Water vapor adsorption in Devonian Marcellus shale kerogen: Experiment and molecular simulation

Water plays significant roles in shale gas accumulation and extraction, yet the interactions between water and shale kerogen are still unclear due to the complexity of the natural pore system and molecular structure of kerogen. This study investigates the adsorption mechanism of water vapor in shale kerogen by analyzing the dynamic vapor sorption (DVS) experimental results, combined with molecular simulations based on a realistic model of Marcellus shale kerogen. Pore characteristics from low-pressure nitrogen and carbon dioxide adsorption, as well as scanning electron microscopy, were used to further understand water vapor adsorption. The experimental and simulation results reveal that the condensation of water vapor exhibits a significant pore size effect. Water vapor condenses more readily in pores smaller than 1.5 nm, while in larger pores, it primarily adsorbs on the surface. Within the pore structure of shale kerogen, the vast majority of water vapor resides in the micropores of the shale kerogen matrix. Mesopores and macropores occupy similar to 10% of the total specific surface area, with less than 5% of the water molecules adsorbed on the surfaces of mesopores and macropores. DVS experiments indicate that the amount of water vapor adsorption increases rapidly when the relative humidity exceeds 80%. Molecular simulation results show that this increase is due to the condensation of water vapor in the ultra-micropores (<1 nm) of the kerogen matrix. The larger free space in mesopores and macropores is not conducive to the condensation of water vapor.