Recovery of Multicomponent Shale Gas from Single Nanopores

The adsorption of multicomponent gas mixtures in shale formations and their recovery are of great interest to the shale gas industry. Here we report molecular dynamics simulations of the adsorption of methane/ethane mixtures in 2 and 4 nm-wide nanopores and their recovery from these nanopores. Surface adsorption contributes significantly to the storage of methane and ethane inside the pores, and ethane is enriched inside the nanopores in equilibrium with bulk methane–ethane mixtures. The enrichment of ethane is enhanced as the pore is narrowed but is weakened as the pressure increases due to entropic effects. These effects are captured by the ideal adsorbed solution (IAS) theory, but the theory overestimates the adsorption of both gases. Upon opening the mouth of the nanopores to gas baths with lower pressure, both gases enter the bath. The production rates of both gases show only weak deviation from the square root scaling law before the gas diffusion front reaches the dead end of the pores. The ratio of the production rate of ethane and methane is close to their initial mole ratio inside the nanopore despite the fact that the mobility of pure ethane is smaller than that of pure methane inside the pores. Scale analysis and calculation of the Onsager coefficients for the transport of binary mixtures of methane and ethane inside the nanopores suggest that the strong coupling between methane and ethane transport is responsible for the effective recovery of ethane from the nanopores