Molecular Dynamics Simulation of Adsorption and Replacement of Methane in Kerogen Micropores

Abstract

Characteristics of CH4 adsorption and CH4 replacement with CO2 in kerogen micropores were investigated by molecular dynamics (MD) simulations to obtain accurate estimates of CH4 volume and reduction of environmental load by applying multi-stage CO2 fracking for shale gas development. Firstly, CH4 density in the kerogen micropores was found to be about 1.8 times higher than in the mesopores outside the kerogen, indicating that an adsorption model accounting for the micropore filling is essential to correctly evaluate the volume of CH4 adsorption. Secondly, CO2 molecules with linear shape easily passed through the throat of the kerogen micropore, whereas CH4 molecules with regular tetrahedron shape did not. Thirdly, CH4 was easily replaced by CO2 in the kerogen micropores due to the higher affinity for CO2 than CH4 of oxygen atoms, which are much more common than other heteroatoms in the kerogen molecule. Finally, H2O molecules in the kerogen micropores and mesopores were aggregated by hydrogen bonding around the heteroatoms and prevented the replacement of CH4 by blocking the pathways.