Molecular simulation of the potential effects of oxygen functionalities on the adsorption and diffusion of methane in kerogen

Oxygen functionalities are vital components in the organic molecules of kerogen and could influence methane adsorption and diffusion through polarity, hydrogen bonds, and intermolecular interactions. The influence of various oxygen functionalities, generally including –COOH, –OH, –C[dbnd]O, and –O–, on CH₄ behaviors remains elusive. Here we built 3D macromolecule models (K) of highly mature kerogen from Marcellus shale containing no oxygen or only a singular oxygen functionality to explore their specific influence on CH₄ adsorption and diffusion. This was achieved by examining their pore structure, pore compressibility, activation energy, CH₄ adsorption amount, and self- and mass diffusion coefficients. The results reveal that oxygen functionalities promote CH₄ adsorption but impede diffusion. They enhance the pore surface area, pore volume compressibility, activation energy, and polarity of kerogen, providing more adsorption sites for CH₄ and increasing the probability of CH₄ molecules collision with the pore surface, but these components restrict the CH₄ diffusion. Among the oxygen functionalities, the mass diffusion coefficients of K–OH and K–COOH are lower than those of other kerogen models, which is attributed to their larger pore surface area, higher pore volume compressibility, and stronger polarity. The increase in the gas mass diffusion coefficient caused by rising temperature (from 278 to 338 K) is about 20 times as much as reducing pressure (from 10 to 1 MPa), suggesting that increasing reservoir temperature would be more conducive to enhancing the gas recovery than reducing the pressure alone. These observations provide essential theoretical support for enhancing gas recovery from shale and coal reservoirs.