Mechanisms of shale gas adsorption: Insights from a comparative study on a thermodynamic investigation of microfossil-rich shale and non-microfossil shale

Abstract Understanding the methane adsorption mechanism in shale, especially for shale with abundant microfossils, is significant for guiding the exploration and development of shale gas. In this study, two groups of organic-rich shale samples derived from the Wufeng-Lungmachi formations were selected for methane adsorption experiments: one core sample contains abundant radiolarian fossils, while the other core sample does not contain any radiolarian fossils. Through a series of calculations and comparisons of thermodynamic studies between radiolarian-rich shale and non-radiolarian shale, the results show that the absolute adsorption of methane in radiolarian-rich shale is greater than the absolute adsorption of methane in non-radiolarian shale at the same pressure and temperature. In addition, the difference in the thermodynamic parameters such as the isosteric heat of adsorption (Qst), enthalpy change (ΔH), standard entropy change (ΔS0) and Gibbs free energy change (ΔG) between radiolarian-rich shale and non-radiolarian shale indicates that the interaction between radiolarian-rich shale and methane molecules is strong, and that radiolarian-rich shale is likely to have more adsorbed methane than non-radiolarian shale under similar geological conditions. Two plausible explanations are proposed to explain the methane adsorption mechanisms in radiolarian-rich shale. On the one hand, the biological pores developed in radiolarian fossils provide some adsorption sites for methane molecules. On the other hand, the amorphous silica in the radiolarian fossils can combine with organic molecules to form a new functional group called trimethylsilane, which possesses a hydrophobic character. The organic-inorganic pore surface provides a new perspective for interpreting methane adsorption mechanisms in shale.