Gas Adsorption Capacity of Type-II Kerogen at a Varying Burial Depth

Kerogen is the main host of methane in most shale formations. Although many studies focused on its isothermal adsorption, the relationship between its methane adsorption capacity and structural characteristics has yet been well addressed. The pore structures and methane adsorption of four kinds of type-II kerogens with different maturities and at a varying burial depth are studied using a combined approach of molecular dynamics and grand canonical Monte Carlo simulations. Attributed to the combined effect of temperature, pressure, and the surface structures of kerogen, the methane adsorption capacity varies with the kerogen maturity and the burial depth. A maximum methane adsorption is predicted at the depth of 1-3 km, which agrees with the observations. Meanwhile, a mismatch between the porosity change and the adsorption change of kerogens is noted and then interpreted by the surface structure evolution of kerogens. Analysis on the amorphous kerogens reveals that the surface and bulk compositions of kerogens are different, and the difference varies with the kerogen maturity. Methane adsorption depends not only on the porosity but also on the proportion of aromatic and aliphatic carbon atoms on the surface. The simulations are expected to be useful for the reserve prediction and interpretation of shale gas formations.

Automated summary

The pore structures and methane adsorption of different maturity kerogens at varying burial depths were studied using molecular dynamics and grand canonical Monte Carlo simulations. The results show that methane adsorption capacity varies with kerogen maturity and burial depth, and there is a mismatch between surface structure evolution and pore change in kerogens.