Methane dual-site adsorption in organic-rich shale-gas and coalbed systems

• Texture of organic-rich shale and coal bed samples are characterized.
• A lattice DFT based model is developed for dual site adsorption.
• Methane adsorption on the samples was quantified by the model.
• We determined the extend of methane adsorbed in the organic matter.

Herein, we report on a novel dual-site adsorption model to determine state of methane in shale formations and coalbeds. By considering two surface energy terms, the model distinguishes methane adsorption in the intrinsic micropore volume of the organic matter from that of the inorganic structure. One energy term describes methane adsorption in micropores (< 2 nm) whereas the other one concerns with meso-macropores. This study showed that at storage conditions, 150 bar and 50.4 °C, where methane is in supercritical state, the surface energy plays the major role to influence methane adsorption capacity whereas the effect of lateral forces between adsorbed molecules has a minor influence on overall adsorption capacity. We applied the model on experimental methane adsorption data in three organic-rich samples including Woodford shale, Woodford isolated kerogen and Cameo coal to quantify their methane storage state. Textural properties of shale and coal samples were analyzed by N2 porosimetry at − 196 °C. The samples had a weak micropore peak followed by a broader peak stretched over meso and macropore regions up to nearly 120 nm. Fitting the dual-site model to experimental methane adsorption data revealed that, among all, the coal sample, with the average heat of adsorption of 24.5 kJ/mol, had the highest interaction energies with both micro- and mesopore sites. Further, we find that 37%, 81% and 83% of methane was stored in the organic matter of Woodford shale, Woodford isolated kerogen and Cameo coal, respectively. Considering low maturity, and hence low micropore volume of the samples, and based on significant methane storage in the organic content we estimated that more than 50% of methane gas is being dissolved into the organic matter rather than just being adsorbed into the micropore volume.

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