Methane adsorption mechanisms in deep/ultra-deep shale and its insights for adsorption model

Y Li and GH Chen and ZX Cai and LJ Yu and HX Yan and SF Lu and SM Guo and YJ Zhang and NW Zhou and WB Li and PF Zhang, FUEL, 398, 135593 (2025).

DOI: 10.1016/j.fuel.2025.135593

Limited understanding of adsorption mechanisms for shale gas in deep and ultra-deep formations makes accurate prediction of adsorption capacity using mathematical models challenging. This research uses GCMC and MD methods to investigate methane occurrence in organic nanopores at varying depths and pore sizes. With increasing burial depth, interactions between adsorption layers strengthen, which explains the presence of a distinct third adsorption layer in deep and ultra-deep shale formations. Pore size significantly affects methane adsorption behavior. In micropores, the potential overlap effect reduces methane mobility. This overlapping effect weakens as pore size increases, and free methane begins to appear at around 2 nm. Next, we discuss the coupled effects of temperature and pressure on the flow and diffusion rates of different adsorption layers. As burial depth increases, the dominant factor influencing methane diffusion shifts from pressure to temperature. The results also indicate that the average density and flow rate of methane in the third adsorption layer are similar to those in the bulk, which is why it is excluded from the adsorption calculations. Finally, the classic Langmuir model is modified based on the adsorption mechanisms, which is one of the innovations of this study. Statistical results of adsorbed phase density/thickness show that treating them as constants for fitting is not entirely appropriate. Replacing the adsorbed phase density with volume and transforming it into a pressuredependent logarithmic function results in good agreement between the model predictions and measured values, with an average relative error under 10 %.

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