations were then performed at the canonical ensemble (NVT), and the self-diffusion coefficients of gas components calculated using the Einstein relationship from the slope of the molecular mean squared displacements versus time curve. It was proven that ethane’s diffusion coefficient is half the diffusion coefficient of methane, in every structure independently of its porosity characteristics. Limiting pore diameter is the only porosity characteristic that has a strong effect on the observed diffusion coefficients, while the maximum pore diameter has no effect at all (Figure 8). There is also some dependence on the accessible surface area of the pore. A similar behaviour was observed for the other gases examined for both temperatures (298.15 K and 398.15 K) and pressures (1 atm and 250 atm) considered. A comparison of the transport properties of mixture components under confinement with their corresponding pure components values, showed that each component is not affected by the presence of other molecules in the mixture. MPD (Å) LPD MPD Linear (LPD) 14 DCH4 (10 cm s ) 10 4 2 LPD (Å) Figure 8: Maximum component of the diffusion coefficient of pure methane in a variety of type II kerogen models as function of the limiting (primary horizontal axis) and maximum (secondary horizontal axis) pore diameters of the structure at 298.15 K and 250 atm. A linear trend line is fitted to the D vs LPD points.
