Two-way coupling dynamics of CH4 adsorption and coal matrix deformation: Insights from hybrid GCMC/MD simulations

Abstract

• CH 4 adsorption-induced matrix deformation is studied by a hybrid GCMC/MD method. • The volumetric strain correlates linearly with CH 4 loading in various samples. • Pore expansion occurs by creating additional pores of 3.3 ∼ 5.2 Å size. • Matrix compacts macroscopically without major changes to the carbon framework. • Flexible matrix absorbs heat from CH 4 adsorption, lowering isosteric heat. CH 4 adsorption can deform coal microporous structures, subsequently altering adsorption isotherms. To unravel this intricate interplay at a microscopic level, we use a hybrid Grand Canonical Monte Carlo/Molecular Dynamics (GCMC/MD) simulation at 313.15 K and pressures up to 500 bar on five independent amorphous coal matrix models. Our results reveal that CH 4 adsorption increases pore volume and porosity primarily by generating additional pores of similar sizes to those present in coal matrices, thereby maintaining a consistent average pore size across different pressures. The volumetric strain has a linear correlation with CH 4 loading, with volumetric swelling amount approximating the expansion of CH 4 -occupiable pore volume, but less than He-occupiable pore volume which results in increased matrix skeletal density. Local radial density distribution of carbon atoms indicates that the immediate environment around carbon atoms remains unchanged. In a flexible matrix, the energy released during CH 4 adsorption is partially absorbed by matrix deformation, resulting in a lower isosteric heat of adsorption compared to a rigid matrix, which suggests easier desorption. This study provides new insights into the mutual relationship between CH 4 adsorption and coal matrix deformation, shedding lights on the complex interactions of various hydrocarbons with geomaterials.